176 CAS WELL GRAVE. 



assist them in feeding until the tube feet have grown sufficiently 

 to assume the function. 



THE HYPOTHETICAL BILATERAL ANCESTOR. 

 Although numerous papers have been written on the subject 

 of the phylogeny of the echinoderms there are but few which 

 retain their vitality at the present time. In these, notwithstand- 



-P.A.P. 



-V 



FIG. 8. Outline of the young ophiuran which metamorphoses from the pluteus 

 shcfwn in Fig. 7. Original. I., II., III.. IV. and V., arms of the ophiuran; 

 3, 4 and 5, ciliated rings. P. PL, remnant of the posterior end of the pluteus. P. 

 A. P., long posterior arms of the pluteus which are never absorbed but are finally 

 dropped. P., madreporite. 



ing the fact that many differences in detail exist, there is a very 

 great similarity in the views set forth and I may state in this 

 connection that the facts of this paper and many of my unpub- 

 lished observations are an additional support to the hypothesis 

 which has been gradually developed by Bury, McBride and 



