THE DEVELOPMENT OF ASTERINA GIBBOSA. 349 
posterior coelom from the ccelom of the przoral lobe, which 
we may now call the anterior celom (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 ccelom, 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 hydrocele, has appeared (as 
will be related immediately a similar rudiment appears on the 
right side, but ‘ hydrocele”’ alone means left hydroceele). It 
originates as an outgrowth from the hinder end of the anterior 
celom ; and whilst it is as yet but faintly marked off from 
this cavity, indications of its five primary lobes are seen. 
These are arranged ina 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. 8, and the 
the most ventral No. 5 (see figs. 32—34). Their mutual re- 
lations are well shown in the sagittal section (Pl. 20, fig. 47), 
though this represents a somewhat later stage. 
We have seen that the division of the right posterior ccelom 
from the anterior ccelom 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 hydroceele ; so we see that the 
colom 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 hydroceele is 
shown in PI, 238, 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 colom. 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. 
