396 E. W. MACBRIDE. 
which still persist in the adult as sharply separated cavities, 
only the most posterior pair, viz. the right and left posterior 
celoms (oral and aboral) of the adult having partially fused 
with each other. The development of Antedon seems to answer 
this question in the affirmative. In its case the hydroceele is 
budded off quite independently of the posterior ccelomic sacs. 
Adopting, then, the view that the ccelomie sacs of the Ente- 
ropneusta and Asterids correspond, we find that the hydrocele 
represents the collar cavity. Now in Cephalodiscus the collar 
cavities are produced into long pinnately branched tentacles, 
comparable to the radial water-vascular canals, and further a 
branch from the central nervous system accompanies each 
tentacle, just as the radial nerves accompany the radial canals 
in Echinoderms. Now, if we suppose that the two hydroceles 
of Asterina were equally developed and approximated in the 
mid-dorsal line, the fusion of the anterior portion of the two 
nerve “‘rings,”’ which of course would in this case be only open 
curves (since a ring-form is attained through the preponderating 
srowth of one side) would give rise to a mid-dorsal nervous 
system like that of Cephalodiscus. Nor is that all; Professor 
Spengel (21) has shown in his monograph of the Enteropneusta 
that the currents in the proboscis-pore and collar-pore are 
inwards, and that by this means the animal inflates the proboscis 
and collar so as to render them efficient locomotor organs. 
We have seen that the function of the stone-canal is a similar 
one. 
We conclude, then, that the free-swimming ancestor of 
Echinoderms, for which we may adopt the name Dipleurula, 
and the Tornaria ancestor of Balanoglossus, were closely allied, 
This involves the assumption that they were allied to the Pro- 
tochordata, for, as I have elsewhere stated (16), Professor 
Spengel’s attempt to refute the Chordate affinities of Balano- 
glossus has been, in my opinion, futile. Although it may seem 
somewhat fanciful, I cannot help seeing hints of Vertebrate 
peculiarities in the anatomy of Echinoderms. Where else 
among all animals of higher grade than the Coelenterates do 
we find calcareous ossicles in the dermis? Where else 
