402 



ZOOLOGY 



SECT. IX 



of a few Crinoids, is probably less primitive than the attached, 

 since in other phyla the radial symmetry is co-ordinated with, 

 and seems to be developed on account of, a fixed, usually stalked 

 condition. Probably then stalked Echinoderms were the pro- 

 genitors of the existing free forms. The fossil Cystoidea, some of 

 which were stalked, and in which the radial symmetry is not 

 always strongly marked, are to be looked upon as the most primi- 

 tive of all the classes of Echinodermata, and a probable genealogy 

 of the whole phylum is indicated in the diagram below. 



Cysfoidea 



PrimiNve Echinoderms 



FIG. 317. Diagram to illustrate the relationships of the classes of Echinodermata. 



According to another view, however, the most primitive 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 supposed to have been derived a form 

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

 stalked Echinoderm, again, the remainder of the free classes the 

 Echinoidea, Asteroid ea and Ophiuroidea are regarded as having 

 been descended. 



