PRIMARY LARVAL PLATES OF BRAOHIATE ECHINODERMS. 29 



the radials and the terminals. The orals and some of the 

 accompanying plates of the actinal hemisome next appear, 

 before any trace of the dorsocentral plate is present. At the 

 stage when the dorsocentral is finally formed, the plates above- 

 mentioned constitute the whole of the plating of the test. It is 

 interesting to compare the great preponderance of the radial 

 plates at this early period of the young Ophiuran (given 

 in diagram in fig. 13) with the proportions of the same 

 plates in the Crinoid at a much later period of growth (fig. 10). 

 It almost seems like an exaggeration of the character of the 

 same plan of development and suggests the idea that the 

 young Ophiurid commenced its development with an exaggerated 

 presentment or ultimatum of what had been the general scheme 

 and whole tenour of the process of development as exhibited in 

 a late stage of the young Crinoid larva. 



At a later stage the homologues of the basal plates appear, 

 internal to, and alternating with, the circle of radials ; and 

 these are followed by other intermediate and interradially 

 placed plates. Ultimately the under-basals are developed 

 (%. 12;2). 



Proceeding now to the consideration of the Asterid larva, 

 little difficulty will be found in the stage we have selected for 

 fig. 14 in homologating the various plates with those of the 

 two preceding groups. Here as in the larval Crinoid and 

 Ophiurid we have a dorsocentral plate (1), surrounded by a 

 circle of five interradially placed plates (3, 3) of large size in 

 the Asterid, corresponding to the basal plates of the Crinoid. 

 Outside and alternating with these are five smaller plates (4, 4), 

 radially placed, which I consider to be homologous with the 

 first radials of the Crinoid calyx. At the extremity of the rays 

 are large terminal plates (t, t). These plates are homologous 

 with the terminal plates which occupy a similar position in the 

 Ophiurid, and like them are, in my opinion, without a repre- 

 sentative in the Crinoid. 



All previous writers who have dealt with this subject have, 

 without exception, considered the terminal or so-called ocular 

 plate at the extremity of the ray of an Asterid as homologous 



