Classification

figure 38-2

This sea lily, Cenocrinus (a), and these feather stars, Oxycomanthus bennetti (b), are members of the class Crinoidea. Notice their adaptations for filter feeding.

Taxonomists divide the 7,000 species of echinoderms into six classes, five of which will be described here: Crinoidea (kri-NOYD-ee-uh), Ophiuroidea (OH-fee-yoor-OYD-ee-uh), Echinoidea (EK-uh-NOYD-ee-uh), Holo-thuroidea (HOH-loh-thuh-ROYD-ee-uh), and Asteroidea (AS-tuh-ROYD-ee-uh).

Class Crinoidea

Members of the class Crinoidea, called crinoids (KRI-noydz), include the sea lilies and feather stars, which are shown in Figure 38-2. The name crinoid means "lily-like." Sea lilies most closely resemble the fossils of ancestral echinoderms from the Cambrian period. They are sessile as adults, remaining attached to rocks or the sea bottom by means of a long stalk. Feather stars, in contrast, can swim or crawl as adults, although they may stay in one place for long periods.

This sea lily, Cenocrinus (a), and these feather stars, Oxycomanthus bennetti (b), are members of the class Crinoidea. Notice their adaptations for filter feeding.

In both types of crinoids, five arms extend from the body and branch to form many more arms—up to 200 in some feather star species. Mucus-covered tube feet located on each arm filter small organisms from the water. The tube feet also serve as a respiratory surface across which crinoids exchange oxygen and carbon dioxide with the water. Cilia on the arms transport trapped food to the crinoid's mouth at the base of the arms. The mouth faces up in crinoids, but in most other echinoderms the mouth faces downward.

Class Ophiuroidea

The 2,000 species of basket stars and brittle stars make up the largest echinoderm class, Ophiuroidea, which means "snake-tail." Members of this class are distinguished by their long, narrow arms, which allow them to move more quickly than other echinoderms. As you can see in Figure 38-3, the thin, flexible arms of basket stars branch repeatedly to form numerous coils that look like tentacles. Brittle stars, so named because parts of their arms break off easily, can regenerate missing parts.

Basket stars and brittle stars live primarily on the bottom of the ocean, often beneath stones or in the crevices and holes of coral reefs. They are so numerous in some locations that they cover the sea floor. Some species feed by raking in food with their arms or gathering it from the ocean bottom with their tube feet. Others trap suspended particles with their tube feet or with mucous strands located between their spines.

Class Echinoidea

The class Echinoidea consists of about 900 species of sea urchins and sand dollars. Echinoidea means "spinelike," a description that applies especially well to many of the sea urchins, such as the ones shown in Figure 38-4. In both sea urchins and sand dollars, the internal organs are enclosed within a fused, rigid endoskeleton called a test.

The spherical sea urchins are well adapted to life on hard sea bottoms. They move by means of their tube feet and feed by scraping algae from hard surfaces with the five teeth that surround their mouth. The teeth and the muscles that move them are part of a complex jawlike mechanism called Aristotle's lantern. The spines that protrude from the test may be short and flat, long and thin, or wedge shaped, depending on the species. In some sea urchins, the spines are barbed, and in others, they are hollow and contain a venom that is dangerous to predators as well as swimmers.

Sand dollars live along seacoasts. As their name implies, they are usually found in sandy areas and have the flat, round shape of a silver dollar. Their shape is an adaptation for shallow burrowing. The short spines on a sand dollar are used in locomotion and burrowing, and they help clean the surface of the body. Sand dollars use their tube feet to capture food that settles on or passes over their body.

figure 38-3

This basket star, Astrophyton muricatum, has long, flexible arms with many coiled branches.

figure 38-3

This basket star, Astrophyton muricatum, has long, flexible arms with many coiled branches.

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www.scilinks.org Topic: Echinoidea Keyword: HM60459

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Maintained by the National Science .TeachenAnociation figure 38-4

The long, sharp spines that cover these sea urchins, Strongylocentrotus, provide protection against most predators.

figure 38-4

The long, sharp spines that cover these sea urchins, Strongylocentrotus, provide protection against most predators.

figure 38-5

Tentacles around the mouth of this sea cucumber collect food and bring it to the animal's mouth. Five rows of tube feet that run along the body are evidence of the sea cucumber's pentaradial symmetry.

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