Chapter Outline

Molluscs are bilaterally symmetrical eumetazoans. They have a true coelom (eucoelom) which is formed by the splitting of embryonic mesodermal masses (schizocoely). Development is protostomous.

An important characteristic of most molluscs is the head-foot region. Most molluscs are strongly cephalized; that is, they have a well-developed head, in which is located a mouth and a concentration of nervous and sensory functions. Adjacent to the head is a large, muscular foot formed from the ventral body wall. Used primarily in locomotion, the surface of the foot is sometimes ciliated and laden with numerous mucous glands.

Another characteristic of most molluscs is the mantle. This sheath of tissue is formed from the dorsal body wall. It surrounds the mantle cavity, which houses the gills or lungs if they are present, and its surface may assist in gas exchange. The mantle also secretes the shell in those forms that possess one.

Internally, molluscs have a complicated digestive system, with a mouth in the head and the anus emptying into the mantle cavity. A rasping organ used in feeding, the radula, is present in all groups except bivalves and Aplacophora. The radula is usually toothed and its structure may be very complex. It is adapted to a wide variety of feeding styles in different species, including scraping, tearing, stabbing, and cutting.

Molluscs have an open circulatory system (except cephalopods, in which it is closed), complete with a heart, blood vessels, and respiratory pigments. Gas exchange is via gills, lungs, or the body surface. Excretion takes place by means of kidneys that, like the digestive tract, pump waste into the mantle cavity.

A fairly complicated nervous system is present, including several ganglia and a system of nerves; sensory and nervous systems are cephalized in at least some kinds of molluscs. Some molluscs have complex, extremely sensitive eyes.

The majority of molluscs have a shell of some kind. This calcareous structure is secreted by the mantle, and in living snails it is covered by a horny layer called a periostracum. Its shape, size, and color are widely used by taxonomists, and they has also made molluscs a popular pursuit of collectors. Further, shells decay slowly and fossilize well, and the fossil record of molluscs is excellent and ancient.

Most molluscs are dioecious. Many pass through free-swimming larval stages, called trochophore and veliger larvae.

Molluscs are an extremely diverse group of organisms. Over 50,000 living species have been named, making Mollusca second only to the Phylum Arthropoda in number of species known. The majority of molluscs are marine, but large numbers of species occupy freshwater and terrestrial habitats. Molluscs are also extremely diverse in their food habits, ranging from species that graze on microscopic algae, to those that eat the leaves of terrestrial plants, to predators that capture fish and other molluscs. Many species of molluscs are important to humans. A large number of bivalves and some snails are important sources of protein. Oysters produce pearls. Other species are pests in gardens and to crops. Some are essential components in the life cycles of human parasites, including devastating diseases such as schistosomiasis. And a few, such as oyster drills, cause problems by preying on other molluscs that are important to us.

Finally, molluscs play a wide variety of essential ecological roles. As common herbivores, they can have a significant impact on the plant species present in an area. As predators, they may have a similar effect on animals, especially other molluscs. As prey, they provide food for a large number of organisms, including many vertebrates (some of which, such as sea otters, are strikingly specialized for feeding on them). An example of the ecological (not to mention economic) impact of molluscs is given by the recent introduction of zebra mussels to the Great Lakes. Zebra mussels live in large numbers on hard surfaces, and they feed by filtering particles from the water. The effectiveness of their feeding is remarkable; water clarity in some of the Great Lakes has improved dramatically in the years since zebra mussels first appeared. Unfortunately, the same material that the mussels are efficiently removing is the basis of a complicated food chain for other species, including economically-important game fishes. Fisheries biologists in the Great Lakes states are very concerned about the future of the species they tend. The impact of these changes on less conspicuous species, such as the many kinds of native bivalves, is not known, but their populations are certainly at risk.

The chitons are another relatively small group, with around 600 species placed in three Orders. They are flattened, elongated animals with eight overlapping dorsal shell plates or "valves," bordered by a thick "girdle" formed from the mantle. The mantle cavity encloses the foot, with which chitons cling to rocky surfaces. Between foot and mantle on each side of the body is a row of gills.

Chitons have a single heart, a pair of kidneys, a simple nervous system comprising two pairs of nerve cords that connect near the mouth, and sense organs including eyes and chemosensory organs.

Chitons are dioecious. Sperm released by males into the water are taken up by females. Fertilized eggs are shed singly or in gelatinous strings. These develop into a trochophore larva; no veliger is present.

Most chitons are small, but one species reaches 30cm in length. All are marine. Most species live in the rocky intertidal zone, but some can be found at great depths. They graze on algae, scraping algal cells from the substrate with a radula.

