0762 C29CO BDOL-829900 8/4/04 10:12 PM Page 762 Echinoderms and Invertebrate Chordates What You’ll Learn ■ You will compare and contrast the adaptations of echinoderms. ■ You will distinguish the fea- tures of chordates by examin- ing invertebrate chordates. Why It’s Important By studying how echinoderms and invertebrate chordates func- tion, you will enhance your understanding of the begin- nings of vertebrate evolution. Understanding the Photo Echinoderms obtain food by a variety of methods. They can be carnivores, herbivores, scav- engers, or filter feeders. These sea stars are carnivorous echino- derms. They extend the stomach from the mouth and engulf mussels, filter-feeding mollusks. Visit ca.bdol.glencoe.com to • study the entire chapter online • access Web Links for more information and activities on echinoderms and invertebrate chordates • review content with the Interactive Tutor and self- check quizzes 762 David Wrobel/Visuals Unlimited 0763-0769 C29S1 BDOL-829900 8/4/04 8:54 PM Page 763 Echinoderms 29.1 SECTION PREVIEW Objectives Echinoderms Make the following Foldable to compare and Compare similarities and contrast the characteristics of sea urchins and brittle stars. differences among the classes of echinoderms. STEP 1 Fold one sheet STEP 2 Fold into thirds. Interpret the evidence of paper lengthwise. biologists have for deter- mining that echinoderms are close relatives of chordates. Review Vocabulary STEP 3 STEP 4 endoskeleton: internal Unfold and draw over- Label the ovals skeleton of vertebrates lapping ovals. Cut the top sheet along as shown. and some invertebrates the folds. (p. 684) Sea Both Brittle New Vocabulary Urchins Stars ray pedicellaria water vascular system Construct a Venn Diagram As you read Chapter 29, list the characteristics madreporite unique to sea urchins under the left tab, those unique to brittle stars under the tube foot right tab, and those characteristics common to both under the middle tab. ampulla What is an echinoderm? Members of the phylum Echinodermata have a number of unusual char- acteristics that easily distinguish them from members of any other animal phylum. Echinoderms move by means of hundreds of hydraulic, suction- cup-tipped appendages and have skin covered with tiny, jawlike pincers. echinoderm from Echinoderms (ih KI nuh durmz) are found in all the oceans of the world. the Greek words echinos, meaning Echinoderms have endoskeletons “spiny,” and derma, meaning “skin”; If you were to examine the skin of several different echinoderms, you Echinoderms are would find that they all have a hard, spiny, or bumpy endoskeleton cov- spiny-skinned animals. ered by a thin epidermis. The long, pointed spines on a sea urchin are pedicellariae from obvious. Sea stars, sometimes called starfishes, may not appear spiny at the Latin word first glance, but a close look reveals that their long, tapering arms, called pediculus, meaning rays, are covered with short, rounded spines. The spiny skin of a sea “little foot”; Pedicellariae cucumber consists of soft tissue embedded with small, platelike structures resemble little feet. that barely resemble spines. The endoskeleton of all echinoderms is made primarily of calcium carbonate, the compound that makes up limestone. Some of the spines found on sea stars and sea urchins have become modified into pincerlike appendages called pedicellariae (PEH dih sih LAHR ee ay). An echinoderm uses its jawlike pedicellariae for protection and for cleaning the surface of its body. You can examine these structures in the MiniLab on the following page. 29.1 ECHINODERMS 763 0763-0769 C29S1 BDOL-829900 8/4/04 8:55 PM Page 764 Echinoderms have radial symmetry You may remember that radial Observe and Infer symmetry is an advantage to animals Examining Pedicellariae Echino- that are stationary or move slowly. derms have tiny pincers on their Radial symmetry enables these ani- skin called pedicellariae. mals to sense potential food, preda- tors, and other aspects of their Procedure environment from all directions. ! Observe a slide of sea star Observe the radial symmetry, as well pedicellariae under low- as the various sizes and shapes of power magnification. spines, of each echinoderm pictured CAUTION: Use caution when in Figure 29.1. working with a microscope Pedicellariae and slides. The water vascular system @ Record the general appearance of one pedicellaria. What does it look like? Another characteristic unique to # Make a diagram of one pedicellaria under low-power echinoderms is the water vascular sys- magnification. tem that enables them to move, exchange gases, capture food, and Analysis excrete wastes. The water vascular 1. Observe Describe the general appearance of one system is a hydraulic system that pedicellaria. operates under water pressure. Water 2. Infer What is the function of this structure? enters and leaves