426 
H. BUBY. 
real water-pore is always median or even somewhat to the 
right, except in the very earliest stages, but it is certainly not 
easy to see in the opaque Spatangid Plutei. What Fewkes 
describes as a movement of the pore is a growth of the water- 
tube (stone-canal) to join the pore-canal. 
Asterids. — In this group the primary opening of the 
hydroccel into the anterior enterocoel continues to exist after 
the former has acquired its five primary tentacular lobes; but 
it does not, as in Echinids, give rise directly to the water-tube. 
The latter arises after the appearance of the tentacular pouches, 
and in the small Bipinnaria above described, in which the 
anterior and posterior enterocoel are distinct, runs close to the 
surface of the stomach in the mesentery separating these two 
cavities, and opens into the anterior enterocoel beside the water- 
pore : it occupies a correspondiug position in other Bipinnariae 
and in Asterina, in which the mesentery in question is in- 
complete. 
Not only is the primary connection of the hydroccel with 
the anterior enterocoel distinct from the water-tube, but they 
are in different interradii ; this has been proved by Ludwig 
for Asterina, and is apparently true also of Bipinnaria, though 
I cannot assert this with any confidence. 
Crinoids. — The hydrocoel forms a ring (long, incomplete) 
through which the oesophagus grows. The water-tube (stone- 
canal) starts from one end of this incomplete ring, and opens 
into the anterior enterocoel (7, p. 21). It is, of course, im- 
possible to say whether this new opening is in the same position 
as the primary one, since the latter is closed before the forma- 
tion of the tentacular lobes. 
Holothurians. — In Auricularia the primary opening be- 
tween the anterior enterocoel and the hydrocoel persists as the 
water-tube (stone-canal), but instead of remaining short, as in 
figs. 22 and 24, it elongates rapidly just before metamorphosis 
into the “Pupa,” and forms a tube with columnar epithelium. 
At the same time the cells forming the wall of the anterior 
enterocoel become rounded and increase in number, at the 
expense of the cavity, so as to form a bunch of cells, which 
