File: <mollusca

Mollusca are a large well-defined group that is fundamentally bilaterally symmetrical even though the distortion may be great. There is no segmentation and name means "soft-bodied." Outstanding body features are the shell or calcareous exoskeleton, the mantle, which is a fold of the body wall that lines and secretes the shell, and the foot, which is a muscular organ usually concerned with locomotion. A true coelom exists although it may be reduced in size. The animals are primarily marine, but many, like some clams and snails, are found in freshwater and terrestrial.

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The Class Pelecypoda, meaning "hatchet foot," includes the bivalves, clams, oysters and mussels. They are bilaterally symmetrical and laterally compressed. There are right and left bivalve shells, which are hinged along the mid-dorsal line, and the ventral side opens. The mantle lines both valves of the shell. There is no head and the foot is laterally compressed. There are two pairs of lamellate gills.

Characteristics of the freshwater clam Genus Anodonta are discussed as follows:

Habitat.-- Anodonta lies on the bottom or is partially buried in mud of freshwater streams and lakes

Body Plan.-- There is a shell the exterior of which is covered with conspicuous concentric growth rings or lines. In older specimens these growth lines become eroded.

The interior of the shell bears ridges and projections called "hinge teeth", which align the shells during closing. The hinge is elastic. Muscle scars are the anterior and posterior adductor scars and anterior and posterior retractors of the foot. A mantle scar is derived from the pallial line, which is the point of attachment of the mantle. The pallial sinus is an indentation in the mantle of saltwater species and is a siphon scar.

The mantle consists of a pair of flaps, which enclose everything within: (1) a proteinaceous periostracum, (2) a prismatic or crystalline layer, (3) a nacreous layer or "Mother of Pearl." The latter two being calcareous.

A hinge ligament is a thickened area of the periostracum. There are poorly developed siphons in freshwater clams, which are located in an area where the mantle does not reach. Tubes may be formed in marine Mollusca where they project out of the bivalve. This is formed of a fusion of the mantle and may be longer than the shell itself.

The mouth is a tiny opening just behind the anterior adductor muscle. The anus opens into a dorsal excurrent siphon. Labial palps are triangular flaps on each side of the mouth. A foot is present but there is no sharp line between the foot and the visceral mass.

Body Wall.-- The entire body is covered by a single layer of epidermis, which is extensively ciliated. Ciliation is especially strong in siphons, gills and labial palps. The dermis is mixed up muscle and connective tissue that fills in all available space. It is not solid but filled with channels for circulation of the blood. A pericardial cavity is all that is left of the true coelom.

Food & Digestion.-- This is accomplished entirely ciliary activity, and only microscopic organisms are ingested. The food enters via a current of water, which comes in through the incurrent siphon. The food is strained out of the water at the surface of the gills. Here it is entangled in a mucous sheath, which moves forward to the labial palps. It ends up at the mouth. Passage of food into the mouth and through the digestive tract is also by means of the cilia.

The route the food takes is shown in the following diagram:

A crystalline style of gelatinous consistency is present, which is constantly being rotated and pushed forward up into the stomach. It abrades against a horny pad in the stomach known as the gastric shield. The style contains the enzyme amylase.

Circulation.-- The heart has one ventricle and there are two auricles on either side. There are an anterior and a posterior aorta. The vena cava is a short vessel that leads to the kidney. Here the blood is collected and goes to the gills. Then it ends up at the auricles of the heart again. The aortas divide and end in blood sinuses or spaces left between the muscle and connective tissue. This is strictly a percolation or open circulatory system and there are no capillaries. The blood is colorless in most species. There are a few examples of where haemoglobin is dissolved in the plasma and hemocyanin may also occur.

Respiration.-- The gills consist of U-shaped tubes. Blood circulates through the tubes and between them. Water circulates on the outside of the tubes.

Water flows through the gills and moves up to the suprabranchial chamber. Also food sticks to the surface of gills whence it is moved by cilia to the mouth.

Excretion.-- One pair of kidneys lies immediately ventral to the pericardial cavity. These are essentially a nephridium since they lead from the coelom to the outside.

Motion & Locomotion.-- These animals plow through the substrate by muscular movements of the foot, and some species are especially active. The retractor muscle pulls the body after the foot. Blood enters and leaves the foot in a swelling and reduction process.

