STUDIES IN THE EMBRYOLOGY OE ECHINODERMS. 429 
In 0 phi u rids it is formed from the posterior enterocoel. 
In Echinids probably from the anterior enterocoel. 
In Asterids (Bipinnaria) from the anterior enterocoel; the 
case of Asterina is easily reducible to this. 
In Crinoids it and the anterior enterocoel come off together 
from the gut, and then separate. 
In Holothurians it and the anterior enterocoel are not at 
first distinguishable from one another, and are always 
connected. 
From the fact that in the first three groups there is a period 
in which the hydrocoel does not yet exist, though the anterior 
enterocoel is already formed, we may assume that the former 
is of later phylogenetic origin ; so that in Crinoids and Holo- 
thurians, where the two cavities arise together, the hydrocoel 
may be regarded as a derivative of the anterior enterocoel. 
The frequency with which this origin of the hydrocoel occurs 
in ontogeny might seem to indicate that it was of phylogenetic 
significance, but it is not easy to see why, if so, there should 
be any departure from this condition in Ophiurids, which, in 
possessing two well-developed anterior enterocoels, seem some- 
what primitive. On the other hand, the variation in the 
ontogeny of the hydrocoel may mean that when it originally 
appeared the anterior and posterior enterocoels were connected, 
as they are in Asterina and some Bipinnarise, and that in 
separating them the other groups have adopted different 
methods of producing the hydrocoel ; but the fact that the 
Bipinnaria in which these two divisions of the coelom are 
separated is in other respects primitive, is opposed to this view. 
A slightly more satisfactory result is arrived at by regarding 
the condition found in Ophiurids as primitive. Then we must 
assume that, on account of its physiological importance to the 
free-swimming larva, an opening was formed at an early stage 
into the anterior enterocoel (before the water-tube could arise), 
and that this in time gave rise to the ontogenetic derivation 
of the hydrocoel from the anterior enterocoel, such as we find 
in all forms except Ophiurids. The fact that this communica- 
tion is not kept open in Crinoids till the formation of the 
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