11.8: Mollusks - Biology

11.8: Mollusks - Biology

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Fish or squid?

Neither. This is a mollusk, a cuttlefish to be specific. What is a mollusk? Well, to start, mollusks are aquatic species that are not fish. There are over 100,000 different mollusks, so there are bound to be some interesting looking organisms, like this one.


Have you ever been to the ocean or eaten seafood? If you have, then you probably have encountered members of the phylum Mollusca. Mollusks include snails, scallops, and squids, as shown in Figure below. There are more than 100,000 known species of mollusks. About 80 percent of mollusk species are gastropods.

This figure shows some of the more common and familiar mollusks.

Structure and Function of Mollusks

Mollusks are a very diverse phylum. Some mollusks are nearly microscopic. The largest mollusk, a colossal squid, may be as long as a school bus and weigh over half a ton! The basic body plan of a mollusk is shown in Figure below. The main distinguishing feature is a hard outer shell. It covers the top of the body and encloses the internal organs. Most mollusks have a distinct head region. The head may have tentacles for sensing the environment and grasping food. There is generally a muscular foot, which may be used for walking. However, the foot has evolved modifications in many species to be used for other purposes.

Basic Mollusk Body Plan. The basic body plan shown here varies among mollusk classes. For example, several mollusk species no longer have shells. Do you know which ones?

Two unique features of mollusks are the mantle and radula (see Figure above). The mantle is a layer of tissue that lies between the shell and the body. It secretes calcium carbonate to form the shell. It forms a cavity, called the mantle cavity, between the mantle and the body. The mantle cavity pumps water for filter feeding. The radula is a feeding organ with teeth made of chitin. It is located in front of the mouth in the head region. Herbivorous mollusks use the radula to scrape food such as algae off rocks. Predatory mollusks use the radula to drill holes in the shells of their prey.

Mollusks have a coelom and a complete digestive system. Their excretory system consists of tube-shaped organs called nephridia (see Figure above). The organs filter waste from body fluids and release the waste into the coelom. Terrestrial mollusks exchange gases with the surrounding air. This occurs across the lining of the mantle cavity. Aquatic mollusks “breathe” under water with gills. Gills are thin filaments that absorb gases and exchange them between the blood and surrounding water.

Mollusks have a circulatory system with one or two hearts that pump blood. The heart is a muscular organ that pumps blood through the circulatory system when its muscles contract. The circulatory system may be open or closed, depending on the species.

The major classes of mollusks vary in structure and function. You can read about some of their differences in Figure below.

Use this figure to compare and contrast gastropods, bivalves, and cephalopods

Mollusk Reproduction

Mollusks reproduce sexually. Most species have separate male and female sexes. Gametes are released into the mantle cavity. Fertilization may be internal or external, depending on the species. Fertilized eggs develop into larvae. There may be one or more larval stages. Each one is different from the adult stage. Mollusks have a unique larval form called a trochophore. It is a tiny organism with cilia for swimming.

Ecology of Mollusks

Mollusks live in most terrestrial, freshwater, and marine habitats. However, the majority of species live in the ocean. They can be found in both shallow and deep water and from tropical to polar latitudes. Mollusks are a major food source for other organisms, including humans. You may have eaten mollusks such as clams, oysters, scallops, or mussels.

The different classes of mollusks have different ways of obtaining food.

  • Gastropods may be herbivores, predators, or internal parasites. They live in both aquatic and terrestrial habitats. Marine species live mainly in shallow coastal waters. Gastropods use their foot to crawl slowly over rocks, reefs, or soil, looking for food.
  • Bivalves are generally sessile filter feeders. They live in both freshwater and marine habitats. They use their foot to attach themselves to rocks or reefs or to burrow into mud. Bivalves feed on plankton and nonliving organic matter. They filter the food out of the water as it flows through their mantle cavity.
  • Cephalopods are carnivores that live only in marine habitats. They may be found in the open ocean or close to shore. They are either predators or scavengers. They generally eat other invertebrates and fish.

KQED: Cool Critters: Dwarf Cuttlefish

What's the coolest critter in the ocean under 4 inches long? The Dwarf Cuttlefish! Cuttlefish are marine animals that belong to the class Cephalopoda. Despite their name, cuttlefish are not fish but mollusks. Recent studies indicate that cuttlefish are among the most intelligent invertebrates, with one of the largest brain-to-body size ratios of all invertebrates. Cuttlefish have an internal shell called the cuttlebone and eight arms and two tentacles furnished with suckers, with which they secure their prey.

KQED: The Fierce Humboldt Squid

The Humboldt squid is a large, predatory invertebrate found in the waters of the Pacific Ocean. A mysterious sea creature up to 7 feet long, with 10 arms, a sharp beak and a ravenous appetite, packs of fierce Humboldt Squid attack nearly everything they see, from fish to scuba divers. Traveling in groups of 1,000 or more and swimming at speeds of more than 15 miles an hour, these animals hunt and feed together, and use jet propulsion to shoot out of the water to escape predators. Humboldt squid live at depths of between 600 and about 2,000 feet, coming to the surface at night to feed. They live for approximately two years and spend much of their short life in the ocean's oxygen-minimum zone, where very little other life exists. Because they live at such depths, little is known about these mysterious sea creatures. The Humboldt squid usually lives in the waters of the Humboldt Current, ranging from the southern tip of South America north to California, but in recent years, this squid has been found as far north as Alaska. Marine biologists are working to discover why they have headed north from their traditional homes off South America.

Where's the Octopus?

When marine biologist Roger Hanlon captured the first scene in this video, he started screaming. Hanlon, senior scientist at the Marine Biological Laboratory in Woods Hole, studies camouflage in cephalopods: squid, cuttlefish and octopuses. They are masters of optical illusion.


  • Mollusks are invertebrates such as snails, scallops, and squids.
  • Mollusks have a hard outer shell. There is a layer of tissue called the mantle between the shell and the body.
  • Most mollusks have tentacles for feeding and sensing, and many have a muscular foot.
  • Mollusks also have a coelom, a complete digestive system, and specialized organs for excretion.
  • The majority of mollusks live in the ocean.
  • Different classes of mollusks have different ways of obtaining food.


  1. List the three major classes of mollusks.
  2. Describe the basic body plan of a mollusk.
  3. Describe the mantle and radula.
  4. What are gills? What is their function?
  5. Create a Venn diagram to show important similarities and differences among the three major classes of mollusks.

11.8: Mollusks - Biology

This laboratory exercise covers the following animals. You should learn this classification scheme and be able to classify the animals into these categories.

  • Phylum: Mollusca (Mollusks)
    • Class: Polyplacophora (Chitons)
    • Class: Gastropoda (snails)
    • Class: Bivalvia (Clams)
    • Class: Cephalopoda (Nautilus, Squid, Octopus)

    All mollusks have a visceral mass, a mantle, and a foot. The visceral mass contains the digestive, excretory, and reproductive organs. The mantle is a covering. It may secrete a shell. The foot is muscular and is used for locomotion, attachment, and/or food capture.

    The mantle and foot can be seen in the figure 1. The visceral mass is underneath the gill.

    There may be a radula, a structure that resembles a tongue but contains hard plates and is often used for scraping food. The coelom is reduced and limited to the region near the heart.

    Most mollusks have an open circulatory system but cephalopods (squids, octopus) have a closed circulatory system. The blood pigment of mollusks is hemocyanin, not hemoglobin. The heart of a clam can be seen in the photograph below. Bivalves have three pairs of ganglia but do not have a brain.

    Most mollusks have separate sexes but most snails (gastropods) are hermaphrodites. Some marine mollusks have a ciliated larval form called a trochophore.

    8.2 Platyhelminthes

    The flatworms, flat worms, Platyhelminthes, Plathelminthes, or platyhelminths (from the Greek platy, meaning “flat” and helminth-, meaning “worm”) are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion (intake of nutrients) and egestion (removal of undigested wastes) as a result, the food cannot be processed continuously.

    Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments, such as leaf litter. Cestodes (tapeworms) and trematodes (flukes) have complex life-cycles, with mature stages that live as parasites in the digestive systems of fish or land vertebrates, and intermediate stages that infest secondary hosts. The eggs of trematodes are excreted from their main hosts, whereas adult cestodes generate vast numbers of hermaphroditic, segment-like proglottids that detach when mature, are excreted, and then release eggs. Unlike the other parasitic groups, the monogeneans are external parasites infesting aquatic animals, and their larvae metamorphose into the adult form after attaching to a suitable host.

    Over half of all known flatworm species are parasitic, and some do enormous harm to humans and their livestock. Schistosomiasis, caused by one genus of trematodes, is the second-most devastating of all human diseases caused by parasites, surpassed only by malaria. Neurocysticercosis, which arises when larvae of the pork tapeworm Taenia solium penetrate the central nervous system, is the major cause of acquired epilepsy worldwide. The threat of flatworm parasites to humans in developed countries is rising because of the popularity of raw or lightly cooked foods, and imports of food from high-risk areas. In less developed countries, people often cannot afford the fuel required to cook food thoroughly, and poorly designed water-supply and irrigation projects increase the dangers presented by poor sanitation and unhygienic farming.

    In traditional medicinal texts, Platyhelminthes are divided into

    Catalytic in vivo protein knockdown by small-molecule PROTACs

    The current predominant therapeutic paradigm is based on maximizing drug-receptor occupancy to achieve clinical benefit. This strategy, however, generally requires excessive drug concentrations to ensure sufficient occupancy, often leading to adverse side effects. Here, we describe major improvements to the proteolysis targeting chimeras (PROTACs) method, a chemical knockdown strategy in which a heterobifunctional molecule recruits a specific protein target to an E3 ubiquitin ligase, resulting in the target's ubiquitination and degradation. These compounds behave catalytically in their ability to induce the ubiquitination of super-stoichiometric quantities of proteins, providing efficacy that is not limited by equilibrium occupancy. We present two PROTACs that are capable of specifically reducing protein levels by >90% at nanomolar concentrations. In addition, mouse studies indicate that they provide broad tissue distribution and knockdown of the targeted protein in tumor xenografts. Together, these data demonstrate a protein knockdown system combining many of the favorable properties of small-molecule agents with the potent protein knockdown of RNAi and CRISPR.


