414 
H. BURY. 
means that they have never become entirely separated. 
However this may be, in considering the difference between 
Metschnikoff’s account and nomenclature and mine, it is im- 
portant to notice that on the right side of the oesophagus there 
still exists a cavity exactly comparable to that which I have 
called the left anterior enterocoel (except that it has no pore), 
whereas the structure to which I have here confined the term 
hydrocoel is entirely unrepresented on the right side. 
Asterids. — In the young Bipinnaria we find beside the 
oesophagus a pair of peritoneal vesicles, one of which early 
opens to the exterior by a pore situated at its posterior end. 
Both vesicles soon extend back to the stomach, over which 
these posterior lobes spread dorsallv and ventrally, while the 
anterior lobes, lying beside the oesophagus, have no such dorso- 
ventral extension; it is at the junction of the anterior and 
posterior lobes on the left side that the water-pore lies. It is 
clear that we have here the representatives of the anterior and 
posterior enterocoels of Ophiurids and Echinids, though they 
are not as a rule separated from one another. 
In many forms of Bipinnaria and in Brachiolaria the two 
anterior lobes of the enterocoels grow forwards and unite in 
front of the mouth, and are then continued as a common cavity 
into the large prseoral lobe ; but they are always separate in 
young larvse, and in certain forms of Bipinnaria described by 
J. Muller (21) and Metschnikoff (18, pp. 32 — 40) they never 
unite at all. I obtained a few examples of such a form at 
Naples, and they are so instructive that it will be well to 
describe their anatomy in some detail. 
In fig. 14 one of these larvae is represented as seen from the 
dorsal side. The hydrocoel is already present as a pouch 
opening into the left anterior enterocoel, though its exact mode 
of origin was not traced. But the great peculiarity of this 
larva lies in the fact that on the left side the anterior and 
posterior enterocoels are entirely separated from one 
another. In Asterina (17) there is a dorsal communication 
of these two cavities just above the hydrocoel and just behind 
the pore, and a ventral communication just below the hydrocoel; 
