424 
H. BURY. 
it is important to notice that fig. 4 represents a stage in 
which the hydrocoel has already formed a ring round the oeso- 
phagus with five tentacular outgrowths, while the rest of the 
body still retains its bilateral form. 
Echinids. — After the stage represented in fig. 8 the hydro- 
coel pushes its way farther back (not forward, as in Ophiurids) 
until it comes to lie in the centre of the left side. In doing 
this it does not pass on the outside of the posterior enterocoel, 
but lies close to the wall of the stomach, while the posterior 
enterocoel forms a kind of horse-shoe round it, as represented 
in fig. 7. 
In this figure the hydrocoel already possesses five lobes, aud 
is itself curved into the form of a ring, incomplete towards the 
posterior end. The tube connecting it with the anterior en- 
teroccel is now much longer than before, and has acquired a 
columnar epithelium ; it is, as already mentioned, the water- 
tube (= stone-canal), and enters the hydrocoel ring anteriorly 
and slightly dorsally. Shortly after this the hydrocoel ring 
closes completely, leaving a central perforation (19) through 
which, at a much later period, the oesophagus grows. 
We must now return to the anterior enterocoel. It has 
usually been stated (1, p. 714 ; 18, p. 42 ; 14, p. 40 ; 9, p. 137) 
that the water-tube (= stone-canal) opens directly to the 
exterior by the water-pore, the hydrocoel being supposed to be 
formed either by a direct metamorphosis of the left oesophageal 
enterocoel (1 and 18), or as an outgrowth from the left posterior 
enterocoel (11, p. 609; 17, p. 141 ; 26, p. 49), but in his last 
note on the subject (19) MetschnikofF tells us that, though 
most of the cavity by the oesophagus goes to form the hydro- 
coel, a considerable portion forms a pulsating vesicle into which 
the pore at first opens. It might be supposed that this pul- 
sating vesicle was the anterior enterocoel of my account, but 
this does not appear to me to be the case. 
In fig. 9 I have given a view of the pore aud its surround- 
ings so far as I have been able to make them out. On the left 
is seen the water-tube (= stone-canal) coming up from the 
hydrocoel, and opening into a swollen portion of the anterior 