    Proteolysis targeting chimeras (PROTACs). (…

    Proteolysis targeting chimeras (PROTACs). ( a ) Proposed model of PROTAC-induced degradation. Von…

    PROTACs downregulate the protein levels…

    PROTACs downregulate the protein levels of their respective targets. ( a ) PROTAC_ERRα…

    PROTACs induce the catalytic ubiquitination…

    PROTACs induce the catalytic ubiquitination of their target protein in a reconstituted E1-E2-VHL…

    PROTACs are highly specific for…

    PROTACs are highly specific for their respective target. ( a ) PROTAC_RIPK2 is…

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    The words mollusc and mollusk are both derived from the French mollusque, which originated from the Latin molluscus, from mollis, soft. Molluscus was itself an adaptation of Aristotle's τὰ μαλάκια ta malákia (the soft ones < μαλακός malakós "soft"), which he applied inter alia to cuttlefish. [7] [8] The scientific study of molluscs is accordingly called malacology. [9]

    The name Molluscoida was formerly used to denote a division of the animal kingdom containing the brachiopods, bryozoans, and tunicates, the members of the three groups having been supposed to somewhat resemble the molluscs. As now known, these groups have no relation to molluscs, and very little to one another, so the name Molluscoida has been abandoned. [10]

    The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. Many have a calcareous shell. [11]

    Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult. [12] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical. [13] The following are present in all modern molluscs: [14] [16]

    • The dorsal part of the body wall is a mantle (or pallium) which secretescalcareousspicules, plates or shells. It overlaps the body with enough spare room to form a mantle cavity.
    • The anus and genitals open into the mantle cavity.
    • There are two pairs of main nerve cords. [15]

    Other characteristics that commonly appear in textbooks have significant exceptions:

    Whether characteristic is found in these classes of Molluscs
    Supposed universal Molluscan characteristic [14] Aplacophora [15] ( p291–292 ) Polyplacophora [15] ( p292–298 ) Monoplacophora [15] ( p298–300 ) Gastropoda [15] ( p300–343 ) Cephalopoda [15] ( p343–367 ) Bivalvia [15] ( p367–403 ) Scaphopoda [15] ( p403–407 )
    Radula, a rasping "tongue" with chitinous teeth Absent in 20% of Neomeniomorpha Yes Yes Yes Yes No Internal, cannot extend beyond body
    Broad, muscular foot Reduced or absent Yes Yes Yes Modified into arms Yes Small, only at "front" end
    Dorsal concentration of internal organs (visceral mass) Not obvious Yes Yes Yes Yes Yes Yes
    Large digestive ceca No ceca in some Aplacophora Yes Yes Yes Yes Yes No
    Large complex metanephridia ("kidneys") None Yes Yes Yes Yes Yes Small, simple
    One or more valves/ shells Primitive forms, yes modern forms, no Yes Yes Snails, yes slugs, mostly yes (internal vestigial) Octopuses, no cuttlefish, nautilus, squid, yes Yes Yes
    Odontophore Yes Yes Yes Yes Yes No Yes

    Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species. [1] The total number of described species is difficult to estimate because of unresolved synonymy. In 1969 David Nicol estimated the probable total number of living mollusc species at 107,000 of which were about 12,000 fresh-water gastropods and 35,000 terrestrial. The Bivalvia would comprise about 14% of the total and the other five classes less than 2% of the living molluscs. [18] In 2009, Chapman estimated the number of described living mollusc species at 85,000. [1] Haszprunar in 2001 estimated about 93,000 named species, [19] which include 23% of all named marine organisms. [20] Molluscs are second only to arthropods in numbers of living animal species [17] — far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000. [15] ( pFront endpaper ) About 200,000 living species in total are estimated, [1] [21] and 70,000 fossil species, [14] although the total number of mollusc species ever to have existed, whether or not preserved, must be many times greater than the number alive today. [22]

    Molluscs have more varied forms than any other animal phylum. They include snails, slugs and other gastropods clams and other bivalves squids and other cephalopods and other lesser-known but similarly distinctive subgroups. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems. Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes. [12] About 80% of all known mollusc species are gastropods. [17] Cephalopoda such as squid, cuttlefish, and octopuses are among the neurologically most advanced of all invertebrates. [23] The giant squid, which until recently had not been observed alive in its adult form, [24] is one of the largest invertebrates, but a recently caught specimen of the colossal squid, 10 m (33 ft) long and weighing 500 kg (1,100 lb), may have overtaken it. [25]

    Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of nonmarine molluscs vary widely, partly because many regions have not been thoroughly surveyed. There is also a shortage of specialists who can identify all the animals in any one area to species. However, in 2004 the IUCN Red List of Threatened Species included nearly 2,000 endangered nonmarine molluscs. For comparison, the great majority of mollusc species are marine, but only 41 of these appeared on the 2004 Red List. About 42% of recorded extinctions since the year 1500 are of molluscs, consisting almost entirely of nonmarine species. [26]

    Because of the great range of anatomical diversity among molluscs, many textbooks start the subject of molluscan anatomy by describing what is called an archi-mollusc, hypothetical generalized mollusc, or hypothetical ancestral mollusc (HAM) to illustrate the most common features found within the phylum. The depiction is visually rather similar to modern monoplacophorans. [12] [16] [27]

    The generalized mollusc is bilaterally symmetrical and has a single, "limpet-like" shell on top. The shell is secreted by a mantle covering the upper surface. The underside consists of a single muscular "foot". [16] The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc. It contains the body organs. [13]

    Mantle and mantle cavity Edit

    The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis, and is exposed, according to habitat, to sea, fresh water or air. The cavity was at the rear in the earliest molluscs, but its position now varies from group to group. The anus, a pair of osphradia (chemical sensors) in the incoming "lane", the hindmost pair of gills and the exit openings of the nephridia ("kidneys") and gonads (reproductive organs) are in the mantle cavity. [16] The whole soft body of bivalves lies within an enlarged mantle cavity. [13]

    Shell Edit

    The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs [13] ) that consists of mainly chitin and conchiolin (a protein hardened with calcium carbonate), [16] [28] except the outermost layer, which in almost all cases is all conchiolin (see periostracum). [16] Molluscs never use phosphate to construct their hard parts, [29] with the questionable exception of Cobcrephora. [30] While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells. [31]

    The shell consists of three layers: the outer layer (the periostracum) made of organic matter, a middle layer made of columnar calcite, and an inner layer consisting of laminated calcite, often nacreous. [13]

    In some forms the shell contains openings. In abalones there are holes in the shell used for respiration and the release of egg and sperm, in the nautilus a string of tissue called the siphuncle goes through all the chambers, and the eight plates that make up the shell of chitons are penetrated with living tissue with nerves and sensory structures. [32]

    Foot Edit

    The underside consists of a muscular foot, which has adapted to different purposes in different classes. [33] The foot carries a pair of statocysts, which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement. In forms having only a top shell, such as limpets, the foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell. [16] In bivalves, the foot is adapted for burrowing into the sediment [33] in cephalopods it is used for jet propulsion, [33] and the tentacles and arms are derived from the foot. [34]

    Circulatory system Edit

    Most molluscs' circulatory systems are mainly open. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton. [13] The blood of these molluscs contains the respiratory pigment hemocyanin as an oxygen-carrier. The heart consists of one or more pairs of atria (auricles), which receive oxygenated blood from the gills and pump it to the ventricle, which pumps it into the aorta (main artery), which is fairly short and opens into the hemocoel. [16] The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coelom as urine. A pair of nephridia ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity. [16]

    Exceptions to the above are the molluscs Planorbidae or ram's horn snails, which are air-breathing snails that use iron-based hemoglobin instead of the copper-based hemocyanin to carry oxygen through their blood.

    Respiration Edit

    Most molluscs have only one pair of gills, or even only a singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart. [16]

    Eating, digestion, and excretion Edit

    Members of the mollusc family use intracellular digestion to function. Most molluscs have muscular mouths with radulae, "tongues", bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. The radula primarily functions to scrape bacteria and algae off rocks, and is associated with the odontophore, a cartilaginous supporting organ. [13] The radula is unique to the molluscs and has no equivalent in any other animal.

    Molluscs' mouths also contain glands that secrete slimy mucus, to which the food sticks. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so the mucus forms a long string called a "food string". [16]

    At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone of feces and mucus, which is rotated by further cilia so it acts as a bobbin, winding the mucus string onto itself. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it. [16]

    The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum (a pouch with no other exit) to be digested. The sorting process is by no means perfect. [16]

    Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large. The anus, in the part of the mantle cavity, is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems. [16]

    As the head has largely disappeared in bivalves, the mouth has been equipped with labial palps (two on each side of the mouth) to collect the detritus from its mucus. [13]

    Nervous system Edit

    The cephalic molluscs have two pairs of main nerve cords organized around a number of paired ganglia, the visceral cords serving the internal organs and the pedal ones serving the foot. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The ganglia above the gut are the cerebral, the pleural, and the visceral, which are located above the esophagus (gullet). The pedal ganglia, which control the foot, are below the esophagus and their commissure and connectives to the cerebral and pleural ganglia surround the esophagus in a circumesophageal nerve ring or nerve collar. [16]

    The acephalic molluscs (i.e., bivalves) also have this ring but it is less obvious and less important. The bivalves have only three pairs of ganglia— cerebral, pedal, and visceral— with the visceral as the largest and most important of the three functioning as the principal center of "thinking". Some such as the scallops have eyes around the edges of their shells which connect to a pair of looped nerves and which provide the ability to distinguish between light and shadow.

    Reproduction Edit

    The simplest molluscan reproductive system relies on external fertilization, but with more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. Two gonads sit next to the coelom, a small cavity that surrounds the heart, into which they shed ova or sperm. The nephridia extract the gametes from the coelom and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization. Some molluscs use internal fertilization and/or are hermaphrodites, functioning as both sexes both of these methods require more complex reproductive systems. [16]

    The most basic molluscan larva is a trochophore, which is planktonic and feeds on floating food particles by using the two bands of cilia around its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. New tissue grows in the bands of mesoderm in the interior, so the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by a veliger stage in which the prototroch, the "equatorial" band of cilia nearest the apical tuft, develops into the velum ("veil"), a pair of cilia-bearing lobes with which the larva swims. Eventually, the larva sinks to the seafloor and metamorphoses into the adult form. While metamorphosis is the usual state in molluscs, the cephalopods differ in exhibiting direct development: the hatchling is a 'miniaturized' form of the adult. [36] The development of molluscs is of particular interest in the field of ocean acidification as environmental stress is recognized to affect the settlement, metamorphosis, and survival of larvae. [37]

    Feeding Edit

    Most molluscs are herbivorous, grazing on algae or filter feeders. For those grazing, two feeding strategies are predominant. Some feed on microscopic, filamentous algae, often using their radula as a 'rake' to comb up filaments from the sea floor. Others feed on macroscopic 'plants' such as kelp, rasping the plant surface with its radula. To employ this strategy, the plant has to be large enough for the mollusc to 'sit' on, so smaller macroscopic plants are not as often eaten as their larger counterparts. [38] Filter feeders are molluscs that feed by straining suspended matter and food particles from water, typically by passing the water over their gills. Most bivalves are filter feeders, which can be measured through clearance rates. Research has demonstrated that environmental stress can affect the feeding of bivalves by altering the energy budget of organisms. [37]

    Cephalopods are primarily predatory, and the radula takes a secondary role to the jaws and tentacles in food acquisition. The monoplacophoran Neopilina uses its radula in the usual fashion, but its diet includes protists such as the xenophyophore Stannophyllum. [39] Sacoglossan sea-slugs suck the sap from algae, using their one-row radula to pierce the cell walls, [40] whereas dorid nudibranchs and some Vetigastropoda feed on sponges [41] [42] and others feed on hydroids. [43] (An extensive list of molluscs with unusual feeding habits is available in the appendix of GRAHAM, A. (1955). "Molluscan diets". Journal of Molluscan Studies. 31 (3–4): 144. .)

