26 W. PERCY SLADEN. 



the latter being equivalent to the dorsocentral plate through- 

 out the series. The brachial plates are in process of formation. 

 The oral plates, which are entirely on the actinal hemisome, 

 are separated from the abactinal hemisome by the development 

 of the intermediate perisomatic membrane and are rapidly 

 being resorbed. 



At a much earlier stage of development than the one we 

 have been considering, the circle of basal and the circle of oral 

 plates are alone present, and constitute respectively the whole 

 of the actinal and abactinal hemisomes of the calyx. At a 

 later stage the radials are developed, and their first appearance 

 is figured by Allman,i Thompson,^ and Dr. Carpenter.^ They 

 are represented in Herbert Carpenter's diagram,* given here 

 in fig. 11. 



Comparing this stage with the one given in fig. 10, it is 

 interesting to note the astonishing alterations which have taken 

 place during the process of development in the relative pro- 

 portions of the various plates; the chief feature being the 

 enormous development of the first radial plates, which have 

 increased in size at a much greater ratio than any of the other 

 plates. 



Passing next to the Ophiurid larva, as in many respects the 

 nearest phylogenetic ally of the Crinoid, we are able to indi- 

 cate with little hesitation the plates which are homologous to 

 the radial plates of the Crinoid. They are situated on the 

 disc between the base of the rays and the dorsocentral plate 

 (marked 4 in fig. 12). Adcentral to the circle of radial 

 plates is another circle of interradially placed plates, which 

 are equivalent to the basals of a Crinoid (marked 3, 3, in the 

 diagram). In the form under notice still another inner ring 

 of plates is present ; these are radially placed and are homo- 

 logous with the under-basals of the dicyclic Crinoids (e.g. 



' 'Trans. Roy. Soc. Edin.,' vol. xxiii. 



2 'Phil. Trans.,' vol. civ, pi. xxv, et seq. 



3 ' Phil. Trans.,' vol. clvi, pi. xxix. 



^ ' Quart. Journ. Micr. Sci.,' vol. xxii, " Notes on Echinoderm Morphology," 

 No. v, p. 11 (separate copy), fig. iii. 



