420 
H. BURT. 
anterior and posterior enterocoels of the left side are distinct, 
is, as already remarked, opposed to the idea that the other 
Asterids are primitive in this respect. If union of the cavities 
originally obtained (as is not improbable), there is strong 
reason for supposing that before the separation of the existing 
groups of Echinoderms, the segmentation of each lateral 
cavity into an anterior and a posterior part had already 
occurred. 
(2) The hydrocoel is generally formed distinctly later than 
the other cavities ; indeed, the only apparent exception to this 
is afforded by the Holothurians, and in these it is quite as 
reasonable to consider the anterior vesicle to be an anterior 
enterocoel as to follow previous writers in regarding it as the 
hydrocoel. This being so, it seems probable that the hydrocoel 
is of later phylogenetic origin than the enterocoels. The entire 
absence of any trace of a right hydrocoel makes it improbable 
that this organ was ever paired ; but we must not lay too 
much stress on this evidence, seeing that in Holothurians and 
Crinoids the right anterior enterocoel has entirely disappeared. 
In its mode of origin the hydrocoel varies, but its normal posi- 
tion, when formed, seems to be between the anterior and 
posterior enterocoels, not separated by either of them from the 
wall of the stomach. 
(3) The water-pore always (with the possible exception of 
Holothurians) arises in connection with the posterior end of 
the left anterior enterocoel, and only communicates indirectly, 
if at all, with the hydrocoel. In all pelagic larvae it ap- 
pears exceedingly early — probably always before the hydro- 
coel (except, perhaps, in Holothurians). In Asterina, on the 
other hand, it is formed later, simultaneously with the hydrocoel ; 
while in Antedon it does not make its appearance till after the 
hydrocoel has become five-lobed, and the larva has escaped from 
the vitelline membrane. It might be thought from this that 
the water-pore was really a late development, which had become 
precociously formed in pelagic larvae on account of its physio- 
logical importance, and that its real time of formation was 
about the same as, or later than, that of the hydrocoel. This 
