290 



ECHES'ODERMATA.. 



CLASS II. ASTEROIDEA (STARFISHES).* 



Echinoderms with dorso-ventrally compressed pentagonal or star- 

 shaped body. The ambulacral feet are confined to the ventral surface. 

 Internal skeletal pieces in the ambulacra articulated together like 

 vertebra. 



The Star-fishes are chiefly characterised by the predominating 

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



FIG. 235. EcTiinaster sentus, from the oral surface (after A. Agassiz). O, mouth; 

 Af, ambulacral 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. These latter consist of 

 transversely arranged, paired calcareous plates (ambulacral ossicles), 



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

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



