47: 



ECHINODERMATA ASTEROIDEA 



forms. Amongst the former is Culcita, in which tlie body is a 

 pentagonal disc, all outer trace of the arms being lost ; Pentaceros 

 is a long-armed form. 



The family Pentagonasteridae furnishes the key to the 

 understanding of most of the forms contained in this order. It 

 contains genera such as Astrogonium which possess on the back 

 unmistakable paxillae, whilst on the under surface they have the 



-TTTrrCy, 





Fig. 204. — Pentar/onastcr jajjon icus. 



(After Sladen.) 



characteristic covering of granules ; these genera seem to be 

 closely allied to the short-armed species of the Archasteridae, 

 from which they are distinguished chiefly l)y the granular 

 covering of the marginals. From a study of tliese cases it 

 seems clear that the plates of the dorsal skeleton of tlie Valvata 

 correspond to the supporting knobs of the paxillae much 

 broadened out, and the granules correspond to the spinelets of 

 the paxillae increased in number and diminished in size. 



