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TABLE 1

Echinoderms and Chordates

Class

Asteroidea(sea stars)

Ophiuroidea(brittle stars)

Echinoidea(sea urchins)

Holothuroidea(sea cucumbers)

Crinoidea (sea lilies)

Characteristics

Star-shaped forms with broad-based arms; ambulacral grooves with tube feet thatare used for locomotion

Star-shaped forms with narrow-based arms that are used for locomotion;ambulacral grooves covered with ossicles or absent

Globular forms without arms; movable spines and tube feet for locomotion;mouth with five teeth

Cucumber-shaped forms without arms or spines; tentacles around mouth

Body attached by stalk from aboral side; five arms witt) ciliated ambulacralgrooves and tentacle like tUbeJeetfor foodcollectio[1i spines absent

The central disc is largely filled with the digestivetract. The mouth opens into the cardiac stomach,which leads to the pyloric stomach, from which apair of digestive glands extends into each arm. Avery short intestine leads to the anus. A pair ofgonads also extends from the central disc into eacharm near the oral surface. The nervous and circulatory

A

Radial canal

D

systems are greatly reduced and will not be consid-ered here.

Sea stars feed on bivalves. Persistent pressurepulling against the valves of the shell weakens the ad-ductor muscles and opens the valves slightly. The seastar then extrudes its stomach into the mantle cavity sothe clam is digested within its own shell.

Ring canal Pedicellariae

Figure 1 A sea star (Asterias). A. Water vasculary system. B. Skin gills andpedicillariae. C. Dissection. D. Arm cross-section.