Vol. XXII, pp. 183-184 October 30, 1909 



PROCEEDINGS ' 



OF THE 



BIOLOGICAL SOCIETY OF WASHINGTON 



A PROPOSED DIVISION OF THE PHYLUM ECHINO- 



DERMATA. 



BY AUSTIN HOBART CLARK. 



Many attempts have been made to elucidate the interrelations 

 between the various echinoderm classes, but none of the pro- 

 posed arrangements have been able to stand the test of critical 

 investigation. I have recently shown that addition to the 

 ambulacral post-radial series of ossicles in the crinoids takes 

 place by the interpolation of ossicles between the first two (the 

 first two brachials of the free arm) and the radials (" interpo- 

 lated division series"); each division series of two ossicles 

 represents /Wr ambulacral plates as found in the urchins ; two 

 fuse, forming an axillary, and one disappears, its proximal and 

 distal articular faces being thereby projected upon each other, fus- 

 ing, and producing the non-muscular articulations by which the 

 components of division series of two ossicles (or of the two pairs 

 in cases where the division series are of four ossicles) are united. 

 The method of ambulacral increase is, therefore, similar in the 

 crinoids and echinoids. In addition, the crinoids add segments 

 monoserially at the extreme tip of their long and slender arms. 

 It has been supposed that this increase was comparable to that 

 of ophiuroids and asteroids, but in reality it is merely a distant 

 analogy, for in the asteroids and ophiuroids the plates are added 

 biserially at the end of the arm, but just proximal to a perma- 

 nent terminal plate. The ossicles up to and including the sec- 

 ond brachial of the undivided crinoid arm are strictly homolo- 

 gous to the entire ambulacral series in an urchin ; the remainder 

 of the free arm represents the auricles of the urchin. A critical 

 comparison of the anatomy of urchins and crinoids shows them 

 to be closely related, and very different in almost every way 



29— Pkoc. Biol. Soc. Wash., Vol. XXII, 1909. (183) 



