CHAPTER XXI. 

 BRANCH ECHINODERMATA. 



THE echinoderms include the starfishes, brittle stars, 

 sea urchins, sea lilies, feather stars, and sea cucumbers, all 

 exclusively marine. 



CLASS I. ASTEROIDEA. 

 Example. The Common Starfish. 



Occurrence. The common starfish is found all along 

 the Atlantic coast from Labrador to Florida. It is more 

 abundant in the shallower water, especially on the oyster 

 beds. Other kinds of starfishes are found deeper. 



General Appearance. The common starfish is a five- 

 rayed star. The central body is called the disk and the 

 arms are the rays. In the center of the more flattened 

 surface is the mouth ; hence this surface is called the 

 "oral" surface in distinction from the opposite "aboral" 

 surface, which is more convex. 



The Skeleton. One of the noticeable features of the 

 starfish is its roughness. This is due to the limy skeleton, 

 which consists of many small pieces (ossicles) of calca- 

 reous material loosely and, in the main, irregularly joined 

 together by more or less muscular tissue, so that the star- 

 fish can turn or twist the rays about to a considerable ex- 

 tent. The skeleton is embedded in the tough body wall, 

 which is more or less ciliated, externally and internally. 

 Many of the ossicles bear rigid projecting spines, while on 



