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16.1 Mollusks. Objectives. Describe the characteristics of mollusks. Compare mollusks to other invertebrates. Describe the features of each class of mollusks. Snails and clams are invertebrates called mollusks MAHLuhsks). All mollusks have soft bodies, and many are covered by hard shells.
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Objectives • Describe the characteristics of mollusks. • Compare mollusks to other invertebrates. • Describe the features of each class of mollusks.
Snails and clams are invertebrates called mollusks MAHLuhsks). • All mollusks have soft bodies, and many are covered by hard shells.
Characteristics of Mollusks • Mollusks live mostly in the ocean, but some can be found in freshwater habitats. • A number of mollusks have also adapted to life on land. • Snails and slugs are common in damp places in your backyard.
There are more than 100,000 species of mollusks. • They range in size from tiny snails only a few millimeters across to the 20-m-Iong giant squid.
No matter what their shape or size, all mollusks have the same basic body plan. • Like the bodies of segmented worms, mollusks' bodies contain organs organized into systems. • However, their bodies are not built of segments like the segmented worms. • Instead, mollusks have most of their organs in one area called a visceral (VIS ur uhl) mass. • The visceral mass contains the digestive, excretory, circulatory, respiratory, and reproductive organs.
Another characteristic of the mollusk body plan is a muscular foot. • The foot is used by most mollusks for movement. • In addition, a mollusk's soft body parts are covered by a skinlike tissue called a mantle. • In many mollusks, the mantle produces a hard, protective shell.
Evolution of Mollusks • The oldest known mollusk fossil is a single-shelled sluglike organism that lived about 570 million years ago. • After this time, mollusks evolved a variety of body forms. • Many were common in the ancient oceans. • Then about 225 million years ago, most mollusk species became extinct. • The survivors are the ancestors of the kinds of mollusks alive today.
Many oceanic mollusks have larvae similar to the larvae of segmented worms, or annelids. • For this reason, some scientists think mollusks evolved from segmented worms. • However, because of mollusks' lack of segmentation, other scientists hypothesize that mollusks evolved from ancient worms more similar to flatworms.
Diversity of Mollusks • You can tell that snails are very different from clams and that both differ from an octopus. • Would you classify these three mollusks in different groups? • That is what scientists do. • Each of these kinds of mollusks is a member of a different group, or class.
Gastropods • Mollusks that glide along on a foot underneath their bodies are called gastropods (GAS troh PAHDZ). • Snails, limpets, slugs, and the sluglike nudibranchs are all gastropods. • Most gastropods have one coiled shell. • The shells of limpets, however, are flattened cones. • Slugs and nudibranchs have no shell at all.
When you watch a snail moving on the ground, you might think it is scooting on its stomach. • Actually, a gastropod's muscular foot spreads out under its body and is separate from the stomach. • A rippling motion of the foot muscle moves the snail along.
Many gastropods eat plants and algae. • Some eat other invertebrates. • As an adaptation for getting their food, gastropods have a tonguelike organ covered with rows of teeth. • It is called a radula (RAJ 00 luh).
A gastropod moves its radula back, and forth to scrape off and scoop up food. • Gastropods that live in water have respiratory organs called gills. • As water flows over the gills, oxygen is extracted. • Gastropods living on land have evolved a cavity inside their Bodies that serves as a simple lung.
Bivalves • Mollusks with two shells hinged together make up a second class called the bivalves (BYvalvz). • Clams, oysters, mussels, and scallops are all bivalves. • Like gastropods, bivalves have a muscular foot. • It is normally hidden inside the two shells. • Some bivalves move by hooking the foot in sand, and then pulling themselves along.
Bivalves have no head, and most move very little. • Some remain attached to one place. • Movement isn't important because they are filter feeders. • Cilia move water across the gills. • Tiny food particles, such as plankton, are trapped by mucus on the gills. • Cilia push the food into the animal's mouth.
The gills in bivalves have a large surface area and a rich supply of blood. • Oxygen from the water passing over the gills diffuses into the blood. • Carbon dioxide, a waste product of animals, diffuses from the blood into the water.
In some kinds of bivalves, water flows over the gills when the shells are open. • In clams, the water enters through a muscular tube called a siphon. • After it flows over the gills, it leaves through another siphon.
Cephalopods • The octopus and squid are the major members of a third class of mollusks called cephalopods (SEF uh loh PARDZ). • In these mollusks, the foot is divided into tentacles. • The tentacles are located at the head, away from the rest of the body.
Cephalopods are the only mollusks to have a closed circulatory system. • In a closed system, the blood stays in blood vessels.
Modern cephalopods all evolved from now-extinct mollusks with external shells. • In the octopus, the shell has disappeared during the process of evolutionary change. • Squids have only small internal shells. • Only cephalopods called nautiluses (NAWTuh luhs uhz) still have the external shells of their ancestors.
Cephalopods live in the oceans and most move about freely. • Unlike the filter-feeding bivalves, they are active predators. • Their tentacles are covered with suction cups for grasping prey. • The octopus lives mostly on the ocean floor, crawling around in search of prey. • Squid swim in the open water.