    Opinions vary about the number of classes of molluscs for example, the table below shows seven living classes, [19] and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and Solenogasters into one class, the Aplacophora. [27] [15] ( p291–292 ) Two of the commonly recognized "classes" are known only from fossils. [17]

    Class Major organisms Described living species [19] Distribution
    Gastropoda [15] ( p300 ) all snails and slugs including abalone, limpets, conch, nudibranchs, sea hares, sea butterflies 70,000 marine, freshwater, land
    Bivalvia [15] ( p367 ) clams, oysters, scallops, geoducks, mussels, rudists† 20,000 marine, freshwater
    Polyplacophora [15] ( pp292–298 ) chitons 1,000 rocky tidal zone and seabed
    Cephalopoda [15] ( p343 ) squid, octopuses, cuttlefish, nautiluses, Spirula, belemnites†, ammonites† 900 marine
    Scaphopoda [15] ( pp403–407 ) tusk shells 500 marine 6–7,000 metres (20–22,966 ft)
    † Cricoconarida extinct
    Aplacophora [15] ( pp291–292 ) worm-like molluscs 320 seabed 200–3,000 metres (660–9,840 ft)
    Monoplacophora [15] ( pp298–300 ) ancient lineage of molluscs with cap-like shells 31 seabed 1,800–7,000 metres (5,900–23,000 ft) one species 200 metres (660 ft)
    Rostroconchia† [44] fossils probable ancestors of bivalves extinct marine
    Solenogastres small, worm-like, shell-less molluscs
    Helcionelloida† [45] fossils snail-like molluscs such as Latouchella extinct marine

    Classification into higher taxa for these groups has been and remains problematic. A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora. [46] Additionally, it suggests a sister taxon relationship exists between the Bivalvia and the Gastropoda. Tentaculita may also be in Mollusca (see Tentaculites).

    Fossil record Edit

    Good evidence exists for the appearance of gastropods (e.g., Aldanella), cephalopods (e.g., Plectronoceras, ?Nectocaris) and bivalves (Pojetaia, Fordilla) towards the middle of the Cambrian period, c. 500 million years ago , though arguably each of these may belong only to the stem lineage of their respective classes. [47] However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated.

    Debate occurs about whether some Ediacaran and Early Cambrian fossils really are molluscs. Kimberella, from about 555 million years ago , has been described by some paleontologists as "mollusc-like", [48] [49] but others are unwilling to go further than "probable bilaterian", [50] [51] if that. [52]

    There is an even sharper debate about whether Wiwaxia, from about 505 million years ago , was a mollusc, and much of this centers on whether its feeding apparatus was a type of radula or more similar to that of some polychaete worms. [50] [53] Nicholas Butterfield, who opposes the idea that Wiwaxia was a mollusc, has written that earlier microfossils from 515 to 510 million years ago are fragments of a genuinely mollusc-like radula. [54] This appears to contradict the concept that the ancestral molluscan radula was mineralized. [55]

    However, the Helcionellids, which first appear over 540 million years ago in Early Cambrian rocks from Siberia and China, [56] [57] are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest trilobites. [45] Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more limpet-like shapes. The tiny specimens have been suggested to be juveniles and the larger ones adults. [58]

    Some analyses of helcionellids concluded these were the earliest gastropods. [59] However, other scientists are not convinced these Early Cambrian fossils show clear signs of the torsion that identifies modern gastropods twists the internal organs so the anus lies above the head. [15] ( pp300–343 ) [60] [61]

    Volborthella, some fossils of which predate 530 million years ago , was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are. Volborthella ' s classification is uncertain. [62] The Late Cambrian fossil Plectronoceras is now thought to be the earliest clearly cephalopod fossil, as its shell had septa and a siphuncle, a strand of tissue that Nautilus uses to remove water from compartments it has vacated as it grows, and which is also visible in fossil ammonite shells. However, Plectronoceras and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside, and had stripes and blotches on what is thought to be the upper surface. [63] All cephalopods with external shells except the nautiloids became extinct by the end of the Cretaceous period 65 million years ago . [64] However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant today. [65]

    The Early Cambrian fossils Fordilla and Pojetaia are regarded as bivalves. [66] [67] [68] [69] "Modern-looking" bivalves appeared in the Ordovician period, 488 to 443 million years ago . [70] One bivalve group, the rudists, became major reef-builders in the Cretaceous, but became extinct in the Cretaceous–Paleogene extinction event. [71] Even so, bivalves remain abundant and diverse.

    The Hyolitha are a class of extinct animals with a shell and operculum that may be molluscs. Authors who suggest they deserve their own phylum do not comment on the position of this phylum in the tree of life. [72]

    Phylogeny Edit

    The phylogeny (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether Kimberella and any of the "halwaxiids" were molluscs or closely related to molluscs, [49] [50] [53] [54] debates arise about the relationships between the classes of living molluscs. [51] In fact, some groups traditionally classified as molluscs may have to be redefined as distinct but related. [75]

    Molluscs are generally regarded members of the Lophotrochozoa, [73] a group defined by having trochophore larvae and, in the case of living Lophophorata, a feeding structure called a lophophore. The other members of the Lophotrochozoa are the annelid worms and seven marine phyla. [76] The diagram on the right summarizes a phylogeny presented in 2007 without the annelid worms.

    Because the relationships between the members of the family tree are uncertain, it is difficult to identify the features inherited from the last common ancestor of all molluscs. [77] For example, it is uncertain whether the ancestral mollusc was metameric (composed of repeating units)—if it was, that would suggest an origin from an annelid-like worm. [78] Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments, [12] while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized. [51] In one particular branch of the family tree, the shell of conchiferans is thought to have evolved from the spicules (small spines) of aplacophorans but this is difficult to reconcile with the embryological origins of spicules. [77]

    The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into a cuticle. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills. [45] Eventually, the cuticle would have become mineralized, [45] using the same genetic machinery (engrailed) as most other bilaterian skeletons. [78] The first mollusc shell almost certainly was reinforced with the mineral aragonite. [28]

    The evolutionary relationships within the molluscs are also debated, and the diagrams below show two widely supported reconstructions:

    Morphological analyses tend to recover a conchiferan clade that receives less support from molecular analyses, [79] although these results also lead to unexpected paraphylies, for instance scattering the bivalves throughout all other mollusc groups. [80]

    However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic in particular, Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes the traditional phylum Mollusca is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded. [75] A 2010 analysis recovered the traditional conchiferan and aculiferan groups, and showed molluscs were monophyletic, demonstrating that available data for solenogastres was contaminated. [81] Current molecular data are insufficient to constrain the molluscan phylogeny, and since the methods used to determine the confidence in clades are prone to overestimation, it is risky to place too much emphasis even on the areas of which different studies agree. [82] Rather than eliminating unlikely relationships, the latest studies add new permutations of internal molluscan relationships, even bringing the conchiferan hypothesis into question. [83]

    For millennia, molluscs have been a source of food for humans, as well as important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, sea silk, and chemical compounds. Their shells have also been used as a form of currency in some preindustrial societies. A number of species of molluscs can bite or sting humans, and some have become agricultural pests.

    Uses by humans Edit

    Molluscs, especially bivalves such as clams and mussels, have been an important food source since at least the advent of anatomically modern humans, and this has often resulted in overfishing. [84] Other commonly eaten molluscs include octopuses and squids, whelks, oysters, and scallops. [85] In 2005, China accounted for 80% of the global mollusc catch, netting almost 11,000,000 tonnes (11,000,000 long tons 12,000,000 short tons). Within Europe, France remained the industry leader. [86] Some countries regulate importation and handling of molluscs and other seafood, mainly to minimize the poison risk from toxins that can sometimes accumulate in the animals. [87]

    Most molluscs with shells can produce pearls, but only the pearls of bivalves and some gastropods, whose shells are lined with nacre, are valuable. [15] ( pp300–343, 367–403 ) The best natural pearls are produced by marine pearl oysters, Pinctada margaritifera and Pinctada mertensi, which live in the tropical and subtropical waters of the Pacific Ocean. Natural pearls form when a small foreign object gets stuck between the mantle and shell.

    The two methods of culturing pearls insert either "seeds" or beads into oysters. The "seed" method uses grains of ground shell from freshwater mussels, and overharvesting for this purpose has endangered several freshwater mussel species in the southeastern United States. [15] ( pp367–403 ) The pearl industry is so important in some areas, significant sums of money are spent on monitoring the health of farmed molluscs. [88]

    Other luxury and high-status products were made from molluscs. Tyrian purple, made from the ink glands of murex shells, "fetched its weight in silver" in the fourth century BC, according to Theopompus. [89] The discovery of large numbers of Murex shells on Crete suggests the Minoans may have pioneered the extraction of "imperial purple" during the Middle Minoan period in the 20th–18th centuries BC, centuries before the Tyrians. [90] [91] Sea silk is a fine, rare, and valuable fabric produced from the long silky threads (byssus) secreted by several bivalve molluscs, particularly Pinna nobilis, to attach themselves to the sea bed. [92] Procopius, writing on the Persian wars circa 550 CE, "stated that the five hereditary satraps (governors) of Armenia who received their insignia from the Roman Emperor were given chlamys (or cloaks) made from lana pinna. Apparently, only the ruling classes were allowed to wear these chlamys." [93]

    Mollusc shells, including those of cowries, were used as a kind of money (shell money) in several preindustrial societies. However, these "currencies" generally differed in important ways from the standardized government-backed and -controlled money familiar to industrial societies. Some shell "currencies" were not used for commercial transactions, but mainly as social status displays at important occasions, such as weddings. [94] When used for commercial transactions, they functioned as commodity money, as a tradable commodity whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurable inflation if more efficient transport or "goldrush" behavior appeared. [95]

    Bioindicators Edit

    Bivalve molluscs are used as bioindicators to monitor the health of aquatic environments in both fresh water and the marine environments. Their population status or structure, physiology, behaviour or the level of contamination with elements or compounds can indicate the state of contamination status of the ecosystem. They are particularly useful since they are sessile so that they are representative of the environment where they are sampled or placed. [96] Potamopyrgus antipodarum is used by some water treatment plants to test for estrogen-mimicking pollutants from industrial agriculture.

    Harmful to humans Edit

    Stings and bites Edit

    Some molluscs sting or bite, but deaths from mollusc venoms total less than 10% of those from jellyfish stings. [98]

    All octopuses are venomous, [99] but only a few species pose a significant threat to humans. Blue-ringed octopuses in the genus Hapalochlaena, which live around Australia and New Guinea, bite humans only if severely provoked, [97] but their venom kills 25% of human victims. Another tropical species, Octopus apollyon, causes severe inflammation that can last for over a month even if treated correctly, [100] and the bite of Octopus rubescens can cause necrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated. [101]

    All species of cone snails are venomous and can sting painfully when handled, although many species are too small to pose much of a risk to humans, and only a few fatalities have been reliably reported. Their venom is a complex mixture of toxins, some fast-acting and others slower but deadlier. [102] [98] [103] The effects of individual cone-shell toxins on victims' nervous systems are so precise as to be useful tools for research in neurology, and the small size of their molecules makes it easy to synthesize them. [102] [104]

    Disease vectors Edit

    Schistosomiasis (also known as bilharzia, bilharziosis or snail fever), a disease caused by the fluke worm Schistosoma, is "second only to malaria as the most devastating parasitic disease in tropical countries. An estimated 200 million people in 74 countries are infected with the disease – 100 million in Africa alone." [105] The parasite has 13 known species, two of which infect humans. The parasite itself is not a mollusc, but all the species have freshwater snails as intermediate hosts. [106]

    Pests Edit

    Some species of molluscs, particularly certain snails and slugs, can be serious crop pests, [107] and when introduced into new environments, can unbalance local ecosystems. One such pest, the giant African snail Achatina fulica, has been introduced to many parts of Asia, as well as to many islands in the Indian Ocean and Pacific Ocean. In the 1990s, this species reached the West Indies. Attempts to control it by introducing the predatory snail Euglandina rosea proved disastrous, as the predator ignored Achatina fulica and went on to extirpate several native snail species, instead. [108]

    External anatomy

    The shell can be very different in size and shape depending on the species.