The Class Gastropoda includes the snails and slugs. Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups. Gastropods are characterized by "torsion," a process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted. Torsion takes place during the veliger stage, usually very rapidly. Veligers are at first bilaterally symmetric, but torsion destroys this pattern and results in an asymmetric adult. Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms. Many snails have an operculum, a horny plate that seals the opening when the snail's body is drawn into the shell.

Gastropods have a muscular foot which is used for "creeping" locomotion in most species. In some, it is modified for swimming or burrowing. Most gastropods have a well-developed head that includes eyes, 1-2 pairs of tentacles, and a concentration of nervous tissue (ganglion).

Gastropods are dioecious, and some forms are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species ends with each individual sending a sperm-containing dart into the tissues of the other. Marine species have veliger larvae.

Torsion in gastropods has the unfortunate result that wastes are expelled from the gut and nephridia near the gills. A variety of morphological and physiological adaptations have arisen to separate water used for respiration from water bearing waste products.

Gastropods are by far the largest group of molluscs. Their 40,000 species comprise over 80% of living molluscs. Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior. Some graze, some browse, some feed on plankton, some are scavengers or detritivores, some are active carnivores.

This very diverse group includes octopus, squid, nautilus, and cuttlefish. A shell is sometimes present. If present, it is divided into chambers, and the animal occupies the last-formed chamber, with a filament of tissue extending to the oldest chamber. The foot of cephalopods, as their name suggests, lies close to the head. It is modified to form a funnel, through which water is expelled from the mantle cavity. This sort of jet propulsion results in very fast locomotion in some species. Around their mouths, cephalopods have a ring of tentacles that bear powerful suckers in some species. The tentacles are used to search out and grasp food.

Most cephalopods have chromatophores, special pigment cells, in their skins, which allow them to change color rapidly. Most also have a sac that secretes an ink-like substance called sepia; this is expelled when the animal is alarmed, providing concealment as the animal flees.

Cephalopods have the most complex brain of any invertebrate. Most species also have excellent eyes that are astonishingly similar to those of vertebrates. Most species have a single pair of gills, a mostly-closed circulatory system, and accessory or brachial hearts. The gills are ventilated by the muscular action of the mantle wall.

Cephalopods are dioecious. They copulate when the male uses one of his arms (specially modified for reproduction in some species) to remove a spermatophore from his mantle cavity and place it in the mantle cavity of a female. Fertilized eggs are attached to the substrate and are sometimes tended by the female.

The annelid worms are thought to have evolved from a primitive coelomate* worm-like ancestor which developed metameric segmentation. The development of a coelom conferred many advantages, including acting as a hydrostatic* means of locomotion.

However, in the ancestral coelomate the force of muscle contractions in one area was carried throughout the body and so precise control of body movements was not possible.

The phylum Annelida is divided into 3 classes:

Polychaeta ; bristleworms, mainly marine Oligochaeta ; earthworms, mainly terrestrial and freshwater Hirudinea ; leeches, mainly freshwater but with marine and terrestrial species. Of these, the polychaetes are thought to be closest to the ancestral form, (although, as we shall see, some of the polychaetes are highly specialized).

Since it is the least specialized of the bristle worms,

Nereis sp. is usually taken to represent a typical polychaete, and this worm will be examined in detail in the practical class.

The metameric body plan observed in annelids, where the coelom is divided into similar parts or segments, was a major evolutionary advance.

Components of all the major body systems, excretory, circulatory, reproductive and nervous are repeated in each segment - the gut is the only exception. Internally, each segment is separated from its neighbor by vertical mesenteries* and a longitudinal mesentery running from one end of the animal to the other.

Metamerism not only increased the efficiency of burrowing but it made possible sophisticated movement of separate segments, independent of others. The need for greater control over these movements in turn led to a more highly developed nervous system.

The bristle worms show remarkable adaptive radiation of a basic body form which has enabled them to colonize a range of different habitats. They include forms which are: active borrowers; sedentary borrowers; tube-dwelling in sediment; tube-dwelling on a hard substrate; errant, beneath stones etc. on rocky shores; and pelagic forms. They also show an associated range of feeding strategies, from active carnivores to suspension feeding and sediment eating.

This radiation probably took place at an early stage in their evolution and has resulted in a large number of different families. In this unit we want you to look at a representative sample of these forms.

For each of the families listed below make notes on how you think the worm is adapted to its habitat. Examine both the living or preserved specimen and the photographs on display, and use diagrams where necessary to identify specific features.

1. The prominent feature of lophophorate animals is the lophophore, a circular or U-shaped ridge around the mouth, with one or two rows of ciliated hollow tentacles. This functions in food collection and as a gas exchange organ. The coelomic cavity is within the lophophore and tentacles.

