350 E. W. MACBRIDE. 
Later stages of this organ are seen in figs. 85 and 86. In 
fig. 35 it is a conspicuous solid bud; in fig. 36 it has acquired 
a lumen, and is connected with the anterior ceelom by a string 
of cells, which soon atrophies, and it is then left as an isolated 
vesicle in the midst of the mesenchyme. Bury (2), indeed, has 
seen it in this stage, and called it ‘‘ a mesenchymatous vesicle ;”” 
and Field (5) has described what I believe to be an homologous 
structure in the larva of Asterias. The right hydrocele persists 
in the adult as a closed sac just under the madreporite, and 
has been seen here by Cuénot (8), and Leipoldt (9) has described 
a similar sac in Echinids. It may seem rather a rash assump- 
tion to regard this organ as the fellow of the water-vascular 
system, but a complete proof that this is really its nature will 
be given when abnormal larve are described. 
Stage D, the summit of the development of the larva, is 
reached on the seventh day, according to Ludwig (PI. 18, figs. 
10 and 11). The preoral lobe and the larval organ have 
greatly increased in size, the former having acquired a large 
ventral as well as a dorsal lobe. The internal changes are 
more striking than the external. The separation of gut from 
celom was practically complete in Stage C, the last trace of 
connection being shown in fig. 86. The right posterior ceelom 
is entirely separated from the anterior ccelom, but, strange to 
say, the septum between the left posterior celom and the ante- 
rior coelom has become broken down in two places. This occurs 
by the two layers of epithelium of which it is composed fusing, 
and then thinning out to a film. Of these two secondary com- 
munications between the two sacs, one is situated dorsal to the 
left hydroceele (Pl. 20, fig. 42), and the other ventral to it 
(Pl. 19, fig. 41). Figs. 42 and 43 belong to the same series ; 
we see that the dorsal opening is formed before the separation 
of the right posterior coelom is complete ; the ventral opening 
is formed at the same time. Not having had the opportunity 
when I wrote my preliminary account (15) of observing younger 
larvee than these, I imagined that the segmentation of the 
ceelom of the left side was incomplete ab initio, a mistake 
which was the more excusable as both the breaches in the 