    To analyze the external anatomy of snails, we will divide their body into the shell and the soft body that holds it. The former is a solid spiral-shaped structure carried on the back, made of a single piece and consisting mostly of calcium carbonate. The central layer of the shell, called ostracum, has two layers of crystals of the same substance, calcium carbonate. The Hipostracum is below, and the most superficial layer is the periostracum, composed of a lot of proteins.

    The shell of a land snail can be very different in size and shape depending on the species. Some of them are cone-shaped while others are round. However, all of them have a spiral design, caused by the way land snails produce and growth their shells.

    This structure protects the snail from the environment and even from predators. It is made up of calcium carbonate which makes it strong and remains that way as long as the snail consumes food with calcium.

    Its surface can show different colors with fringe designs, but they usually are brown or yellow. The shell protects the body and internal organs of the animal and has an opening to one side, usually the right.

    Image under GNU license. Author Original by Wikimedia Commons User Al2, English captions and other edits by Jeff Dahl

    The rest of the body is soft, with a viscous texture and dark colors with gray or light spots. It lacks legs but moves thanks to a “muscular ventral foot.” The foot has a wave-shaped movement produced by muscular contractions that make the snail “glide” while the foot secretes a slippery mucus that reduces the friction on the surface in which it moves. This mucus is the “trace” that leaves the mollusk on the ground as it moves.

    When snails sense danger around them, they hide into the shell.

    The head, at one end of the body, has one to two pairs of tentacles (retractable and provided with tactile receptors), which have the eyes at the tips. The lower pair works as olfactory organs to smell. It also has an outer skin fold of tissue, which covers the internal organs and also usually covers the shell and the mantle cavity. You may not always see their tentacles because all land snails have the ability to retract them.

    Some land species secrete a layer of mucus, which when hardened blocks the entrance of the shell and is called epiphragm.

    When snails sense danger around them, they hide into the shell. Snails spend a long time in their shell when the weather is hot and dry. Otherwise, their moist bodies could dry out.

    Snails vary in size and color. The largest are members of the family Achatinidae, of which the species Achatina achatina can reach a length up to 11.8 inches and a diameter up to 5.9 inches.

    Octopus Gonadotropin-Releasing Hormone

    Hiroyuki Minakata , in Handbook of Hormones , 2016


    A peptide with structural features similar to those of chordate GnRHs was originally isolated from the central nervous system of the Japanese common octopus Octopus vulgaris , and was named oct-GnRH. Oct-GnRH has also been characterized from the central nervous systems of the common cuttlefish Sepia officinalis, and the swordchip squid Uroteuthis edulis. Oct-GnRH induces gonadal maturation and oviposition by regulating sex steroidogenesis and a series of egg-laying behaviors, as a key peptide in the octopus analog of the hypothalamo-hypophysial–gonadal axis. Localization of oct-GnRH mRNA expression and immunoreactive fibers in the brain, and distribution of oct-GnRH receptor in the brain and the peripheral tissues, suggest that oct-GnRH acts as a multifunctional modulatory factor in higher brain function such as feeding behavior, the touch and visual memory system, cardiac control, arm movement, postural regulation, and sensory and autonomic functions.

    1. Term Paper on the Introduction to Lower Animals
    2. Term Paper on the Sources of Diversity of Lower Animals
    3. Term Paper on the Sources of Continuity of Lower Animals
    4. Term Paper on the Question of Size of Lower Animals
    5. Term Paper on Phylum Porifera: Sponges
    6. Term Paper on Phylum Coelenterata: Polyps and Jellyfish
    7. Term Paper on Phylum Platyhelminthes: Flatworms
    8. Term Paper on Phylum Rhynchocoela: Ribbon Worms
    9. Term Paper on Phylum Nematoda: Roundworms
    10. Term Paper on Phylum Annelida: Segmented Worms
    11. Term Paper on Phylum Mollusca: Mollusks
    12. Term Paper on Phylum Echinodermata: Starfish

    Term Paper # 1. Introduction to Lower Animals:

    Animals are many-celled heterotrophs. They depend directly or indirectly for their nourishment on photosynthetic autotrophs-algae or plants. Most digest their food in an internal cavity, and most store food as glycogen or fat. Their cells do not have walls. Most move by means of contractile cells (muscle cells) containing characteristic proteins. Reproduction is usually sexual.

    As adults, most are fixed in size and shape, in contrast to plants, in which growth often continues for the lifetime of the organism. The higher animals-the arthropods and the vertebrates—are the most complex of all organisms, with many kinds of specialised tissues, including elaborate sensory and neuromotor mechanisms not found in any of the other kingdoms.

    For most of us, animal means mammal, and mammals are, in fact, the chief focus of attention in other section. However, the mammals, or even the vertebrates as a whole, represent only a small fraction of the animal kingdom. More than 90 percent of the different species of animals are invertebrates-that is, animals without backbones and most of these are insects. Indeed, the enormous variety displayed by the invertebrates is partly why they are so endlessly fascinating to study. They are, in addition, of great ecological importance the insects, for example, have long challenged human dominance of the earth.

    Finally, and perhaps most important, the invertebrates, confronted with the same biological problems that we face, demonstrate a spectrum of ingenious solutions. In this way, they illuminate the essential nature of these problems and so help us to understand and evaluate the solutions arrived at by mammals.

    Term Paper # 2. The Sources of Diversity of Lower Animals:

    The tremendous versatility of the prokaryotes, as exemplified by the wide range of environments they inhabit and the many ways in which they satisfy their energy requirements.

    Among the invertebrates, we see, on a slightly different scale, this same pattern of adaptation to many different ways of life. Thus, for instance, on a single coral head, only a meter or two in diameter, one finds a dazzling array of different forms-sponges, jellyfish, starfish, sea urchins, anemones, and the coral animals themselves.

    Similarly, to take terrestrial examples, a single spadeful of soil turns up earthworms, pillbugs, spiders, nematodes, and various other tiny animals and the branch of a single tree may harbor a dozen different kinds of insects. Given the relentless force of natural selection, why do not the larger ones crowd out the smaller? Why are not the “lower” animals replaced by “higher” animals with superior strength or intelligence?

    Darwin, again, offered the answer: The different organisms, he noted, “occupy different positions in the economy of nature.” Each has been shaped by the long process of evolution to occupy a different niche in the environment. Natural selection has worked not to make one “superior” to the other but to continuously adapt the different forms to different ways of life.

    This process of adaptation, of course, continues. Every species, including our own, is a traveler through time, caught for only an instant in the present.

    Term Paper # 3. The Sources of Continuity of Lower Animals:

    Through the patterns of diversity, there is a strong theme of continuity. One reason is simply that of “descent.” We are all related not only are we made of the same atoms and molecules and even macromolecules, but from E. coli to elephant, we even share many of the same enzymes.

    Although the evolutionary relationships between us and the invertebrates are obscure, we can read into them traces of our own biological beginnings. In the twitch of a tiny segment of an earthworm’s artery, we sense the echo of our own heartbeat.

    Second, all organisms face the same set of problems. These problems can best be defined by recalling that an organism is a cell or group of cells. A primary need is to supply the cell or cells with, first of all, a source of energy. Also, most cells-on this planet, at least-require water, oxygen, a source of nitrogen, fixed carbon (in the case of heterotrophs), and a few ions.

    Another requirement is to eliminate wastes, including excess carbon dioxide, nitrogenous wastes from the breakdown of amino acids, and, in some cases, excess water. Cells that live individual or colonial lives in a watery environment can solve these problems in relatively simple ways, but as organisms get larger, thicker, and more complex, the problem of servicing each individual cell becomes correspondingly more complicated.

    Another set of problems that a multicellular organism must solve in order to exist arises from the fact that it is more than just a group of cells. It is, in fact, a complex society of cells, in which the needs of each individual cell are subordinated to the needs of the society. In a population of Paramecium, the organisms have common requirements, but each is in competition with the others.

    In a society of even a few thousand cells—a small crustacean, for instance-the individual cells are dependent on the existence of the group and are organised in a system of mutual cooperation. The second group of problems faced by organisms, therefore, relates to the organisation or integration of activities.

    Hormones are one of the chief means of integration in both plants and animals. In the animals, another, more rapid integrating mechanism has evolved: the nervous system, by which the organism keeps in touch with its environment and coordinates its own activities.

    Term Paper # 4. A Question of Size of Lower Animals:

    At this point, one might well ask – Considering the problems faced by larger organisms, why did larger animals evolve? What selective advantages do the multicellular, more complex animals have as compared with the smaller ones? Some answers to these questions are obvious and simple.

    Larger animals are, in general, more likely to eat than be eaten. Larger organisms, especially those that live underwater or on land, are generally able to travel faster and farther than small ones and this is an advantage.

    A small ciliate, for instance, might starve only a few centimeters from a food supply. On the other hand, its requirements are very modest.

    Perhaps even more important, however, than mobility and edibility is what the French physiologist Claude Bernard called the milieu interior, the internal environment of the animal, as distinct from the external environment that surrounds it.

    A single-celled organism is as cold or as hot and as wet or as dry as its surroundings, whereas a larger animal is more independent and, to some extent, controls the environment in which its cellular society lives. Control of the internal environment is more readily achieved by the many-celled animal because of the simple surface-to-volume geometry.

    Exchanges between a cell and its surroundings take place across a cell’s available surface area. This is a principal reason why a cell, which depends for its existence on the exchange of substances with its environment, cannot be very large.

    On the other hand, since it may be advantageous to conserve certain substances, such as water and heat, an organism may be better off, within limits of weight and mobility, if its relative surface area is reduced. One-celled animals can live successfully only in water or as parasites in the bodies of other organisms, which amounts to the same thing.

    Many-celled animals can live not only in water but also on land, in the sky, and even, as we are now beginning to discover, in outer space-which is a logical extension of an old evolutionary trend. In this article, we shall discuss the so-called lower invertebrates and some that must clearly be considered higher, such as the clever and highly emotional octopus.

    Term Paper # 5. Phylum Porifera: Sponges:

    Sponges seem to have had a different origin from other members of the animal kingdom and to have traveled a solitary evolutionary route. For this reason, they are often classified in a sub-kingdom of then-own, the Parazoa (“alongside of animals”).

