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KEY CONCEPT Most mollusks have shells, and echinoderms have spiny skeletons. Sunshine State STANDARDS SC.F.1.3.1: The student understands that living things are composed of major systems that function in reproduction, growth, maintenance, and regulation. SC.F.1.3.7: The student knows that behavior is a response to the environment and influences growth, development, maintenance, and reproduction. SC.F.2.3.3: The student knows that generally organisms in a population live long enough to reproduce because they have survival characteristics. VOCABULARY mollusk p. 426 gill p. 427 lung p. 427 echinoderm p. 429 VOCABULARY Choose a strategy from earlier chapters, such as a word triangle, or one of your own to take notes on the term mollusk. BEFORE, you learned NOW, you will learn • Body shape affects how animals move and behave • Cnidarians have radial symmetry and simple body systems • Worms have bilateral symmetry and segmented body systems • About different types of mollusks and their features • About different types of echinoderms and their features THINK ABOUT How does a snail move? Snails belong to a group of mollusks called gastropods. The name means “belly foot.” Snails are often put into aquariums to clean up the algae that can build up along the walls of the tank. If you get a chance, look at a snail moving along the glass walls of an aquarium, observe how it uses its foot to move. How would you describe the action of the snail’s foot? foot Mollusks are soft-bodied animals. One characteristic that is shared by all mollusks is a soft body. Many of these invertebrate animals also have an outer shell to protect their body. Oysters, clams, snails, and mussels are all mollusks. So are octopuses, squids, and slugs. Mollusks live on land and in freshwater and saltwater environments. You will read about three groups of mollusks: bivalves, gastropods, and cephalopods. Most mollusks have well-developed organ systems. They have muscles, a digestive system, a respiratory system, a circulatory system, and a nervous system with sensory organs. Mollusks reproduce sexually, and in most species there are distinct male and female organisms. Two adaptations distinguish mollusks as a group. First, all mollusks have a muscular foot. A mollusk’s head is actually attached to its foot. Second, all mollusks have a mantle, a layer of folded skin that protects its internal organs. 426 Unit 3: Diversity of Living Things Bivalves Bivalves are named for a hard shell that is made up of two matching halves. Clams, mussels, scallops, and oysters are all bivalves. The shell, when it is closed, completely encloses the body. If you’ve ever seen a raw oyster, you know that a bivalve’s body looks like a mass of tissue. Bivalves do not have a distinct head, but they do have a mouth and sensory organs. The scallop shown in the photograph has light-sensitive organs that look like tiny eyes. Bivalves are filter feeders, they filter food from the surrounding water. To move, a bivalve balances upright, opens its shell, and extends its foot. The animal moves by pushing the foot in and out. The foot is also used for burrowing, digging down into the sand. Bivalve Most of this blue-eyed scallop’s body is inside its two-part shell. The invertebrates you’ve studied so far—sponges, cnidarians, and worms—take in oxygen all along the surface of their bodies. A bivalve takes in oxygen through a pair of gills. A gill is an organ that filters dissolved oxygen from water. The gill is an adaptation that allows an organism to take in a lot of oxygen in just one area of its body. It is made up of many folds of tissue that create a large surface area. Blood picks up the oxygen and moves it to the rest of the animal’s body. In most bivalves, the gills also filter food from the water. Check Your Reading RESOURCE CENTER CLASSZONE.COM Discover more about mollusks. What are the two functions of gills and how do those functions relate to where bivalves live? Gastropods Gastropods are the most diverse group of mollusks. Some, such as snails and slugs, live on land. Many live in water, for example, conches, whelks, and periwinkles. Many gastropods are protected by a spiral-shaped shell. To protect itself, a gastropod withdraws into the shell. The gastropod’s head is located at the end of its foot. The head has eyes and specialized tentacles for sensing. Many gastropods have a cutting mouth part, called a radula, that shreds their food. Some gastropods eat animals, but most feed on plants and algae. Gastropods that live in water have gills. Some gastropods that live on land have lungs. A lung is an organ that absorbs oxygen from the air. Like gills, lungs have a large surface area. Gastropod This brown-lipped snail extends most of its body out of its shell when it moves. Chapter 12: Invertebrate Animals 427 Mollusks and Echinoderms How do mollusk shells compare with echinoderm skeletons? PROCEDURE 1 SKILL FOCUS Observing MATERIALS Closely observe the mollusk shells and skeletons of sea stars and sand dollars you are given. 