438 ZOOLOGY sect, ix 



According to another view, however, the most prinaitive of 

 existing Echinoderms are Synapta and its allies (Holothuroidea 

 apoda). The other Holothuroids are supposed, according to this 

 conception of the relationships of the various classes, to have been 

 derived from a Synapta-like ancestor. From the primitive stock 

 of the Holothuroids is s\ipposed to have been derived a form 

 which gave origin to all the stalked classes. From this ancestral 

 stalked Echinoderm, agaid, the remainder of the free classes — the 

 Echinoidea, Asteroidea and Ophiuroidea — are regarded as having 

 been descended. 



Possible relationships between the Echinodermata and the 

 Chordata will be referred to in the discussion of the afSnities of 

 the latter phylum. 



