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Phylum Mollusca BIO 102 The phylum Mollusca is the second-largest animal phylum, with over 100,000 species. The molluscs include many familiar animals, including clams, snails, slugs, and squid, as well as some less familiar animals, like tusk shells and chitons. Molluscs are found in nearly all freshwater and marine environments, and some are found also on land. The marine molluscs are probably the best-known and easily recognized members of the phylum. Many of their shells are highly valued by collectors. There are four major groups within the phylum Mollusca: Class Polyplacophora consists of chitons, snail-like molluscs with eight-part overlapping scale shells. Class Gastropoda are true snails and slugs. They represent the most diverse class within phylum Mollusca with 60,000 to 80,000 extant species in marine, freshwater, and terrestrial habitats. Class Bivalvia are molluscs with hinged two-part shells. Examples include clams, oysters, mussels, and scallops. Class Cephalopoda are molluscs with large heads, large eyes, and grasping tentacles. Examples include octopus, squid, cuttlefish, and nautiloids. A slug, a snail, a clam, and a squid do not look alike, but they are all molluscs. Although there is no single feature that all molluscs possess, three features are so common in molluscs that they are used to distinguish them from organisms in other phyla: All molluscs have a specialized foot used in digging, grasping, or creeping. The foot is a muscular organ modified into different forms in different molluscan classes Molluscs have a mantle or mass of soft flesh that covers the soft body and encloses the internal organs. In many species, the mantle produces a hard shell. Not all molluscs produce a shell. Many molluscs have a radula, which, in most species, is a rasp-like scraping organ used in feeding. The word derives from the Latin root prefix radulmeaning scraper. Not all molluscs have a radula, but nothing like it is found in any other group of organisms. Bivalve molluscs lack a radula. The foot is a muscular organ found in all molluscs. Polyplacophorans (chitons) and gastropods have a single flat foot used for crawling. Some bivalves, such as clams, have a paddle-shaped foot adapted for digging into soft sediments. Because a sea slug’s stomach is in its foot, it is named Gastropoda, “stomach-foot” (from the Greek root words gastro meaning stomach and pod meaning foot; Fig. 3.53 B). The foot in octopus and squid is modified into many tentacles that are attached to the animal’s head. That feature gave the class its name Cephalopoda (from the Greek root word cephal- meaning head), or the “head-foot” molluscs. Octopus and squid use their tentacles for moving and for grasping and holding the prey they capture for food. In most molluscs, the mantle produces a hard protective shell. The mantle also creates patterns of color on a shell. The shell is an exoskeleton, even though it is completely surrounded by soft tissue in some molluscs. The shell is continually produced and grows with the animal. Chitons are in the class Polyplacophora (poly meaning many; placo meaning plate or shell; p hora meaning bearing). A chiton’s mantle produces eight shell-like plates that cover the body. Joints between the plates allow the chiton to curl up in a ball and to move flexibly A). The class of molluscs called Bivalvia (from Latin root words bimeaning two and -valv meaning folding door) includes clams, oysters, mussels, and scallops. Bivalves produce two shells that are hinged at the top. The mantle of snails (gastropods) produces a single shell in a spiral shape. The mantle itself cannot be seen because it is on the inner surface of the shell. In some gastropods, such as the cowries, the mantle extends over the shell, keeping the shell shiny and new in appearance. In other gastropods, like the sea hares, and in some cephalopods, like the squid and the octopus, the shell is very small and the mantle covers the shell completely. The nudibranchs, or sea slugs (nudi- meaning naked, -branch meaning gill), are gastropods that don’t produce a shell, so these animals are all softbodied. The chambered nautilus is one cephalopod that secretes an external shell. Squid and cuttlefish produce internal shells that are contained within the mantle, and octopus do not produce shells at all. The mouth structures of many molluscs include a specially adapted rasplike tongue called a radula. The radula is a hard ribbon-shaped structure covered in rows of teeth. Herbivorous snails have a mouth with a radula of usually five to seven complex teeth. There is a great diversity of radula forms in the mollusca . The snail uses its radula like a file, rasping it back and forth