THE AFFINITIES OF THE ECHINOIDEA 



AUSTIN HOBART CLARK 



The recent discovery that the crinoids in their ontog- 

 eny increase the number of their ambulacral post-radial 

 ossicles by the interpolation of numerous ossicles (in 

 pairs) between the first two primitive brachials and the 

 radial, forming what are known as "interpolated division 

 series," as well as by the addition of brachials in a linear 

 series at the growing tip of the arm, where only hereto- 

 fore addition to the number of the brachials was sup- 

 posed to occur, 1 has shown that in the manner of increase 

 of the number of ambulacral segments there is a close 

 similarity between the crinoids and the echinoids, both 

 groups adding new plates between those already formed 

 and the radials (oculars), whereas in the ophiuroids and 

 the asteroids new plates are added only at the tip of the 

 arms, not, however, at the extreme tip, as in the crinoid 

 arm, but just proximal to a permanent plate; and the 

 question naturally arises, can the commonly accepted 

 view regarding the interrelations of the various classes 

 of the Echinodermata be maintained in the light of the 

 present state of our knowledge ! 



So long ago as 1821 J. S. Miller remarked on the simi- 

 larity of an inverted Cidaris to a crinoid, and this simi- 

 larity was also noticed by Loven. That this similarity is 

 not superficial but in reality fundamental has become in- 

 creasingly evident to me during the course of my studies 

 on the echinoderms, and I have now no hesitation in sta- 

 ting that the crinoids and the echinoids have much more 

 in common, and are much more closely related to each 

 other, than either group is to the asteroids or the ophiu- 

 roids. 



Considering only the external skeleton, we find that, 

 in the crinoids and echinoids (1) the ambulacrals in- 



x Ttoc. V. S. Na 



113-131. 



