THE DEVELOPMENT OF ASTEEINA GIBBOSA. 281 



posterior coelom from the ccelom of the pr^oral lobe, which 

 we may now call the anterior coelom {a., figs. 32—35), is com- 

 plete. On the right side the separation of the posterior part of 

 the right coelomic lobe, the right posterior coelom, from the 

 anterior coelom has just commenced dorsally (fig. 32). On the 

 left side the rudiment of the water-vascular system, or, as it is 

 convenient to term it, the left hydrocoele, has appeared (as 

 will be related immediately a similar rudiment appears on the 

 right side, but " hydrocoele " alone means left hydrocoele). It 

 originates as an outgrowth from the hinder end of the anterior 

 coelom ; and whilst it is as yet but faintly marked off from 

 this cavity, indications of its five primary lobes are seen. 

 These are arranged in a curve open anteriorly, and throughout 

 all the figures they are denoted by the Arabic numerals ; the 

 most dorsal being No. 1, the most posterior No. 3, and the 

 the most ventral No. 5 (see figs. 32 — 34). Their mutual re- 

 lations are well shown in the sagittal section (PI. XIII, fig. 

 47), though this represents a somewhat later stage. 



We have seen that the division of the right posterior ccelom 

 from the anterior coelom has begun in exactly the same manner 

 as happened in the case of the left posterior ccelom at an earlier 

 stage. This division has not proceeded very far towards the 

 ventral surface, when the anterior coelom buds off a vesicle 

 from its right posterior extremity. This vesicle is homologous 

 to the water-vascular rudiment on the left side, for which 

 reason it will be termed the right hydrocoele ; so we see that the 

 coelom on the right side of the larva undergoes exactly the 

 same changes as that on the left, only that they are retarded 

 in their appearance. The first trace of the right hydrocoele is 

 shown in PI. XVI, fig. 95 ; we see that it consists of a small 

 vesicle of cubical cells arising as a thickening of the coelomic 

 wall. Its lumen is, in this stage, a minute slit; other pre- 

 parations show this slit in open communication with the an- 

 terior coelom. It is important to observe that it originates 

 from the dorsal portion of the hinder end of the anterior 

 coelom, which extends further back ventrally to it, as would be 

 seen if a more ventral section than fig. 95 were shown. 



