ECHINODERM LARV^). 477 



anterior (r.a.c. and l.a.c.) and posterior (r.p.c. and l.p.c.) 

 portions, and are taking up their characteristic positions, 

 the two anterior ones alongside the oesophagus and the 

 two posterior alongside the stomach, as shown in fig. 21, 

 which also is a ventral view of a young pluteus. 



In fig. 23, the first indication of the formation of the 

 five-lobed hydrocoel (%.) is seen. This organ, according 

 to MacBride (11), is budded off from the left anterior 

 coelom, but is not completely separated from it. The 

 anterior portion of the original vesicle, as well as the 

 tubular portion — the future stone-canal — connecting it 

 with the hydrocoel, is but rarely discernible in whole pre- 

 parations, and is not represented in the figures. The 

 hydrocoel now enlarges considerably, and extends along 

 the entire length of the oesophagus (fig. 24), while the 

 right anterior and the two posterior vesicles remain prac- 

 tically of the same size, and, up to this point, have under- 

 gone no further development. The metamorphosis of 

 the larva into the post-larval Ophiurid is now initiated; 

 and the hydrocoel, apparently losing, for the time being, 

 its five-lobed form, proceeds to surround the larval 

 oesophagus (figs. 25 and 28, PL IV). The further details 

 of the metamorphosis cannot be satisfactorily studied 

 without recourse to serial sections. A short account of 

 the development of the primary elements of the skeleton 

 of the adult, as illustrated by a considerable number of 

 metamorphosing larvae, may, however, be here given. 



In fig. 26, PL IV, which shows a larva viewed from 

 the ventral surface, there appear on the right side (left 

 of the figure) five minute, tri-radiate calcareous plates 

 (rl.), three of which lie upon the dorsal surface, and two 

 upon the ventral surface of the larva. These eventually 

 become the radial plates of the disc of I lie adult Ophiurid. 

 On the left side (right of the figure) other five plates (tl.) 