Sense Organs.-- In the roof of the incurrent siphon there is an osphradium or water tester. One pair of statocysts located in the interior of the foot is built on the same plan as those of the Cnidaria.

Numerous sensory cells occur all over the body, and the siphons and edges of the mantle are especially sensitive.

Nervous System.-- There are one pair of cerebral ganglia, posterior or visceral ganglia and pedal ganglia in the foot.

Reproduction.-- Considering Anodonta spp, freshwater clams, the sexes are separate. Two gonads are diffuse, and ducts from them open into the suprabranchial chamber. Sperm leave the body through the excurrent siphon. Eggs are carried into the suprabranchial chamber but they become lodged in the gills in a brood pouch.

Sperm enter the body of the female and pass to the brood pouch where they fertilize eggs. Early development of the egg occurs in the brood pouch producing the glochidium. Glochidia leave the body of the female via the excurrent siphon and become attached to gills of fish where they complete their development. This involves a slow metamorphosis to the adult. At maturity the glochidia hatch out of cysts on the fish gills and fall to the bottom of the body of water as small adults.

Characteristics of the marine clam Genera Vanus, Teredo and Pecten are discussed as follows:

There is much variation in the foot, siphon life cycle and muscles. Some species have only an adductor muscle. Some like the oyster become sessile and secrete a shelly layer to the substrate. Some secrete byssus threads to the substrate (e.g. black mussel).

Shipworms of the genus Teredo bore into wood and are able to digest cellulose.

Scallops of the genus Pecten are swimming clams where movement is accomplished by clapping the valves together. Jets of water on the end opposite to the shell aperture push the clam ahead. Blue eyes are present on flanges of the mantle.

Reproduction.-- This compares with freshwater forms. The sexes are mostly separate. Protandry or "first males" exists. All oysters are males at first and later they become females. Sexes may alternate from season to season. Some freshwater clams such as Sphaerium and Pisidium are also hermaphroditic and viviparous.

Eggs and sperm are dumped into the open sea. Fertilization forms the trochophore larva. The Veliger larva possesses a shell and more elaborate ciliature.

Economic Importance of Pelecypoda.-- Oysters, clams and scallops are a major source of human food. The shells of freshwater clams particularly have been used for buttons. Pearls are secretions of the mantle around an irritant of calcium carbonate and are usually found in some oyster species.

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Please see following plates for Example Structures of the Pelecypoda:

Plate 47 = Phylum: Mollusca: Pelecypoda: Venus sp.

Plate 48 = Phylum: Mollusca: Pelecypoda: Ostea, Pecten, Ensis, Mytilus, Maja, Teredo spp.

Plate 108 = Phylum: Mollusca, Class: Pelecypoda -- Sculpturing on shell surfaces

Plate 111 = Phylum: Mollusca, Class: Pelecypoda -- Shells of various genera

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The Class Gastropoda, meaning, "stomach-foot", includes the snails and slugs. The shell is univalve and almost always coiled. They have a flat, sole-like, creeping foot and a well-developed head with sense organs. The mantle lines the shell. There is a twisting of the internal organs called torsion. The majority of species are marine, but many freshwater and terrestrial snails exist.

There is a tremendous variation in size, shape and sculpturing of the shell and also in reduction of the shell. The mantle in addition to secreting the shell also is especially modified to form a siphon on the left side of the animal. A respiratory chamber is formed, and the mantle is capable of being inflated in some tropical snails, which results in a high polish on the outside of the shell.

The foot is also subject to modification. This is a wing-like structure developed for swimming. The operculum on the foot is for closing the shell like a door (e.g., Busycon sp.)

The internal anatomy is largely variable in the degree of torsion, where all organs in the visceral mass are involved. The evolutionary trend is toward the loss of paired structures, as there is only one gonad, kidney, etc. resulting from the loss of one of the pair through atrophy.

Larval stages are present in marine snails. The trochophore larval stage is always passed through in the egg and these hatch as veliger larvae only, which have already undergone torsion. The entire larval stages may be passed in the egg.

Economic Importance.-- Abalone, snails, etc. have been important food sources for humans. Some species such as the oyster drill, slugs and terrestrial snails are pests. Over the centuries the dye Royal Purple was produced from Mediterranean snails.

