The Development of Echinus esculentus. 273 



At the same time the ectodermic invagination becomes completely 

 closed : its floor consists of a thick layer of cells which is moulded 

 over the outgrowths of the developing hydrocoele, but its walls and 

 roof remain very thin. Its cavity will be called the amniotic cavity. 



The further development of the Pluteus consists chiefly in the 

 enlargement of the hydrocoele and of its outgrowths, and of course of 

 the ectoderm covering the latter, and of the amniotic cavity into 

 which they project. 



At about 23 days five interradial ridges of the ectoderm, which 

 covers the hydroccele, make their appearance, alternating with the five 

 primary tentacles. The sides of these ridges grow out into over- 

 arching lamellae, which soon meet one another, and in this way five 

 epineural canals are formed, opening into a central epineural space 

 (see figs. 5 and 6). 



FIG. 6. Plan of the Hydrocoele (Rudiment of the Water-vascular System) of a 

 Larva about 24 days old. 



The wedges (distinguished by cross-hatching) are the epineural folds, which even- 

 tually meet one another laterally and form a complete covering over the hydro- 

 ccele. 



At 26 or 27 days the left posterior ccelom has extended to the 

 posterior apex of the hump, and has there met the right posterior 

 coelom, forming a thin vertical mesentery. Just above this mesentery 

 the rudiment of the first pedicellaria appears as a little elevation filled 

 with amoebocytes (fig. 5). A little later the rudiments of two other 

 pedicellariae appear on the right side one situated dorsally, the other 

 ventrally. 



The left posterior coelom, which was at first situated entirely posterior 

 to the hydroccele, has by this time grown completely round it in a 

 circle, and in the axis of this circle the stone-canal still connects the 

 " ampulla" or anterior coelom and the hydrocoele (fig. 5). 



At 27 28 days the rudiments of the masticatory system of the so- 

 called " Aristotle's lantern " make their appearance. These are five 

 outgrowths of the left posterior coelom, having the shape of inverted 