    In fact, until the eighteenth century, the sponges were classified as plant-animals (“zoophytes”) since they are all sessile (attached to a substrate) during their adult life. Sponges are found on ocean floors throughout the world. Most live along the coasts in shallow water, but some, such as the fragile glass sponges, are found at great depths, where the water is almost motionless. A few types are found in fresh water.

    A sponge is essentially a water-filtering system, made up of one or more chambers through which water is driven by the action of numerous flagellated cells. Sponges are made up of a relatively few cell types, the most characteristic of which are the choanocytes, or collar cells, the flagellated cells that line the interior cavity of the sponge.

    Similar cells, the choanoflagellates, are found among the ciliated protozoans, and it is possible that the sponges arose from such organisms. All of a sponge’s digestive processes are carried out intracellularly hence, even a giant sponge-and some stand taller than a person-can eat nothing larger than microscopic food particles.

    The outer surface of the sponge is covered with epithelial cells. Among these epithelial cells are cells that contract in response to touch or to irritating chemicals, and in so doing, close up pores and channels. Each cell acts as an individual, however there is little coordination among them.

    Between the epithelial cells and the choanocytes is a middle, jellylike layer, and in this layer are amoeba-like cells, amoebocytes, which carry out various functions. Some amoebocytes carry food particles from the choanocytes to the epithelial cells. Amoebocytes also secrete skeletal materials.

    Sponges are grouped into three classes, according to their skeletal structure. In some species, the skeleton consists of individual spicules of calcium carbonate. Some, the glass sponges, have spicules of silica fused in a continuous and often very beautiful structure.

    The third and largest group has unfused silica spicules, or a tough, fibrous keratin like protein called spongin, or a combination of the two. The skeleton of sponges serves only for protection, stiffening, and support, but not for locomotion, since the adult forms are sessile.

    The sponge shown in Figure 11.1 is a small and simple one. In larger sponges, the body plan, although it is essentially the same, looks far more complex. These sponges have greatly increased feeding and filtering surfaces, owing to their highly folded body walls.

    We have already encountered this evolutionary stratagem for increasing biological work surfaces at the cellular level—as in the inner membrane of the mitochondrion-and we shall be encountering it again as we examine the structure of gills and lungs.

    Sponges are somewhere between a colony of cells and a true multicellular organism. The cells are not organised into tissues or organs each leads an independent existence. Yet there is a form of recognition among the cells that holds them together and organises them.

    If the sponge Microciona prolifera is squeezed through a fine sieve or a piece of cheesecloth, the body of the sponge is separated into individual cells and small clumps of cells. Within an hour, the isolated sponge cells begin to re-aggregate, and as these aggregations get larger, canals, flagellated chambers, and other characteristics of the body organisation of the sponge begin to appear.

    This phenomenon has been used as a model for the analysis of cell adhesion, recognition, and differentiation, all of which are basic biological features of development in higher organisms.

    Most kinds of sponges are hermaphroditic that is, they have male and female reproductive organs in the same individual. Gametes appear to arise from an enlarged amoebocyte, but there are reports that choanocytes can also form gametes. A sperm enters another sponge in a current of water.

    It is captured by a choanocyte and transferred to an amoebocyte, which then transfers it to a ripe egg (a method of fertilisation unique to the sponges). The fertilised egg develops into a ciliated, free-swimming larva. After a short life among the plankton, the larva settles and becomes sessile.

    Sponges also reproduce asexually, either by fragments that break off from the parent animal, or by gemmules, aggregations of amoeba like cells within a hard, protective outer layer. Production of such resistant forms is found, in general, only among freshwater organisms.

    In the ocean, conditions are relatively unchanging, but the freshwater environment is much harsher. Invertebrates that live in fresh water are more likely to have protected embryonic forms than even closely related marine species.

    Term Paper # 6. Phylum Coelenterata: Polyps and Jellyfish:

    The coelenterates are a large and often strikingly beautiful group of aquatic organisms. Their adult-form is generally radially symmetrical that is, their body parts are arranged around a central axis, like spokes around a hub. As you can see in figure 11.3, the basic body plan is a simple one: The animal is essentially a hollow container, which may be either, vase-shaped, the polyp, or bowl-shaped, the medusa. The polyp is usually sessile the medusa, motile.

    Both consist of two layers of tissue: ectoderm and endoderm (from the Greek ektos, “outer,” and endon, “inner,” plus derma, “skin”). Between the two layers is a gelatinous filling, the mesoglea (“middle jelly”), which is made of a collagen like material. In the polyp form, the mesoglea is sometimes very thin, but in the medusa, it often accounts for the major portion of the body substance.

    Most coelenterates go through both a polyp and a medusa stage in their life cycles. In such species, polyps reproduce asexually and medusas sexually. This sort of life cycle, in which the sexually reproductive form is distinctly different from the asexual form, superficially resembles alternation of generations in plants.

    There is, however, no alternation between haploid and diploid forms as there is in plants the only haploid forms are the gametes.

    Another distinctive feature of the animals in this phylum is the coelenteron, a digestive cavity with only one opening. Within this cavity, enzymes are released that break down food, partially digesting it extra-cellularly, as our own food is digested within the stomach and intestinal tract.

    The food particles are then taken up by the cells lining the cavity they complete the digestive process and pass the products on to the other cells of the animal. Inedible remains are ejected from the single opening.

    A third distinctive feature of coelenterates is the cnidoblast. Coelenterates are carnivores. They capture their prey by means of tentacles that form a circle around the “mouth.” These tentacles are armed with cnidoblasts, special cells that contain nematocysts (thread capsules).

    Nematocysts are discharged in response to a chemical stimulus or touch. The nematocyst threads, which are often poisonous and may be sticky or barbed can lasso prey, harpoon it or paralyze it or some useful combination of all three. The toxin apparently produces paralysis by attacking the lipoproteins of the nerve cell membrane of the prey.

    Cnidoblasts occur only in this phylum, with some interesting exceptions. Certain other invertebrates, including nudibranchs (a kind of mollusk) and flatworms, can eat coelenterates without triggering the nematocysts. The nematocysts then migrate to the surface of the predator and can be fired in their new host’s defense.

    Classes of Coelenterates:

    There are three major classes of coelenterates: Hydrozoa, in which the polyp is usually the dominant form Scyphozoa, predominantly medusoid, exemplified by the common jellyfish and Anthozoa, which includes the sea anemones and the reef-building corals, and has only the polyp.

    One of the most thoroughly studied of the coelenterates is Hydra, which is a small, common freshwater form, convenient to keep in the laboratory. Figure 11.5 shows a small section of the body wall of Hydra. The ectoderm is composed largely of epitheliomuscular cells, which perform a covering, protective function and also serve as muscle tissue.

    Each cell has contractile fibers, myonemes, at its base and so can contract individually, like the contractile epithelial cells of the sponge. The endoderm is mostly made up of cells concerned with digestion these cells also contain contractile fibers.

    In Hydra, as in other polyps, the contractile fibrils of the ectoderm attach lengthwise to the mesoglea and the fibrils of the endoderm cells attach transversely, so the body walls can stretch or bulge, depending on which group is stimulated.

    In addition to cnidoblasts and epithelia muscular cells, which are independent effectors-cells that both receive and respond to stimuli- Hydra contain two other types of nerve cells: sensory receptor cells and cells connected into a network, the nerve net.

    Sensory receptor cells are more sensitive than other epithelial cells to chemical and mechanical stimuli, and when stimulated they transmit their impulses to an adjacent cell or cells. The adjacent cell may be simply an epithelia muscular cell, an effector, which then responds.

    Note that this system is one step more complicated than the epithelia muscular cell or cnidoblast, which acts as both receptor and effector. The nerve net, a loose connection of nerve cells lying at the base of the epithelial layers, is the simplest example of a nervous system that links an entire organism into a functional whole. It coordinates the muscular contractions of Hydra, making possible a wide variety of activities. However, there is no center of operations for the nervous system. This type of conducting system occurs in Hydra and certain other coelenterates.

    Although Hydra has only the polyp form, many hydrozoans have both hydroid (polyp) and medusoid forms at different times in their life cycles. Coelenterates of the genus Obelia, for example, spend most of their lives as colonial polyps.

    The colony arises from a single polyp, which multiplies by budding. The new polyps do not separate but remain interconnected so that their body cavities form a continuous channel, through which food particles are circulated.

    Within the colony are two types of polyps: feeding polyps with tentacles and cnidoblasts, and reproductive polyps from which tiny medusas bud off. These medusas produce eggs or sperm that are released into the water and fuse to form zygotes.

    Thus colonial polyps, with their division of labor between feeding and reproductive forms, are very like a single organism. Such a high degree of specialisation of structure and function among social organisms is seen in other phyla only among the social insects.

    Class Scyphozoa: Jellyfish:

    A second major class of coelenterates is Scyphozoa, or “cup animals,” in which the medusas form is dominant. Scyphozoans, more commonly known as jellyfish, range in size from less than 2 centimeters in diameter up to animals 4 meters across and trailing tentacles 10 meters long.

    In the adult animal, the mesoglea is so firm that a large, freshly beached jellyfish can easily support the weight of a human being. The mesoglea of some jellyfish is filled with wandering, amoeba-like cells, which serve to transport food from the nutritive cells of the endoderm. Unlike Hydra, scyphozoans have true muscle cells these underlie the ectoderm, contracting rhythmically to propel the medusa through the water.

    Nervous System of Medusa:

    In the medusa, there are concentrations of nerve cells in the margin of the bell. These nerve cells connect with fibers innervating (providing the nerve supply for) the tentacles, the musculature, and the sense organs.

    The bell margin is liberally supplied with sensory receptor cells sensitive to mechanical and chemical stimuli. In addition, the jellyfish has two types of true sense organs- Statocysts and light-sensitive ocelli.

    Statocysts are specialised receptor organs that provide information by which an animal can orient itself with respect to gravity. The statocyst, which seems to have been one of the first special sensory organs to have appeared in the course of evolution, has persisted apparently unchanged to the present day, appearing in many animal phyla.

    Ocelli, which may have evolved even earlier, are groups of pigment cells and photoreceptor cells. They are typically located at the bases of the tentacles.

    Anthozoans (“flower animals “)-the corals and sea anemones-are members of a class of coelenterates that, like Hydra, have lost the medusa stage. They differ from Hydra in having a gullet lined with epidermis and a coelenteron divided by vertical partitions.

    In most corals, which are colonies of anthozoans, the epidermal cells secrete protective outer walls, usually of calcium carbonate (limestone), into which each delicate polyp can retreat. The coral- forming polyps are the most ecologically important of the coelenterates.

    The 2,000-kilometer-long Great Barrier Reef off the northeast shores of Australia, and the Marshall Islands in the Pacific are examples of coral-created land masses.

    A coral reef is composed primarily of the accumulated limestone skeletons of coral coelenterates, covered by a thin crust occupied by the living colonial animals. A reef is both the structural and nutritional basis of the complex coral reef community.

    Term Paper # 7. Phylum Platyhelminthes: Flatworms:

    The flatworms are the simplest animals, in terms of body plan, to show bilateral symmetry. In bilaterally symmetrical animals, the body plan is organised along a longitudinal axis, with the right half an approximate mirror image of the left half.

