351 A. H. Clark — Pentamerous Symmetry of Crinoidea. 



tion of the origin of pentamerous symmetry must be sought, 

 and a review of the data gleaned from a study of certain 

 species offers some points of considerable interest. But just 

 because certain of the crinoids appear never to have attained 

 a true and complete radial symmetry, it does not at all follow 

 that the marked bilateral arrangement of all types can be thus 

 explained ; for instance, in the recent representatives of the 

 family Comasteridse there are several species having the arms 

 arising from the two posterior rays much shorter than the 

 others, and devoid of ambulacral grooves or tentacles. This 

 is a bilateral symmetry derived through a pentamerous sym- 

 metry clearly, for in the less specialized species belonging to 

 the same genera, and in their own young before adolescent 

 autotomy, the two posterior rays and the arms arising there- 

 from are just like all the others. 



That classical genus Antedon, the "Comatula" of most 

 text-books, represents a form with the most highly perfected 

 pentamerous symmetry, yet exhibiting no advance whatever 

 toward the more highly spscialized secondary bilateral sym- 

 metry seen in the comasterids. Antedon is strictly penta- 

 merous in all respects, excepting only the digestive system; 

 but that this pentamerism is really quite superficial is strikingly 

 shown by a variant described by Professor Bateson * in which 

 one of each of the posterior pairs of arms was abnormally 

 palmate. It would seem, then, that Antedon was a good 

 genus in which to search for an explanation of the origin of 

 pentamerism. 



Dr. P. H. Carpenter had the good fortune to be able to 

 examine several four-rayed comatulids, most of them belong- 

 ing to the genus Antedon ; and it is a fact of the highest inter- 

 est that in every case it was the anterior ray, the one directly 

 opposite the anal area, that was missing. The U. S. National 

 Museum possesses a four-rayed specimen of that magnificent 

 comasterid Comanthus polyenemis, which was brought from 

 the Philippine Islands ; on examining it I found that here 

 again it was the anterior ray that was missing. Such a remark- 

 able coincidence of observations can mean but one thing ; 

 there must be some good reason why the anterior ray should 

 be deficient rather than any of the other four. Since we can 

 safely say that there is no physiological reason why the ante- 

 rior ray should be suppressed in preference to the others — 

 indeed in the case of comasterids one would think that one of 

 the normally abortive posterior pairs would be the first to 



*Proc. Zool. Soc. London, 1890, p. 534, fig. 4. The eleven and twelve 

 armed specimens of Antedon described by Dendy and Carpenter are not 

 bilaterally arranged ; but probably they arose through accident and are not 

 congenital, not being, therefore, strictly comparable. 