over the substrate to scrape off small bits of food. As radular teeth wear down or break off, new teeth are formed to replace them. The tooth patterns of snail radulas are distinctive to species, and scientists can identify snails by looking at their radulas. Some radulas are highly specialized. A group of gastropods called cone snails are carnivorous (meat-eating) hunters that produce venom in glands near the mouth. Their radulas are shaped into long, hollow teeth, which they thrust one at a time into their prey like harpoons. A barbed radular tooth fires through the proboscis, which is an extension of the mouth. It pierces the prey, paralyzing it with venom and preventing its escape. The cone snail “swallows” the prey by engulfing it with its proboscis. In this way cones stalk and capture worms, molluscs, and even fish. Some cones produce a poison strong enough to kill humans who handle them carelessly. Their poison is a neurotoxin that attacks and destroys nerves. Molluscs breathe with gills called ctenidia that sit in a cavity between the mantle and body mass. In some molluscs, most notably bivalves like oysters and mussels, the ctenidia are also used as filter feeding apparatus to strain particulate food from the water. Molluscs have a complete digestive tract surrounded by a small coelom. The molluscan circulatory system is composed of a series of blood sinuses or cavities, rather than closed, discrete vessels. This is referred to as an open circulatory system. Molluscs display a large diversity of nervous systems, from the rudimentary nervous system of the brainless bivalves to the complex systems of the cephalopods, who have well-developed brains and are considered the most intelligent of invertebrates. Chitons (Polyplacophora) are basal relative to other extant molluscs Their soft bodies are covered with a series of eight shell plates. The joints between these shell plates enable to chitons to roll up for protection. Chitons are mobile and contract their muscular foot in waves to move about. The primarily herbivorous chitons have a well-developed radula. Their nervous system is a series of ladder-like nerves and only a few species have poorly developed ganglia. Chitons are found only marine environments. They are most commonly found in tide pools and rocky intertidal zones. Chitons can tolerate the harsh conditions of these habitats where ocean and land meet. Gastropods are the most diverse group of molluscs. The ones we usually think of are snails and slugs. Most gastropods have a calcareous shell protecting the soft-bodied animal inside. Some gastropods, such as sea slugs, sea hares, and garden slugs, lack a shell or have a reduced shell buried in the folds of their mantle. Most creep about on a flattened foot, but some swim, using extended folds of their mantle as fins. Most snails and terrestrial slugs are herbivorous. Bivalves are more enclosed by their shells than other molluscs. Water enters and leaves a bivalve by way of two tubes called siphons. One siphon takes in water while the other expels water and waste. The water taken in contains oxygen and food particles. Most bivalve species acquire energy and nutrients through filter feeding. Filter feeding or suspension feeding is the process of ingesting water and filtering out food particles. Invertebrate examples of filter feeders include sponges, corals, and bivalve molluscs. Class Cephalopoda The cephalopods are molluscs with large heads and tentacles. Examples of cephalopod molluscs include squid, octopus, cuttlefish, and nautilus. Most cephalopods are relatively small. But the giant octopus (Enteroctopus sp.), which lives along the west coast of the United States, can grow to 1.5 m or more. The giant squid, the largest invertebrate, reaches lengths of 15 m. The foot in this group has specialized by dividing into arms that are attached to the head, thus the name cephalopod, meaning head-foot. Like other molluscs, cephalopods have a mantle and mantle cavity that houses the respiratory ctenidia. The mantle cavity is also used to take in and rapidly expel water to facilitate the jet propulsion swimming mode of most cephalopods. When the mantle closes forcefully, seawater ejected through the siphon propels the animal in short bursts. Both squid and octopus change course by redirecting their siphon. They steer by pressing their arms together and can use their speed to elude an attacking predator. They can also squirt ink from the ink sac into the water, creating an ink cloud for camouflage and confusing the predator. Deep-water cephalopods can even produce luminescent ink. Cephalopods also have a small radula, but the radula is not used for food capture. In the mouth of the squid is a beak shaped much like the beak of a parrot.