2 Examine the shape and texture of each. Sort them by their characteristics. Selection of mollusk shells, sea stars, and sand dollars TIME 15 minutes WHAT DO YOU THINK? How are the shells and skeletons the same? How are they different? CHALLENGE Based on your observations, what can you infer about the bodies of living mollusks, sea stars, and sand dollars? Cephalopods Cephalopods (SEHF-uh-luh-PAHDZ) live in saltwater environments. Octopuses, squids, and chambered nautiluses are cephalopods. Among mollusks, cephalopods have the most well-developed body systems. Cephalopods have a brain and well-developed nerves. They have a pair of eyes near their mouth. The foot, which surrounds the mouth, has tentacles for capturing prey. The mantle is adapted to push water forcefully through a tubeshaped structure called a siphon. This produces a jet of water that moves the animal. Gills take in oxygen, which is picked up by blood vessels and pumped through the body by three hearts. Cephalopods This Maori octopus has a welldeveloped head attached to a foot with eight tentacles. VISUALIZATION CLASSZONE.COM Watch how different cephalopods move. Octopuses and squids do not have protective shells. They do have protective behaviors, however. Some can change body color to match their surroundings. Some release dark clouds of inklike fluid into the water, to confuse their predators. The lack of a shell lets them move freely through the water. The nautilus is the only cephalopod that has a shell. The shell is made up of separate compartments, or chambers. The nautilus itself lives in the outermost chamber. The inner chambers are filled with gas, which makes the animal better able to float. The chambered shell also provides the soft-bodied nautilus with protection from predators. Check Your Reading 428 Unit 3: Diversity of Living Things How is the foot of a cephalopod adapted for hunting? Mollusks show a range of adaptations. You might not think that a clam would belong to the same group as an octopus. These organisms look very different from one another. They also interact with the environment in different ways. The great variety of mollusks on Earth today provides a good example of how adaptations within a group can lead to great diversity. A good example of this is the range of adaptations shown in the shape and function of a mollusk’s foot. The foot of the bivalve is a simple muscular structure that moves in and out of its shell. The foot allows a bivalve to crawl along the ocean floor and to bury itself in the sand. Gastropods have a head at the end of the foot, which runs the length of the body. Muscles in the foot produce ripples that allow the gastropod to glide over a surface as it searches for food. In cephalopods, the foot has tentacles to pull food into its mouth. The tentacles also help some cephalopods move along the ocean floor. foot COMPARE How does the foot of this clam compare with the foot of an octopus? Echinoderms have unusual adaptations. Echinoderms are a group of invertebrates that live in the ocean. In their adult form, their bodies have radial symmetry. Sea stars, sea urchins, sea cucumbers, and sand dollars belong to this group. Echinoderms feed off the ocean floor as they move along. An echinoderm’s mouth is located at the center of the body, on the underside. Some echinoderms, such as sea urchins and sand dollars, filter food from their surroundings. Others, such as sea stars, are active predators that feed on clams, snails, and even other echinoderms. Spines and Skeletons Echinoderm means “spiny-skinned.” Some of the more familiar echinoderms have long, sharp spines, like the sea urchin in the photograph at the bottom of this page. However, some echinoderm species, such as sea cucumbers, have spines that are very small. COMBINATION NOTES Remember to take notes and make sketches for the main idea: Echinoderms have unusual adaptations. This purple sea urchin has very obvious spines. One unusual adaptation that echinoderms have is a type of skeleton. Remember that echinoderms are invertebrates, they have no bone tissue. The echinoderm skeleton is made up of a network of stiff, hard plates. The plates lie just under the surface of the echinoderm’s skin. Some echinoderms, such as sea stars, have skeletons with loosely connected plates and flexible arms. In other echinoderms, such as sand dollars, the plates grow close together, so the skeleton does not allow for much flexibility. 429 tube feet This sea star has captured a bivalve and is using its tube feet to open the shell. FLORIDA Content Preview This sea star’s arms have been pulled up to show how its tube feet are attached to the bivalve’s shell. Water Vascular System and Tube Feet Another adaptation that is unique to echinoderms is a water vascular system. This system is made up of water-filled tubes that radiate out from the center of the echinoderm’s body. Tiny openings along the upper surface of the echinoderm’s body feed water into these tubes. At the base of the tubes is a series of tube feet. reminder In order to survive, animals and plants must adapt to their environment. You will learn more about adaptations in grade 7. Muscles attached to the top of each tube can close the tube off, producing suction at the base of the tube. The tube feet stick to the ocean floor, allowing the echinoderm to pull itself along. The tube feet can also be used for hunting prey. For example, a sea star can surround a clam or oyster with its body, as shown in the photograph on the left. The tube feet pull the shell open. Then, the sea star’s stomach is pushed out through its mouth and into the bivalve’s shell, where it begins to digest the bivalve’s body. Not all echinoderms eat other animals. Some, like sea urchins, feed off algae on the ocean floor. KEY CONCEPTS CRITICAL THINKING 1. What two features do all mollusks have? 4. Analyze For mollusks and echinoderms, what are the advantages and disadvantages of having a shell or spiny skeleton? 2. What are two features all echinoderms have? 3. What are two functions of tube feet in echinoderms? 430 Unit 3: Diversity of Living Things 5. Compare and Contrast Compare the foot of mollusks with the tube feet of echinoderms. CHALLENGE 6. Analyze Animals with lungs or gills can be larger than animals that take in oxygen through their skin. What feature do both lungs and gills have that affect the amount of oxygen they can absorb? What role does the circulatory system play?