270 



THE RISE OF ANIMAL LIFE 



Fig. 12-22. A close-up of the tube feet of the deep sea red starfish (Hippasteria phrygiana) to show the nature of 

 the sucking disc at the tip and the circular muscles in the tube that aid in its action. 



ton. In addition, the larvae of some forms 

 resemble some chordate larvae (Fig. 

 13-4). 



The starfish 



Starfish, found in abundance along most 

 seacoasts, vary greatly in size from tiny 

 species about one-half inch in diameter to 

 the giant starfish, which measures about 

 18 inches. The common starfish, Asterias 

 vulgaris ( Fig. 12-20 ) , is found chiefly upon 

 rocky seashores and bottoms where mol- 

 lusks, its main food, are also most abun- 

 dant. Starfish resemble the conventional 

 five-pointed star pattern, the five radiating 

 arms rising from a central disc. Unlike the 

 higher invertebrate fonns already studied, 

 the starfish is headless, similar to some mol- 

 lusks. It is able to move itself in any direc- 

 tion that one of the five rays may point. 

 However, it usually moves forward with 

 two particular arms, namely, the bivium, 

 which consists of the two arms adjacent to 

 the madreporite, a sieve-like structure 



through which water enters (Fig. 12-21). 

 The upper portion of the body, the aboral 

 surface, is covered with spines, and be- 

 tween these projections are gills, or 

 papulae, which function as respiratory 

 organs for the animal. Along the bases of 

 the spines are small pincerlike structures, 

 the pedicellariae, which serve to keep the 

 body free from foreign material. Because 

 the animal is so well armored with various 

 types of sharp projections, it is little won- 

 der that it is not chosen as food by other 

 animals. The oral side, or under surface, on 

 which the mouth is centrally located, serves 

 two main purposes, locomotion and food 

 collection. 



An outstanding feature of the echino- 

 derms is the appearance of a unique device, 

 the ambulacral system, consisting of two 

 rows of tube feet which extend from the 

 mouth down the oral side of each of the 

 five rays (Fig. 12-22). The tube feet enable 

 the animal to move slowly over rocks or 

 along the ocean floor, to twist and turn its 



