418 
H. BURY. 
its component cells being hardly distinguishable from the 
surrounding mesoderm cells, which lie scattered over the 
surface of the hydrocoel, while other mesoderm cells (purposely 
omitted in. this figure) collected round the water-pore and the 
tube leading from it (“ pore canal ”) further obscure it. A 
lateral view (fig. 24) is far more satisfactory, but is by no 
means easy to obtain ; the only method known to me is to 
place the larva in a watch-glass and roll it over until it assumes 
the required position ; but of course it cannot then be kept 
perfectly steady, and drawing with the camera is impossible. 
To the later stages of development, in which this cavity is 
still easily traceable, I shall return presently. Enough has, I 
think, been said to show that in Auricularia a cavity is 
present between the hydrocoel and the water-pore, which, 
though usually rudimentary, we have reason to regard as the 
representative of the left anterior enterocoel. We shall sub- 
sequently see that it is also present in Cucumaria, though 
perhaps not at such an early stage. 
Summary and Conclusions. 
Ophiurids. — Here we find two pairs of enterocoels, meta- 
merically arranged. The anterior enterocoels retain the 
position of the primary pair of peritoneal vesicles, and one of 
them (the left) opens to the exterior at its posterior end by 
means of the water-pore. Besides these there is formed some- 
what later a hydrocoel, lying on the left side between the 
anterior and posterior enterocoels, and apparently derived 
from the latter ; at this stage it has no communication with 
the anterior enterocoel. 
E chin ids. — These have two pairs of enterocoels and a 
water-pore, as in Ophiurids ; the hydrocoel occupies the same 
position as in that group, but appears to arise from the 
anterior enterocoel, and to retain its communication with it. 
As ter ids. — The anterior and posterior enterocoels are dis- 
tinguishable on both sides, but are not usually separated, 
though they are so on the left side in one form. The water- 
pore and hydrocoel occupy their usual positions, but the latter 
