290 



ECHOs'ODEBMATA. 



CLASS II ASTEKOIDEA (STARFISHES).* 



EcJiinoderms with dorso-ventro.lly comjtressed pentagonal or star- 

 slwtped body. The ambulacra! feet are confined to the ventral surface. 

 Internal skeletal pieces in the ambulacra articulated together lil^-, 

 vertebra?. 



The Star-fishes are chiefly characterised by the predominating 

 pentagonal or star-like discoidal shape of the body, to the ventral 



FIG. 235. F.ch'waster srntus, from the oral surface (after A. Agassiz). O, mouth; 



Af, ambulacra! feet. 



surface of which the ambulacral feet are confined (fig. 235). The 

 radii are long in comparison with the inter-radii, which are very 

 short in consequence of the divergence of the interambulacral 

 rows of plates ; they constitute more or less projecting movable 

 arms, with movable skeletal structures. Those latter consist of 

 transversely arranged, paired calcareous plates (ambulacral ossicles), 



* J. Muller and Troschel, " System der Asteriden," Brunswick, 1841. Com- 

 pare besides the numerous papers of Krohn, Sars, Liitken, L. Agr.ssiz, etc. 