The Subclass Prosobrachia are all marine animals with seprate sexes. They have twisted nerve cords and a gill located anterior to the heart. Torsion of the shell is very prominent. And a veliger larva occurs during development.

The Subgroup Opisthobranchia are all marine and have many bizarre forms. They are hermaphroditic and have straight nerve cords. The gill is located behind the heart, and there is a veliger larva.

The Subgroup Pulmonata have both freshwater and terrestrial species. They are hermaphroditic and the ganglia are clumped around the esophagus. Thre is no gill but a lung chamber instead. Development is direct. Example Genus is Helix.

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An example animal is Helix pomatia, the vineyard snail:

Habitat.-- The species is found in vineyards and gardens.

Body Plan.-- It has a coiled shell lined by the mantle that holds most of the viscera. The foot has a flat sole and the head bears two pairs of tentacles. The eyes are located on the tips of long tentacles. The mouth is ventral to the first pair of tentacles. The anus is situated on the right hand side of the body and thee is one opening into the mantle cavity.

Body Wall.-- As in the clam there is an epidermis and mixed muscle and connective tissue.

Food & Digestion.-- Snails are active feeders in contrast to the passiveness of clams. A buccal mass is housed inside the proboscis, which can be protruded through the mouth. The radula is a horny, rasping ribbon of chitin. A cartilaginous bar, odontophore, is a supporting structure. Muscles, and esophagus and a stomach are also present.

Salivary glands are located on the sides of the stomach. A coiled intestine opens ultimately at the edge of the mantle on the right hand side. The loop results from torsion. A digestive gland, or liver, opens into the stomach by several openings. The food is primarily vegetarian. Digestion is extra- and intracellular. Movement of the food is by ciliary action and muscular contractions, but not strictly ciliary as in clams.

Circulation.-- The heart has a single ventricle and auricle located on the left hand side of the mantle cavity. There is an anterior and a posterior aorta. Arteries ultimately open out in sinuses between muscle and connective tissue and movement of the blood is by percolation. The blood is collected from the sinuses and directed into the vena cava after which it passes through the lung. The lung is a mantle cavity, which has become highly vascularized. From here the blood returns to the ventricle of the heart. Blood is colorless, and has both hemoglobin and hemocyanin in most species

Respiration.-- There are no gills in Helix and oxygenation is in the lung chamber. The pneumatopore is an opening into the lung and is the only opening into the mantle cavity. Muscles that open and close it allowing air to enter control it. Some freshwater snails with lungs conserve air obtained from the water surface, or they may simply fill the lungs with water that contains oxygen.

Excretion.-- There is one kidney near the heart, which is the same as that found in clams. The route is pericardium to mantle cavity to outside the body.

Locomotion & Musculature.-- Snails are slow movers. They creep along on a flat foot. The columellar muscle fastens the animal to the shell, while the retractor muscle serves to pull the snail back into the shell. There are separate muscles for the tentacles, radula, stomach, etc.

Sense Organs.-- The tentacles have a well-developed eye at their tip. There is no osphradium. One pair of statocysts is in the foot and all the rest of the body surface is highly sensitive.

Nervous System.-- Modified from that of the clam the nervous system shows torsion, but in Helix the nerve cords are not completely twisted. A pair of cerebral ganglia is on the sides of the esophagus, and pedal ganglia are in the foot. There are also visceral ganglia and pleural ganglia. All the main ganglia are arranged in a ring around the esophagus.

In Helix the anterior location of the nerve cords avoid the torsion field.

Reproduction.-- Helix is hermaphroditic. The gonad consists of a combination of ovary and testes, called the ovotestis, and there is a joint carrier of eggs and sperm called the hermaphroditic duct. This duct splits into two sections; one for the male and the other for the female parts. Later the system will rejoin at the common genital pore.

The female portion consists of an oviduct, an albumen gland, two mucous glands, a seminal receptacle, a vagina and the common genital pore. The male portion consists of a vas deferens, a flagellum, a penis and the common genital pore.

The two systems are actually connected at their proximal ends (at the oviduct and vas deferens).

Copulation does occur where an individual assumes one sex at a time. Cross-fertilization usually occurs, but self-fertilization may also take place. The eggs are laid with a considerable mass of albumen and they are enclosed in a calcareous shell. They are deposited in clusters in the ground or elsewhere. Development in Helix is direct and there is no larval stage.

