430 
H. BURY. 
water-tube, might be explained by their having compara- 
tively recently acquired a large amount of food-yolk, and 
passing a very short free-swimming existence. But this is 
merely a suggestion, and it is difficult to understand why the 
pelagic larvae of Ophiurids should have been able to do with- 
out the early connection of the hydrocoel with the anterior 
enterocoel, supposed to be of such importance to other larvae ; 
or why this connection should not have been made to coincide 
with the water-tube (stone-canal) in Asterids as it does in 
Echinids and Holothurians. Possibly the true explanation is, 
that the hydrocoel originally arose in some more complicated 
way, which has since been simplified, independently, by each 
of the different groups, and is no longer repeated in the 
ontogeny of any one of them. 
(2) Connection with the Anterior Enterocoel. — The 
hydrocoel never has an external pore of its own, but always at 
some time opens into the anterior enterocoel, and so forms an 
indirect communication with the exterior ; there are two ways 
in which this communication may be established : 
(a) In those cases in which the hydrocoel is derived from the 
anterior enterocoel, there is, of course, a communication from 
the first. 
(b) At some period or other, but usually late, a water-tube 
(stone-canal) is formed as an outgrowth from the hydrocoel ; it 
has a columnar ciliated epithelium. 
These communications (a and b) coincide in Holothurians, 
and probably in Echinids ; but even in these groups the forma- 
tion of a definite columnar epithelium in the water-tube occurs 
somewhat late — after the appearance of the primary tentacles 
in Holothurians (compare figs. 7 and 27). 
In Asterids the two communications coexist but do not 
coincide, being in different interradii. The water-tube is formed 
after the pouching of the hydrocoel. 
In Crinoids (Antedon) the primary communication (a) closes 
too early for its exact position to be determined. The water- 
tube appears after the primary tentacles. 
In Ophiurids the connection (a) never exists at all 