the water vascular 3. Explain How does the structure of pedicellariae assist system of a sea star through the in their function? madreporite (mah druh POHR ite), a sievelike, disk-shaped opening on the upper surface of the echinoderm’s body. This disk functions like the strainer that fits into a sink drain and Figure 29.1 keeps large particles out of the pipes. All echinoderms have radial symmetry as adults and an endoskele- ton composed primarily of calcium carbonate. Explain How does radial symmetry benefit an animal that cannot move fast? A A living sand dollar has an endoskeleton composed of flattened plates that are fused into a rigid framework. B A sea lily’s feathery rays are composed of calcified skeletal plates covered with an epidermis. 764 (t)Kenneth Read/Tom Stack & Associates, (bl)Scott Johnson/Animals Animals, (br)Norbert Wu/Peter Arnold, Inc. 0763-0769 C29S1 BDOL-829900 8/4/04 8:56 PM Page 765 Look at the close-up of the under- side of a sea star in Figure 29.2. You can see that tube feet run along a groove on the underside of each ray. Tube feet are hollow, thin-walled tubes that end in a suction cup. Tube feet look somewhat like miniature droppers. The round, muscular struc- ture called the ampulla (AM pew lah) Figure 29.2 works something like the bulb of a information to the animal. Echino- Tube feet enable sea dropper. The end of a tube foot works stars and other echino- derms have cells that detect light and like a tiny suction cup. You can find derms to creep along touch, but most do not have sensory out how tube feet help a sea star eat the ocean bottom or to organs. Sea stars are an exception. A sea by looking at Figure 29.4 on the next pry open the shells of star’s body consists of long, tapering bivalves. Infer What page. Each tube foot works indepen- rays that extend from the animal’s cen- causes the suction a dently of the others, and the animal tral disk. A sensory organ known as an sea star uses to pry moves along slowly by alternately open shells? eyespot and consisting of a cluster of pushing out and pulling in its tube light-detecting cells is located at the tip feet. You can learn more about the of each arm, on the underside. Eyespots operation of the water vascular sys- enable sea stars to detect the intensity of tem in the Connection to Physics at the light. Most sea stars move toward light. end of this chapter. Sea stars also have chemical receptors Tube feet also function in gas on their tube feet. When a sea star exchange and excretion. Gases are detects a chemical signal from a prey exchanged and wastes are eliminated animal, it moves in the direction of the by diffusion through the thin walls of arm receiving the strongest signal. the tube feet. Figure 29.3 Echinoderms have varied Echinoderms have bilaterally The larval stage of nutrition symmetrical larvae echinoderms is bilat- eral, even though the All echinoderms have a mouth, If you examine the larval stages of adult stage has radial stomach, and intestines, but their echinoderms, you will find that they symmetry. Explain methods of obtaining food vary. Sea have bilateral symmetry. The ciliated What does this characteristic show stars are carnivorous and prey on larva that develops from the fertilized about echinoderms? worms or on mollusks such as clams. egg of an echinoderm is shown in Most sea urchins are herbivores and Figure 29.3. Through metamor- graze on algae. Brittle stars, sea lilies, phosis, the free-swimming larvae and sea cucumbers feed on dead and make dramatic changes in both decaying matter that drifts down to body parts and in symmetry. the ocean floor. Echinoderms are Echinoderms have a simple deuterostomes nervous system Recall that most invertebrates Echinoderms have no head or brain, show protostome development. but they do have a central nerve ring Echinoderms are deuterostomes. that surrounds the mouth. Nerves This pattern of development extend from the nerve ring down each indicates a close relationship ray. Each radial nerve then branches to chordates, which are also into a nerve net that provides sensory deuterostomes. Color-enhanced LM Magnification: 125ϫ 29.1 ECHINODERMS 765 (t)Norbert Wu, (b)Cabisco/Visuals Unlimited 0763-0769 C29S1 BDOL-829900 8/4/04 8:56 PM Page 766 A Sea Star Figure 29.4 If you ever tried to pull a sea star from a rock where it is attached, you would be impressed by how unyielding and rigid the animal seems to be. Yet at other times, the animal shows great flexibility, such as when it rights itself after being turned upside down. The rigidity or flexibility of a sea star is due to its endoskeleton. Critical Thinking How is radial symmetry useful to a sea star? A Endoskeleton A sea star can quickly Blood sea star change from a rigid structure to a flexible one because it has an endoskeleton in the form of calcium carbonate plates just under its epidermis.