Structure And Function Of Echinoderms

The sea star will be used to demonstrate some of the details of echinoderm structure and function. As you read about how echino-derms carry out life functions, consider how they differ from the other groups of invertebrates you have studied.

External Structure

As you can see in Figure 38-7, the body of a sea star is composed of several arms that extend from a central region. Sea stars typically have five arms, but in some species, such as the one shown in Figure 38-6b, there may be as many as 24. Two rows of tube feet run along the underside of each arm. The body is often flattened.

In echinoderms, the side of the body where the mouth is located is referred to as the oral surface. The side of the body that is opposite from the mouth is called the aboral (A-BOHR-uhl) surface. In sea stars, the oral surface is on the underside of the body.

The body of a sea star is usually covered with short spines that give the animal a rough texture. Surrounding each spine in many sea stars are numerous tiny pincers called pedicellariae (PED-uh-suh-LAR-ee-ee), which are shown in Figure 38-7. Pedicellariae help keep the body surface free of foreign objects, including algae and small animals that might grow on the sea star or damage its soft tissues. Pedicellariae are found in sea stars and some sea urchins.

figure 38-7

Sea stars have a number of structural features that are unique to the phylum Echinodermata. Their pentaradial symmetry is indicated by their five arms, each of which contains a division of their internal organ systems. The water-vascular system consists of a network of canals connected to hundreds of tube feet. The inset shows that the sea star's exterior is dotted with short spines, pincerlike pedicellariae, and skin gills.

Sensory tentacles

Tube feet

Spine

Pedicellaria

Sensory tentacles

Spine

Tube feet

Structure Echinodermata

Digestive glands

Radial canal n

Digestive glands

Radial canal

Word Roots and Origins ampulla form the Latin ampulla, meaning "flask"

figure 38-8

This sea star, Asterias rubens, is prying open the shell of a clam to feed on the clam's soft tissues.

figure 38-8

This sea star, Asterias rubens, is prying open the shell of a clam to feed on the clam's soft tissues.

Sea Urchin Pedicellariae

Water-Vascular System

The water-vascular system is a network of water-filled canals that are connected to the tube feet. Use Figure 38-7 to follow the path of water through the water-vascular system. Water enters the system through small pores in the madreporite (MA-druh-PAWR-lET), a sievelike plate on the aboral surface. Water then passes down the stone canal, a tube that connects the madreporite to the ring canal, which encircles the mouth. Another tube, the radial canal, extends from the ring canal to the end of each arm. The radial canals carry water to the hundreds of hollow tube feet. Valves prevent water from flowing back into the radial canals from the tube feet.

The upper end of each tube foot is expanded to form a bulblike sac called an ampulla (am-PUHL-uh). Contraction of muscles surrounding the ampullae forces water into the tube feet, causing them to extend. Contraction of muscles lining the tube feet forces water back into the ampullae and shortens the tube feet. In this way, the sea star uses water pressure to extend and withdraw its tube feet. In many species, small muscles raise the center of each tube foot's disklike end, creating suction when the tube feet are pressed against a surface. These coordinated muscular contractions enable sea stars to climb slippery rocks and capture prey.

Feeding and Digestion

The sea star's mouth is connected by a short esophagus to the cardiac stomach, which the sea star can turn inside out through its mouth when it feeds. The cardiac stomach transfers food to the pyloric stomach, which connects to a pair of digestive glands in each arm. The cardiac stomach, pyloric stomach, and digestive glands break down food with the help of the enzymes they secrete. Nutrients are absorbed into the coelom through the walls of the digestive glands, and undigested material is expelled through the anus on the aboral surface.

Most sea stars are carnivorous, feeding on mollusks, worms, and other slow-moving animals. When a sea star captures a bivalve mollusk, such as a clam, it attaches its tube feet to both halves of the clamshell and exerts a steady pull, as Figure 38-8 shows. Eventually, the clam's muscles tire, and the shell opens slightly. The sea star then inserts its cardiac stomach into the clam and digests the clam's soft tissues while they are still in the shell. The sea star then withdraws the stomach, containing the partially digested food, back into its body, where the digestive process is completed.

Other Body Systems

Like other echinoderms, the sea star has no circulatory, excretory, or respiratory organ systems. Fluid in the coelom bathes the organs and distributes nutrients and oxygen. Gas exchange and waste excretion take place by diffusion through the thin walls of the tube feet and through the skin gills, hollow tubes that project from the coelom lining to the exterior. You can see the skin gills in the inset in Figure 38-7, on the previous page.

Because echinoderms have no head, they also have no brain. The nervous system consists mainly of a nerve ring that circles the mouth and a radial nerve that runs from the nerve ring along each arm. The nerve ring and radial nerves coordinate the movements of the tube feet. If the radial nerve in one arm is cut, the tube feet in that arm lose coordination. If the nerve ring is cut, the tube feet in all arms become uncoordinated, and the sea star cannot move.

Sea stars also have a nerve net near the body surface that controls the movements of the spines, pedicellariae, and skin gills. The end of each arm has an eyespot that responds to light and several tentacles that respond to touch. The tube feet also respond to touch, and other touch-sensitive and chemical-sensitive cells are scattered over the surface of the sea star's body.

Reproduction and Development

Most sea star species have separate sexes, as do most other echinoderms. Each arm of the sea star contains a pair of ovaries or testes. Females produce up to 200 million eggs in one year. Fertilization occurs externally, when the eggs and sperm are shed into the water. Each fertilized egg develops into a bilaterally symmetrical, free-swimming larva called a bipinnaria (BlE-pin-AR-ee-uh). After about two months, the larva settles to the bottom, and begins metamorphosis into a pentaradially symmetrical adult.

Echinoderms have remarkable powers of regeneration. Sea stars can regenerate arms from the central region of their body, even if they lose all of their arms. The process of regeneration is very slow, taking as long as a year. Sea stars use their regenerative ability as a defensive mechanism, automatically shedding an arm at its base when the arm is captured by a predator. As you can see in Figure 38-9, some sea stars can even regenerate a complete, new individual from a detached arm, as long as the arm is attached to a portion of the central region. Certain species reproduce asexually when the body splits through the central region. The two parts that are formed then regenerate the missing structures.

figure 38-9

As long as a sea star retains part of its central region, it can regenerate any arms it loses. The sea star shown here, a member of the genus Echinaster, is regenerating five new arms.

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