Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Echinoderms
Class Asteroidea

The body has the shape of a flat star with 5 or more arms (Fig. 79). Longitudinal grooves run along the underside of the arms, containing numerous tube feet. Locomotion is carried out using both the tube feet and the arms themselves.

The Mouth is located in the center of the lower surface of the body. A short Esophagus leads into a voluminous Stomach (Fig. 80) capable of everting outward. Upon finding large prey (such as a mollusk), the starfish covers it with its body, everts its stomach, and presses it against the prey.

Juices secreted by The Stomach wall digest the food. A short intestine leads to the anus, which lies on the upper surface of the body. Starfish inhabit sea bottoms, descending to great depths. They feed on benthic animals and organic detritus, and cause damage to oyster farms.

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Fig. 79 Starfish

Fig. 80 Internal Structure of a starfish:

1 — mouth; 2 — stomach; 3 — anus; 4 — Liver; 5 — madreporite; 6 — axial organ; 7 — stone canal; 8 — ring canal of the ambulacral system; 9 — its radial canal; 10 — tube feet; 11 — ampullae; 12 — ring vessel of the Circulatory system; 13 — its radial vessel; 14 — circumoral nerve ring; 15 — its radial nerve; 16 — gonad.