    It also has a top and a bottom or, in more precise terms (applicable even when it is turned upside down or, as in the case of humans, standing upright), a dorsal and a ventral surface. Like most bilateral organisms, the flatworm also has a distinct “headness” and “tailness” anterior and posterior.

    Having, one end that goes first-cephalization-is characteristic of actively moving animals. In such animals, many of the sensory cells are also collected into the anterior end. With the aggregation of sensory cells, there came a concomitant gathering of nerve cells this gathering is a forerunner of the brain.

    Flatworms have three distinct tissue layers-ectoderm, mesoderm, and endoderm-characteristic of all animals above the coelenterate level of organisation.

    Moreover, not only are their tissues specialised for various functions, but also two or more types of tissue cells may combine to form organs- for example, the muscular pharynx. Thus, while sponges are made up of aggregations of cells and coelenterates are largely limited to the tissue level of organisation, flatworms can be said to exemplify the organ level of complexity. There are three classes of flatworms the free-living turbellaria and two parasitic forms, flukes and tapeworms.

    The flatworms are believed by some zoologists to have evolved from the coelenterates (or, perhaps, vice versa), not by way of either adult form, however, but from the ciliated larval form.

    Others are persuaded that the flatworms had independent origins among the ciliates. Still another group maintains that they are degenerate annelids. It is unlikely that this matter will soon be resolved.

    The free-living flatworms form a large and varied group, and we shall single out just one for examination, the freshwater planarian. The ectoderm of the planarian is made up of cuboid epithelial cells, many of which are ciliated, particularly those on the ventral surface.

    Ventral ectodermal cells secrete mucus, which provides traction for the planarian as it moves by means of its cilia along its own slime trail. Planarians are among the largest animals that can use cilia for locomotion.

    The cilia in larger species are usually employed for moving water or other substances along the surface of the animal, as in the human respiratory tract, rather than for propelling the animal.

    The planarian has an endoderm composed largely of amoeboid cells, which, although they are phagocytic, are not wandering cells like the amoebocytes of the jellyfish. Between the ectoderm and the phagocytic endoderm is a mesoderm, or middle tissue layer.

    In planarians, as in all other groups to follow, the muscle cells and the principal organ systems are of mesodermal origin.

    The planarian, like the coelenterates, has a digestive cavity (gut) with only one opening, located on the ventral surface. This digestive cavity has three main branches, which is why planarians are placed in the order of flatworms known as Tricladida.

    Like other flatworms, the planarian is carnivorous. It eats either dead meat or slow-moving animals it can fasten itself to or subdue by sitting on, such as smaller planarians. It feeds by means of a muscular tube, the pharynx, which is free at one end. The free end can be stretched out through the mouth opening.

    Muscular contractions in the tube causes strong sucking movements, which tear the meat into microscopic bits and draw them into the internal cavity, where they are phagocytized by the endodermal cells.

    Unlike the sponges or coelenterates, most flatworms have an excretory system. In the planarian, the system is a network of fine tubules that runs the length of the animal’s body. Side branches of the tubules contain flame cells, each of which has a hollow center in which a tuft of cilia beats, flickering like a tongue of flame, moving water along the tubules to the exit pores between the epidermal cells.

    The flame-cell system appears to function largely to regulate water balance most of the metabolic waste products probably diffuse out through the ectoderm or the endoderm.

    Planarians have a complicated reproductive system. The eggs are fertilised internally. At mating, each partner deposits sperm in the copulatory sac of the other partner. These sperm then travel along special tubes, the oviducts, to fertilise the eggs as they become ripe.

    Organisms like planarians, in which both types of gametes are produced by one individual, are known as hermaphrodites (from Hermes and Aphrodite).

    Solitary, slow-moving animals, such as earthworms and snails, are often hermaphrodites these animals may seldom encounter another adult member of the species, but every such encounter can result in a mating. Some types of hermaphroditic animals can fertilise themselves, although they do not usually do so if another individual is present.

    The Planarian Nervous System:

    The evolution of bilateral symmetry brought with it marked changes in the organisation of the nervous system as well as of other systems. Even among primitive flatworms, the neurons (nerve cells) are not dispersed in a loose network, as in Hydra, but are instead condensed into longitudinal cords.

    In the planarians, this condensation is carried further, and there are only two main conducting channels, one on each side of the flat, ribbon like body. These channels carry impulses to and from the aggregation of nerve cells in the anterior end of the body. Such aggregations of nerve cell bodies are known as ganglia (singular, ganglion).

    The ocelli of the planarian are usually inverted pigment cups. They have no lenses, and they cannot form an image. However, they can distinguish light from dark and can tell the direction from which the light is coming.

    Planarians are photonegative if you shine a light on a dish of planarians from the side, they will move steadily away from the source of light.

    Among the epithelial cells are receptor cells sensitive to certain chemicals and to touch. The head region in particular is rich in chemoreceptors. If you place a small piece of fresh liver in the culture water so that its juices diffuse through the medium, the planarians will raise their heads off the bottom and, if they have not eaten recently, will lope directly and rapidly (on a planarian scale) toward the meat, to which they then attach themselves to feed.

    The animal locates the food source by repeatedly turning toward the side on which it receives the stimulus more strongly until the stimulus is equal on both sides of its head. If the chemoreceptor cells are removed from one side of the head, the animal will turn constantly toward the intact side.

    Planarians, with their simple nervous systems, their capacity to react to a variety of stimuli, and their powers of regeneration, have been the subject of a number of experiments. In one group, planarians trained to avoid electric shock were fed to untrained planarians.

    The result, it was claimed, was that the untrained planarians that had eaten the trained ones behaved as if they had been trained. These experiments, still not verified, on the “transfer of training by canniba­lism” led to a great deal of controversy in the 1960s and also, as you might expect, to a number of suggestions for more meaningful student- teacher relationships.

    Phylum Platyhelminthes includes also the tapeworms and the trematodes (flukes), parasitic forms that can cause serious and sometimes fatal diseases among vertebrates. Members of both of these parasitic classes have a tough outer layer of cells that is resistant to digestive fluids and, usually, suckers or hooks on their anterior ends by which they fasten to their victims.

    Trematodes feed through a mouth, but the tapeworms, which have no mouths, digestive cavities, or digestive enzymes, merely hang on and absorb predigested food molecules through their skin. Tapeworms are found in the intestines of many vertebrates, including humans, and may grow as long as 5 or 6 meters.

    They cause illness not only by encroaching on the food supply but also by producing wastes and by obstructing the intestinal tract. The most common human tapeworm, the beef tapeworm, infects people who eat the undercooked flesh of cattle that have eaten fodder contaminated by human feces containing tapeworm segments.

    All parasites, including parasitic flatworms, are believed to have originated as free-living forms and to have lost certain tissues and organs (such as the digestive tract) as a secondary effect of their parasitic existence, while developing adaptations of advantage to the parasitic way of life. Such adaptations also often include a complex life cycle.

    Term Paper # 8. Phylum Rhynchocoela: Ribbon Worms:

    The ribbon worms (sometimes called nemertines), although a small phylum, are of special interest to biologists attempting to reconstruct the evolution of the invertebrates. They appear to be closely related to the flatworms, but with an important difference: They have a one-way digestive tract beginning with a mouth and ending with an anus.

    This is a far more efficient arrangement than the one-opening digestive system of the coelenterates and flatworms. In the one-way tract, food moves assembly-line fashion, with the consequent possibilities- (1) that eating can be continuous and (2) that various segments of the tract can become specialised for different stages of digestion. The ribbon worms also have a circulatory system, usually consisting of one dorsal and two lateral blood vessels that carry the colourless blood.

    This phylum is called Rhynchocoela (“beak” plus “hollow”) because these worms are characterised by a long, retractile, slime-covered tube (proboscis). The proboscis, sometimes armed with a barb, seizes prey and draws it to the mouth where it is engulfed. Some inject a paralyzing poison into their prey.

    Term Paper # 9. Phylum Nematoda: Roundworms:

    The number of species of roundworms (nematodes) has been variously estimated as low as 10,000 and as high as 400,000 to 500,000. Most are free-living, microscopic forms. It has been estimated that a spadeful of good garden soil usually contains about a million nematodes. Some are parasites most species of plants and animals are parasitised by at least one species of nematodes.

    Humans are hosts to about 50 species, including hookworms, pinworms, and Trichinella. This causes trichinosis, which is transmitted by eating uncooked or undercooked pork, a single gram of which may contain 3,000 cysts (resting forms) of Trichinella. Ingestion of only a few hundred of these cysts can be fatal to human beings.

    Nematodes have a three-layered body plan and a tubular gut with a mouth and an anus. They are un-segmented and are covered by a thick, continuous cuticle, which is molted periodically as they grow.

    An interesting, and unique, feature of nematode construction is the absence of circular muscles. The contraction of the longitudinal muscles acting against the tough, elastic cuticle gives the worm its characteristic whipping movement in water. The sexes are usually separate.

    Nematodes may have evolved from early Platyhelminthes, possessing, as they do, a three-layered body plan without a true coelom. They have, however, what is known as a pseudocoelom, a body cavity that is between the endoderm and the mesoderm and lacks the epithelial lining of a true coelom.

    Six other minor (in terms of species and numbers) phyla, mostly small, wormlike animals, have body plans based on the pseudocoelom, but Nematoda is the only major pseudocoelomate phylum.

    Term Paper # 10. Phylum Annelida: Segmented Worms:

    This phylum includes almost 9,000 different species of marine, freshwater, and soil worms, including the familiar earthworm. The term annelid means “ringed” and refers to the most distinctive feature of this group, which is the division of the body into segments, visible as rings on the outside and with partitions on the inside.

    This segmented pattern is found in a modified form in higher animals, too, such as dragonflies, millipedes, and lobsters, which are thought to have evolved from the same ancestors that gave rise to modern annelids.

    The annelids have a three-layered body plan, a tubular gut, and a well-developed circulatory system that transports oxygen (diffused through the skin or through fleshy extensions of the skin) and food molecules (from the gut) to all parts of the body.

    The excretory system is made up of specialised paired tubules, nephridia, which occur in each segment of the body except the head. Annelids have a nervous system and a number of special sense cells, including touch cells, taste receptors, light-sensitive cells, and cells concerned with the detection of moisture. Some also have well-developed eyes and sensory antennae.

    In the flatworms and ribbon worms, the mesoderm is packed solid with muscle and other tissues, but in the annelids there is a fluid-filled cavity, the coelom, (pronounced see-loam) in this middle layer. (Note that the term coelom, although it sounds similar to coelenteron and comes from the same Greek root, meaning “cavity,” refers quite specifically to a cavity within the mesoderm, whereas the coelenteron is a digestive cavity lined by endoderm.)

    Within the coelom, the gut-lined with an epithelium-is suspended by double layers of mesoderm known as mesenteries. The fluid in the coelom constitutes a hydrostatic skeleton for the annelid, stiffening the body in somewhat the same way water pressure stiffens and distends a fire hose.

    The muscles of the earthworm work against this hydrostatic skeleton much as our muscles work against our bony skeleton. Although the opening of a cavity within the mesoderm may seem less dramatic than other evolutionary innovations, it is extremely important.