Ecology.-- Aestivation is possible, which enables Helix to combat a combination of heat and dryness. A mucous membrane is secreted across the mouth of the shell and the snail can remain inactive for months. Moisture is the stimulus for emergence. Hibernation for up to six months through the cold of winter is also possible in some snails.

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Please see following plates for Example Structures of the Gastropoda:

Plate 110 = Phylum: Mollusca, Class: Gastropoda -- Shells of various genera

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The Class Cephalopoda, meaning, "head foot", includes the squids, cuttlefish, octopus and Nautilus. However, a well-developed shell is present only in Nautilus. In the squid and cuttlefish the shell is reduced and is located internally. In the octopus it is completely absent. Tentacles vary in number from eight in the octopus to 10 in the squid and more than 20 in Nautilus. The giant squid is the largest known invertebrate that can reach 17 to 20 meters in length with its tentacles extended. Cuttlefish do not occur in North America. See Inv137 for example of Nautilus.

There is considerable intelligence among many members of the Cephalopoda. Their brain size is especially great in proportion to their total biomass. Extensive experiments with cuttlefish have shown them to be quick learners. They also display an array of defensive behavior, which allows them to change pattern, color and surface texture in accordance with their environment or other phenomena. Some Southern Hemispheric species are able to produce a flashing light display of many colors and luminosity.

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An example animal is Sepia, the cuttlefish. It produces a brown "sepia dye" in an ink gland. It is bilaterally symmetrical but distorted. The foot is displaced forward and modified into tentacles around the mouth. A jet of water from the mantle cavity can be ejected through the funnel, which is also derived from the foot. The funnel has been erroneously referred to as a siphon. The mantle encloses the whole body except the head. The shell is reduced or absent except in Nautilus, and there is an internal cartilaginous skeleton. The nervous system is highly developed. See Figures Inv138 & Inv139 for examples.

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Another example animal is Loligo, the East coast squid. See Inv140 for comparison with octopus.

Habitat.-- Occurs in deep water in winter and shallow water in summer.

Body Plan.-- Similar to Sepia except longer and more streamlined. The ventral end is functionally anterior, while the dorsal end is functionally posterior.

The shell is reduced in size and internal. This is referred to as the "pen" and it is chitinous.

Food & Digestion.-- Fish are the primary food, which is caught by means of arms and tentacles and held by suckers. The food is brought to the mouth. Horny jaws or the "Beak" tear food into smaller pieces. The radula grinds up the food into still smaller pieces. Then the esophagus carries food to the stomach.

Food is mixed with juices from the liver and pancreas glands in the stomach. Food as absorbed by an absorptive surface on the caecum. Salivary glands occur in the head, neck and body and all open into the buccal mass.

Circulation.-- Sinuses and capillaries are emptying regions for the blood arteries (not only the sinuses as in clams and snails). The pericardial cavity is a true coelom. The blood contains hemocyanin as well as the metallic element copper. Its color is pale blue when oxygenated and colorless when reduced.

The parts consist of an anterior and a posterior aorta, veins from gills, a systemic heart, anterior venae cavae, posterior venae cavae, branchial hearts, a kidney mixed with anterior vena cava, afferent and efferent vessels to and from the gills, respectively, etc.

Respiration.-- Two gills serve for respiration.

Excretion.-- Two kidneys with nephridiopores, which open into the mantle cavity directly.

Support & Protection.-- The pen serves to support the animal, and there are cartilages throughout the body but best developed in the head. There is a heavy muscular mantle. These animals have a remarkable ability to change color and patterns with that of their environment, and thus become less obvious to predators.

The ink gland is a protective structure that is derived from a diverticulum off of the posterior part of the rectum. It is divided into a glandular and a storage section. The ink functions not as a coloring fluid, bur rather as an anesthetic to the sense organs of predators. Some species also are extremely poisonous if consumed.

Motion & Locomotion.-- Rapid movement may be attained by jet propulsion. A current of water is squirted out thru the funnel, often referred to as the siphon. The name "funnel" is preferred because it is not derived from the mantle but from the foot.

Flapping of the fins also allows for a slow gliding motion.

