428 



ZOOLOGY 



SECT. IX 



alimentary canal with mouth on ventral surface and anus at 

 posterior end ; that it had a ccelome, originally developed from the 

 archenteron of the gastrula ; and that it had a band of strong 

 cilia running around the concave ventral surface. Such a 

 dipleurula-like form became converted, it is supposed, into a fixed 

 form, such as that represented by some of the extinct class of the 

 Cystoidea. The fixation must be supposed to have become 

 effected through the medium of the pre-oral lobe, and further 

 changes must have involved the shifting of the mouth to about 

 the middle of the free surface. From this primitive Cystoid, thus 

 regarded as the most primitive of all known Echinoderms, the 

 remaining classes, both fixed and free, might have been derived by 

 some such order of succession as that indicated in Fig. 351. 



!!olothuroidea 



Echinoidea 



Asteroidea 



Primitive Cystoid 



Dipleurula 

 Flo. 351. Diagram to illustrate the relationships of the classes of the Echinodermata. 



According to another view, however, the most primitive of 

 existing Echinoderms are Synapta and its allies (Holothuroidea 

 apod a). 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 Echmo- 

 derm, again, 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 th* discussion of the affinities of 

 the latter phylum. 