    Within such a space, organ systems can bend, twist, and fold back on themselves, increasing their functional surface areas and filling, emptying, and sliding past one another, surrounded by lubricating coelomic fluid. Consider the human lung, constantly expanding and contracting in the chest cavity, or the 6 or 7 meters of coiled human intestine neither of these could have evolved until the coelom made room for them.

    The earthworm is the most familiar of the annelids. Figure 11.8 shows a portion of the body of an earthworm. Note how the body is compartmented into regular segments. Most of these segments, particularly the central and posterior ones, are identical, each exactly like the one before and the one after.

    Each identical segment contains four pairs of bristles, or setae two nephridia, excretory tubules that pick up waste materials from the body fluids and excrete them through pores on the ventral surface of the worm four sets of nerves branching off from the central nerve cord running along the ventral surface a portion of digestive tract and a left and right coelomic cavity.

    The chief exceptions to this rule of segmented structure are found in the most forward segments. In these, sensory cells, a cluster of nerve cells (ganglia), and specialised areas of the digestive, circulatory, and reproductive systems are found.

    The tube like body is wrapped in two sets of muscles, one set running longitudinally and the other encircling the segments. When the earthworm moves, it anchors some of its segments by its setae, and the circular muscles of the segments anterior to the anchored segments contract, thus extending its body forward. Then its forward setae take hold, and the longitudinal muscles contract while the posterior anchor is released, drawing the posterior segments forward.

    Digestion in Earthworms:

    The digestive tract of the earthworm is a long, straight tube. The mouth leads into a strong, muscular pharynx, which acts like a suction pump, drawing in decaying leaves and other organic matter, as well as dirt, from which organic materials are extracted.

    The earthworm makes burrows in the earth by passing such material through its digestive tract and depositing it outside in the form of castings, a ceaseless activity that serves to break up, enrich, and aerate the soil. The narrow section of tube posterior to the pharynx, the esophagus, leads to the crop, where food is stored.

    In the gizzard, which has thick, muscular walls lined with protective cuticle, the food is ground up with the help of the ever-present soil particles. The rest of the digestive tract is made up of a long intestine, which has a large fold along its upper surface that increases its surface area. The intestinal epithelium consists of enzyme-secreting cells and ciliated absorptive cells.

    Circulation in Earthworms:

    In the protists and in the smaller and simpler animals, food molecules and oxygen are supplied to cells largely by diffusion, aided by the movement of external fluids and, sometimes, as we saw in coelenterates, by wandering, amoeboid cells.

    A circulatory system that propels extracellular fluid around the body solves the problem of providing each cell with a more direct and rapid line of supply.

    The circulatory system of the earthworm is composed of longitudinal vessels running the entire length of the worm, one dorsal and several ventral. The largest ventral vessel underlies the intestinal tract, collecting nutrients from it and distributing them by means of many small branches to all the tissues of the body and to the three smaller ventral vessels that surround the nerve cord and nourish it.

    Numerous small capillaries in each segment carry blood from the ventral vessels through the tissues to the dorsal vessel. Also in each segment are larger parietal (“along the wall”) vessels transporting blood from the sub-neural vessels to the dorsal vessel. Fluids collected in this way from all over the animal’s body are fed into the muscular dorsal vessel, which propels the blood forward.

    Connecting the dorsal and ventral vessels, and so completing the circuit, are five pairs of hearts, muscular pumping areas in the blood vessels. Their irregular contractions force the blood down to the ventral vessels and also forward to the vessels that supply the more anterior segments.

    Both the hearts and the dorsal vessel have valves that prevent backflow. Note that the blood flows entirely through vessels. Such a system is known as a closed circulatory system. Evolution of a closed circulatory system, in effect, added a new compartment to the body plan and, in so doing, made possible a degree of control, not previously feasible, over the content of the circulating body fluid.

    Excretory System of Earthworms:

    The excretory system consists of pairs of tubules, the nephridia one pair for each segment. Each nephridium consists of a long, convoluted tubule that terminates in a ciliated funnel opening into the coelomic cavity of the anteriorly adjacent segment.

    Coelomic fluid is carried into the funnel by the beating of the cilia and is excreted through an outer pore. As the fluid makes its way through the long tubule, water, sugar, salts, and other needed materials are returned to the coelomic fluid through the walls of the tubule, while other materials are absorbed into the tubule for excretion.

    Thus the excretory system is concerned not only with the problem of water balance, as are the contractile vacuoles of Paramecium and the flame cells of planarians, but also with the homeostatic regulation of the chemical composition of the body fluids.

    Thus, in effect, one segment monitors and regulates this aspect of the physiology of its neighboring segment-a neat trick for integrating the animal as a whole.

    Respiration in Earthworms:

    The earthworm has no special respiratory organs respiration takes place by simple diffusion through the body surface. The gases of the atmosphere dissolve in the liquid film on the surface of the earthworm’s body, which is kept moist by secreted mucus and excreted water.

    Oxygen travels inward by diffusion, since the surface film, exposed to the oxygen-rich atmosphere, contains more oxygen than does the blood in the network of capillaries just underlying the body surface. The oxygen is consumed by body cells as the blood circulates.

    Carbon dioxide moves out to the surface film and then into the air by the same principle. In fact, all gas exchange in animals, whether the organism is land-dwelling or water-dwelling, takes place across moist membranes.

    Nervous System in Earthworms:

    The earthworm has a variety of sensory cells. It has touch cells, or mechanoreceptors. These contain tactile hairs, which, when stimulated, trigger a nerve impulse. Patches of these hair cells are found on each segment of the earthworm. The hairs probably also respond to vibrations in the ground, to which the earthworm is very sensitive.

    The earthworm does not have ocelli-as one might expect, since it lives most of its life in complete darkness-but it does have light-sensitive cells. Such cells are more abundant in its anterior and posterior segments, the parts of its body most likely to be outside of the burrow.

    These cells are not responsive to light in the red portion of the spectrum, a fact exploited by anglers who search for worms in the dark using red-lensed flashlights.

    Among the earthworm’s most sensitive cells are those that detect moisture. The cells are located on its first few segments. If an earthworm emerging from its burrow encounters a dry spot, it swings from side to side until it finds dampness failing that, it retreats.

    However, when the anterior segments are anesthetised, the earthworm will crawl over dry ground. The animal also appears to have taste cells. In the laboratory, worms can be shown to select, for example, celery in preference to cabbage leaves and cabbage leaves in preference to carrots.

    Each segment of the worm is supplied by nerves that receive impulses from sensory cells and by nerves that cause muscles to contract. The cell bodies for these nerves are grouped together in clusters (ganglia). The movements of each segment are directed by a pair of ganglia.

    Movement in each segment is triggered by movement in the adjacent anterior segment thus a headless earthworm can move in a coordinated manner. However, an earthworm without its cerebral ganglia moves ceaselessly in other words, cephalic ganglia modulate activity.

    There are also, as in planarians, conducting channels made up of nerve fibers bound together in bundles, like cables, which run lengthwise through the body. These nerve fibers are gathered together in a double nerve cord that runs along the ventral surface of the body.

    The nerve cords contain fast-conducting fibers that make it possible for the earthworm to contract its entire body very quickly, withdrawing into its burrow when disturbed.

    Reproduction in Earthworms:

    Earthworms are hermaphrodites. Two earthworms, held together by mucous secretions from the clitellum (a special collection of glandular cells), exchange sperm and separate. Two or three days later, the clitellum forms a second mucous sheath surrounded by an outer, tougher protective layer of chitin.

    This sheath is pushed forward along the animal by muscular movements of its body. As it passes over the female gonopores, it picks up a collection of mature eggs, and then, continuing forward, it picks up the sperm deposited in the spermathecas.

    Once the mucous band is slipped over the head of the worm, its sides pinch together, enclosing the now fertilised eggs in a small capsule from which the infants hatch.

    Other annelids resemble the earthworm in that they are cylindrical worms divided into a series of similar segments, and they have a complex circulatory system of blood vessels, a main ventral nerve trunk, a complete digestive tube, and a coelom.

    The phylum is usually divided into three principal classes:

    Oligochaeta is the group that includes the earthworms and related freshwater species. The polychaetes, which are marine animals, differ from the earthworms and other oligochaetes in a number of ways.

    The most striking difference is that they typically have a variety of appendages, including tentacles, antennae, and specialised mouthparts. Each segment contains two fleshy extensions, parapodia, which function in locomotion and also, because they contain many blood vessels, are important in gas exchange.

    Many polychaetes live in elaborately fashioned tubes constructed in the mud or sand of the ocean bottom. Usually the sexes are separate, fertilization is external, and the larvae are free swimming. Hirudinea are the leeches, which have flattened, often tapered, bodies with a sucker at each end.

    Bloodsucking leeches attach themselves to their hosts by their posterior sucker, and then, with their anterior sucker, either slit the host’s skin with their sharp jaws or digest an opening through the skin by means of enzymes. Finally, they secrete a special chemical (hirudin) into the host’s blood to prevent it from coagulating.

    Term Paper # 11. Phylum Mollusca: Mollusks:

    The mollusks constitute one of the largest phyla of animals, both in numbers of species and in numbers of individuals. They are characterised by soft bodies within a hard, calcium-containing shell, although in some forms the shell has been lost in the course of evolution, as in slugs and octopuses, or greatly reduced in size and internalised, as in squids.

    There are three major classes of mollusks:

    (1) The gastropods, such as the snails, whose shells are generally in one piece

    (2) The bivalves, including the clams, oysters, and mussels, which have two shells joined by a hinge ligament and

    (3) The cephalopods, the most active and most intelligent of the mollusks, including the cuttlefish, squids, and octopuses.

    The basic molluscan body plan is shown in figure 11.9. This hypothetical animal was bilaterally symmetrical and segmented.

    Among modern mollusks, only the chitons, a relatively small group, bear any obvious resemblance to the archetypal model, but modern mollusks, although diverse in size and shape, all have the same fundamental body plan.

    There are three distinct body zones-a head-foot, which contains both the sensory and the motor organs a visceral mass, which contains the organs of digestion, excretion, and reproduction and a mantle, which hangs over and enfolds the visceral mass and which secretes the shell.

    The mantle cavity, a space between the mantle and the visceral mass, houses the gills the digestive, excretory, and reproductive systems discharge into it. Water sweeps into the mantle cavity (propelled in the bivalves by cilia on the gills), passing through the gills and aerating them. It then passes by the nephridia, gonopores, and rectum, which are always downstream from the gills.

    Water leaving the mantle cavity carries excreta and, in season, gametes. The digestive tract is far more convoluted and so provides more working surface than that of the annelids.

    In all mollusks, the digestive tract is extensively ciliated, with many different working areas. Food is taken up by the cells lining the digestive glands arising from the stomach and the anterior intestine, and then is passed into the blood.

    A characteristic organ of the mollusk, found only in this phylum, and in all classes except the bivalves, is the radula, a tooth-bearing strap of movable pieces of chitinous tissue covering the tongue.

    The radula apparatus, which operates with a rhythmic back-and- forth movement, serves both to scrape off algae and other food materials and also to convey them backward to the digestive tract. It is also used in combat.

