NOTES ON BCHINODERM MORPHOLOGY. 307 



On the other hand, the terminals of Ophiurids never form a 

 closed ring of plates, but are well separated laterally from their 

 very first appearance, and become more and more separated as 

 the growth of the arms carries them away from the disc. 



2. Sladen has shown that there are both terminal and radial 

 plates in the young Asterid, just as in the young Ophiurid, 

 and, like all his predecessors, he regards the terminals of 

 Asterids as homologous with those of Ophiurids, although the 

 latter have no eye-spot. If this homology be admitted — and I do 

 not know of its ever having been disputed — we have an exactly 

 similar case to that of the Ophiurid radials and the oculars of an 

 Urchin, the one set of plates being associated with an eye-spot 

 and the other not, although they are mutually homologous. 



We may then, I think, assert, without fear of contradiction, 

 that the radial plates are mutually homologous in Ophiurids 

 and Urchins, Asterids and Crinoids, and also that the relative 

 time of their appearance is of no general morphological im- 

 portance. They appear before the basals in the Ophiurids, but 

 after them in the other three groups, being also anticipated in 

 Asterids 1 and Crinoids, 2 and probably in the Urchins as well, 

 by the dorsocentral. 



Thus then, so far as concerns the primary radial plates of 



1 On pp. 124, 127 Fewkes refers to the works of Agassiz and Balfour as appa- 

 rently proving that the radials of Starfishes appear before the dorsocentral. He 

 forgets, however, that the plates which were formerly described as the radials 

 of Starfishes are now known to be the terminals, and that the true radials do 

 not appear till after the basals and the dorsocentral, as shown by Sladen. 



2 Fewkes remarks that " it is difficult to compare the centrodorsal of 

 Crinoids with the dorsocentral of the Ophiuran " (p. 127). But why make 

 the attempt ? There is a dorsocentral in Crinoids perfectly homologous 

 with that of the Ophiuran, as pointed out by Sladen and myself. But it 

 is at the bottom of the larval stem, and altogether different from the cirrus- 

 bearing top stem-joint or centro-dorsal. The distinction between the two 

 was explained in this Journal as long ago as 1878 ; and the homology of 

 the terminal plate at the base of the Crinoid stem with the dorsocentral 

 of the Echinozoa has been frequently noticed since then by Liitken, Duncan 

 and Sladen, Wachsmuth and Springer, Etheridge and myself. I am at a loss 

 to understand therefore why Fewkes refers to the Crinoid centro-dorsal, but 

 omits all notice of the dorsocentral. 