2. The protostomic and deuterostomic features are radial cleavage like deuterostomes, in two phyla, the blastopore becomes a mouth (protostomic) and, in the third, it becomes the anus (deuterostomic). The three phyla in this group are brachiopods (shelled, clam-like organisms), bryozoans (moss-like organisms), and phoronids (tube worms).

3. The basic body plan of a mollusc is a visceral mass with a soft epithelium and a muscular foot, and in the mass is contained digestive, excretory, and reproductive organs; this animal may have a differentiated head at the anterior end. The mantle is outside the visceral mass. This allows the gills to develop from its tissues, and the mantle may secrete the protective shell. In the mantle cavity of aquatic molluscs the water passes through, bringing in oxygen and food and carrying out wastes and gametes. This has developed into the jet propulsion system of squid and octopus.

4. A radula is a (usually) barbed rasping strip that allows molluscs to scrape nutrients off of hard substrates such as rocks (or aquarium walls). In some molluscs, the radula has been secondarily lost (bivalves), and in others it has been modified into a sharp puncturing organ, through which toxic materials or enzymes can be delivered to immobilize or kill prey (cone shells).

5. The circulatory system of most molluscs is an open system with a three-chambered heart. Cephalopod circulation differs from others because it has a closed system of vessels to carry the blood to and from the heart. The coelom represents the area around the heart.

6. The mollusc excretory system is composed of two tubular structures called nephridia; the opening is a funnel-like nephrostome that is lined with cilia that collect fluid from the coelom; and a coiled tubule leads to the bladder which is connected to an excretory pore. This type of excretory system allows for concentration of an excretory product.

7. The reproductive individuals in the molluscs are generally male and female individuals. Trochophores are distinct mollusc larvae with a row of cilia around the middle of the body. A veliger is a second larval stage of marine snails and bivalves with a distinct foot, shell, and mantle.

8. The four characteristics of the bivalves are (1) two hinged shells, (2) a wedge-shaped foot, (3) lack of radula, and (4) tentacles. The adductor muscles open and close the shells. The mantle secretes the shell and forms the in-going and outgoing siphons. Bivalves feed as the cilia on the gills bring in water, and food particles become entangled in the mucus secreted by glands; then the palps direct the mass into the mouth. Bivalves disperse as larvae because adults are relatively fixed and immobile.

9. The annelid nervous system has diverse sensory organs that are concentrated on the anterior end, cerebral ganglion, and a primitive brain that is found in the anterior segment with separate interconnected ganglia in each segment. Annelid and nematode locomotion are similar because muscles move against a hydrostatic skeleton. Annelid locomotion is more advanced because each segment can move independently. Annelid setae are bristles of chitin that serve as anchors in certain crawling movements and may aid in swimming.

10. Annelids are similar developmentally to molluscs because both possess free-swimming trochophore larvae. It is likely that molluscs and annelids are derived from a common unsegmented ancestor. The relationship between segmentation in annelids and arthropods and that in vertebrates is that annelids and arthropods may share a same segmented ancestor whereas vertebrate segmentation evolved independently.

11. The circulatory system of annelids is a closed system with five hearts. They exchange gases with the environment directly through the skin-they lack gills or lungs.

12. Parapodia are feathery outgrowths of the body wall next to the setae in polychaete annelids that function in increased surface area for respiration. Some parapodia may also assist in movement.

13. Earthworms obtain nutrients by sucking soil into their mouth via contraction of the pharynx; then muscles in the gizzard grind up the organic material and the food moves through a long, straight digestive tract, with undigested material deposited as castings. Earthworm sensory structures sense light, touch, and moisture. Earthworms do not have parapodia. Earthworm reproduction is hermaphroditic. They orient head to tail when mating, held together by a mucous band produced by the clitellum. Each exchanges sperm with the other. After separation the clitellum secretes a cocoon, and as it passes the female openings, it receives the eggs. It then picks up the other worm's sperm from the sperm receptacles. Young worms eventually hatch from the cocoon.

14. Leeches and earthworms both possess a clitellum (during breeding season) and are both hermaphroditic with cross-fertilization. Leeches differ in that they have a reduced coelom, they are not divided into individual segments, they have suckers at one or both ends of the body, they generally have no setae, and many are external parasites. The adaptations that allow leeches to suck blood are a mouth with chitinous teeth that rasp through skin, an anticoagulant that prevents the wound from clotting, and powerful sucking muscles that suck up the blood. The anticoagulant as well as leaches are still used in medicine today in reduction of superficial hematomas.

Last modified: 3/2/98

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