Sense Organs.-- The eyes consist of a sclerotic coat and rest in a socket. A corollary coat is pigmented; there is a corona and an anterior chamber, an iris, a lens, a posterior chamber and a retina. This differs from the vertebrate eye as the nerves enter at the rear of the retina and there is no blind spot. The anterior chamber contains salt water and no aqueous humour.

Olfactory slits are present, but their function has been in doubt. Statocysts and the general body surface are sensitive.

Nervous System.-- Concentrated in a ring around the esophagus are the various ganglia all in a single mass. Star-shaped stellate ganglia are on each side where the mantle is attached. These control contraction of the mantle, which in turn regulates the funnel and other structures actively.

Reproduction.-- Males have a single testis, which lies in a sac that is a part of the coelom. The vas deferens is connected with the wall of the sac and not to the testes directly. There is a spermatophoric gland, a spermatophoric sac, and a penis. Sperm are stored in a spermatophore (shown at left side of diagram below).

The female's ovary is located in a sac of the coelom. There is an oviduct, oviducal gland and nidamental glands, the latter secreting the egg cases.

During copulation the male reaches down inside his mantle cavity with specialized tentacles to pick up a mass of spermatophores, which he subsequently transfers to the mantle cavity of the female. After fertilization a case is secreted around the egg by the nidamental gland. Eggs may be brooded by female octopie. Development is direct, and there is no larval stage.

Economic Importance.-- Cephalopods are used for human food, fish bait in saltwater fishing, sepia dye and cuttlebone for caged birds, especially canaries.

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Please see following plates for Example Structures of the Cephalopoda:

Plate 50 = Phylum: Mollusca: Cephalopoda: Loligo sp. -- General Morphology

Plate 51 = Phylum: Mollusca: Cephalopoda: Loligo sp. -- Venous System

Plate 52 = Phylum: Mollusca: Cephalopoda: Loligo sp. -- Arterial & Digestive Systems

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The Class Scaphopoda has a body plan that is bilaterally symmetrical and the shell is open at both ends. The head is without sense organs but may bear exrtensible filaments called captacula. The foot is modified for digging.

The circulatory system is simplified and there is no heart. Respiration is by the mantle and there are no gills. Two kidneys that do not open into the perivisceral coelom are involved in excretion.

The nervous system has separate cerebral and pleural ganglia.

For reproduction the gonad discharges into the right kidney, and there is a trochophore larva.

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The Class Amphineura includes the chitons. Their body plan is bilaterally symmetrical. The mouth and anus are located at opposite ends. The head is without tentacles or eyes. There are a continuous series of shells dorsally located, and there is a flattened foot.

Primitive gills serve for respiration. The nervous system also is very primitive. For reproduction some species have a trochophore larva.

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Please see following plates for Example Structures of the Amphineura:

Plate 109 = Phylum: Mollusca, Class: Amphineura -- Ventral view of Neopilina galathecae

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Class Aplacophora are worm-like animals where the food is absent or represented by a median ridge in a groove on the ventral side. The mantle is enlarged and there are no shell plates by only spicules. A tiny cloacal chamber at the posterior end may represent the mantle cavity. Gills may be either present or absent.

These are simplified Mollusca that have many characteristics of worms but may be distinguished from the Annelida by not having segmentation and possessing a Mollusca type of coelom. A radula is present in some species. Further information may be obtained from Borradaile & Potts (1958).

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Class: Monoplacophora are bilaterally symmetrical with internal metamerism. One solitary shell covers the pallium that extends over the dorsum. The anus is located in the medial posterior area. They have well developed coelomic cavities

The paired auricles deliver blood to two symmetrical long ventricles located on either side of the intestine. Nephridia emanate from coelomic sacs and op0en on the surface in the palial furrow. The gonads are symmetrical and open through the nephridia. The nervous system is very primitive. Sexes are separate.

The animals occur at great depths (3,500+ meters) in the ocean, one specimen having been found off the west coast of Mexico. Further information may be obtained from Borradaile & Potts (1958).

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Classes Caudofoveata, Helcionelloida (extinct class) and Rostroconchia (extinct class) will not be treated in this section at this time.

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Please see following plates for Example Structures of the Mollusca:

Plate 107 = Phylum: Mollusca -- Example stomach with grooves for guiding food

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Bibliography

Introduction Annelida►

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