    Mollusks, have gills. To understand the basic plan of gill structure and function, it is necessary only to recall the moist epidermis of the earthworm, through which oxygen diffuses, and the blood vessel lying close beneath it, which transports the oxygen to other parts of the body. A gill is a structure with an increased amount of surface area, through which gases can diffuse, and a rich blood supply for transport of these gases.

    Oxygen diffuses inward, along the gradient, because the cells of the animal have removed oxygen from the bloodstream by cellular respiration. Carbon dioxide, produced by cellular respiration, diffuses outward.

    Mollusks have three-chambered hearts two of the chambers (atria) collect oxygenated blood from the gills, and the third (the ventricle) pumps it to the oxygen-depleted tissue.

    Except for the cephalopods, mollusks have what is known as an open circulation that is, the blood does not circulate entirely within vessels-as it does in the earthworm, for example-but is oxygenated, pumped through the heart, and released directly into spaces in the tissues from which it returns, deoxygenated, to the gills and then to the heart.

    Such a blood-filled space is known as a hemocoel (“blood cavity”). Cephalopods, which are extremely active animals, have accessory hearts that propel blood into the gills, and a closed circulatory system.

    In the bivalves, the two-shelled mollusks, the body has become flattened between the two shells, and “headness” has generally disappeared. The bivalves are sometimes called Pelecypoda-“hatchet foot”—because the muscular foot is often highly developed in this group.

    A clam, using its “hatchet foot,” can dig itself into sand or mud with remarkable speed. However, the bivalves are largely sessile forms, and many of them secrete strong strands of protein by which they anchor themselves to rocks.

    Most bivalves are filter-feeding herbivores they live largely on microscopic algae. Their gills, which are large and elaborate, collect food particles.

    Water is circulated through the sieve-like gills by the beating of gill cilia. Small organisms and particles of food are trapped in mucus on the gill surface and swept toward the mouth by the cilia the gills also sort particles by size, rejecting sand and other larger particles.

    The shells are held together and opened at the hinge by a strong ligament and are drawn closed by one or two large muscles connecting the two shells.

    Throughout the molluscan phylum, there is a wide range of development of the nervous system. The bivalves have three pairs of ganglia of approximately equal size-cerebral, visceral, and pedal (supplying the foot)-and two long pairs of nerve cords interconnecting them.

    They have statocysts, usually located near the pedal ganglia, and sensory cells for discrimination of touch, chemical changes, and light. The scallop has quite complex eyes a single individual may have a hundred or more eyes located among the tentacles on the fringe of the mantle.

    The lens of this eye cannot focus on images, however, so it does not appear to serve for more than the detection of light and dark and movement.

    The gastropods, which include the snails, whelks, periwinkles, abalones, and slugs, are the largest group of mollusks. They have either a single shell or, as a secondary evolutionary development, no shell. Another feature common to all members of this group, as compared with the ancestral mollusk, is that all of them have undergone torsion.

    In other words, the internal organs, the shell, and the mantle have been twisted 180° so that in the modern animal, the mouth and anus and also the gills share the same comparatively small mantle cavity, now pointing forward instead of toward the rear.

    Third, the stomach and digestive gland have become twisted upward into a spirally coiled visceral mass. In response to this displacement and consequent crowding of the internal organs, the gill and nephridium of the right side have been lost in many species.

    In some close relatives of the snails, such as the slugs, the digestive tract has become straightened out again by another course of evolutionary events, in which the shell was lost but the missing organs were not regained.

    Land-dwelling snails do not have gills but the area in their mantle cavities once occupied by gills is rich in blood vessels, and the snail’s blood is oxygenated there.

    Some snails that were once land dwellers have returned to the water, but they have not regained gills. Instead, they must bob up to the surface at regular intervals to entrap a fresh bubble of air in their mantle cavities.

    Thus the mantle cavity has, in effect, become a lung. Moreover, as with all lungs, the opening is reduced to retard evaporation. Gastropods, which lead a more mobile, active existence than bivalves, have a ganglionated nervous system with as many as six pairs of ganglia connected by nerve cords.

    There is a concentration of nerve cells at the anterior end of the animal, where the tentacles, which have chemoreceptors and touch receptors, and the eyes, are located. In some of the animals, the eyes are quite highly developed in structure they appear, however, to function largely in the detection of changes in light intensity, like the eyes of the scallop.

    The cephalopods (the “head-foots”) are the most highly developed mollusks. The large head has conspicuous eyes and a central mouth surrounded by arms, some 70 or 80 in the chambered nautilus, 10 in the squid, and 8 in the octopus.

    The nautilus, as the only modem shelled cephalopod, offers an indication of some of the steps by which this class disposed of the shell entirely. The animal occupies only the outermost portion of its elaborate and beautiful shell, the rest of which serves as a flotation chamber.

    In the squid and its relative, the cuttlefish, the shell has become an internal stiffening support, and in the octopus, it is lacking entirely.

    The octopus body seldom reaches more than 30 centimeters in diameter (except on the late show), but giant squids sometimes attain sea-monster proportions. One caught in the Atlantic some hundred years ago was 15 meters long, not counting the tentacles, and was estimated to weigh 2 tons.

    Freedom from the external shell has given the mantle more flexibility. The most obvious effect of this is the jet propulsion by which cephalopods dart through the water. Usually, water taken into the mantle cavity bathes the gills and is then expelled slowly through a tube-shaped structure, the siphon but when the cephalopod is hunting or being hunted, it can contract the mantle cavity forcibly and suddenly, thereby squirting out a sudden jet of water.

    Contraction of the mantle cavity muscles usually shoots the animal backward, head last, but the squid and the octopus can turn the siphon in almost any direction they choose. In addition to the siphon, cephalopods have sacs from which they can release a dark fluid that forms a cloud, concealing their retreat and confusing their enemies.

    These coloured fluids were at one time a chief source of commercial inks. Sepia is the name of the genus of cuttlefish from which a brown ink used to be obtained.

    The cephalopods have well-developed brains, composed of many groups of ganglia, in keeping with their highly developed sensory systems and their lively, predatory behaviour. These large brains are covered with cartilaginous cases.

    The rapid responses of the cephalopods are made possible by a bundle of giant nerve fibers that control the muscles of the mantle. Many of the studies on conduction of the nerve impulse are made with the giant axon of the squid, which is large enough to permit the insertion of an electrode.

    Evolutionary Affinities of the Mollusks:

    Although the annelids and the mollusks are quite different in their basic body plans, there are some similarities between them that seem to suggest evolutionary links. One of these is the trochophore larva. Many of the annelids (the oligochaetes and hirudineans excepted) have this very distinct larval form.

    Most marine mollusks (except the cephalopods) also pass through a trochophore stage in their development. Until fairly recently, the lack of unequivocal traces of segmentation in the mollusks seemed to argue against the evidence of close affinity provided by the trochophore.

    In the 1950s, however, 10 living specimens of Neopilina, a genus of mollusks previously known only from Cambrian fossils, were dredged from a deep ocean trench off the coast of Costa Rica. Neopilina, which is little more than 2.5 centimeters long, resembles a combination of gastropod and chiton, with a single large shell but five pairs of gills, five pairs of retractor muscles, and six pairs of nephridia, all arranged in what seems to be a distinctly segmental pattern.

    A third link between mollusks and annelids is the pattern of embryonic development-protostomes vs. deuterostomes.

    Term Paper # 12. Phylum Echinodermata: Starfish:

    The starfish and its relatives are known as echinoderms, or “spiny skins.” Adult echinoderms are radially symmetrical, like most coelenterates, although the symmetry is imperfect with some traces of bi-laterality in the adults and with bilaterally symmetrical larvae.

    The most familiar of the echinoderms is the starfish, whose body consists of a central disk from which radiate a number of arms. Most starfish have five arms, which was the ancestral number, but some have more.

    A starfish has no head, and any arm may lead in its sluggish, creeping movements along the sea bottom. The central disk contains a mouth on the ventral surface, above which is the stomach. Like all echinoderms, the starfish has an interior skeleton that typically bears projecting spines, the characteristic from which the phylum derives its name.

    The skeleton is made up of tiny, separate calcium-containing plates held together by the skin tissues and-by muscles. Each arm contains a pair of digestive glands and also a nerve cord, with an eyespot at the end.

    These latter are the only sensory organs, strictly speaking, of the starfish, but the epidermis contains thousands of neurosensory cells (as many as 70,000 per square millimeter) concerned with touch and chemoreception. Each arm also has its own pair of gonads, which open directly to the exterior through small pores.

    The circulatory system consists of a series of channels within the coelomic cavity. Respiration is accomplished by many small fingerlike projections, the skin gills, which are protected by spines. Amoeboid cells circulate in the coelomic fluid, picking up the wastes and then escaping through the thin walls of the skin gills, where they are pinched off and ejected.

    The water vascular, or hydraulic, system is a unique feature of the phylum. Each arm of a starfish contains two or more rows of water- filled tube feet. These tube feet are interconnected by a central ring and radial canals. Water filling the soft, hollow tubes makes them rigid enough to walk on.

    Each tube foot connects with a rounded muscular sac, the ampulla. When the ampulla contracts, the water is forced under pressure through a valve into the tube foot this extends the foot, which attaches to the substrate by its sucker. When, the muscles at the base of the tube feet contract, the animal is pulled forward.

    If the tube feet are planted on a hard surface, such as a rock or a clam shell, the collection of tubes will exert enough suction to pull the starfish forward or to pull apart a bivalve mollusk, a feat that will be appreciated by anyone who has ever tried to open an oyster or a clam.

    When attacking bivalves, which are its staple diet, the starfish averts its stomach through its mouth opening and then squeezes the stomach tissue through the minute space that the starfish has made between the bivalve shells. The stomach tissues can insinuate themselves through a slit as narrow as 0.1 millimeter and begin to digest the soft tissue of the prey.

    The echinoderms are believed to have evolved from an ancestral, bilateral, mobile form that settled down to a sessile life, and then became radially symmetrical. The sea lilies represent this second hypothetical stage.

    In the third evolutionary stage, some of the animals, as represented by the starfish and sea urchins, became mobile again. Following this line of reasoning, one might expect an eventual return to bilateral symmetry in this group, and, in fact, this is seen to some extent in the soft, elongated bodies of sea cucumbers.

    The ancestral bilateral form, like most other hypothetical ancestors, was wormlike. It had a coelom and a one-way digestive tract. However, it differed from the ancestor of the mollusks and annelids in what zoologists consider a very fundamental way, its early embryonic development.

    Among mollusks, annelids, and also arthropods, the early cell divisions of the zygote are spiral, occurring in a plane oblique to the long axis of the egg. In the echinoderms, the cleavage pattern is radial, parallel to and at right angles to the axis of the egg.

    The second difference appears when the embryo becomes a hollow sphere of cells. In the embryos of both groups, an opening, the blastopore, appears. Among the mollusks, annelids, and arthropods, the mouth (stoma) of the animal develops at or near the blastopore, and this group is called the protostomes—”first the mouth.”

    In the echinoderms, which are called deuterostomes, the anus forms at or near the blastopore and the mouth forms secondarily. The chordates, the phylum to which we vertebrates belong, share these characteristics with the echinoderms.

    Watch the video: Mollusks for kids - Invertebrate animals - Science for kids (December 2022).