396 MR, HANCOCK ON THE ANATOMY OF THE FRESHWATER 
indeed the tentacles of the one have died out, and the branchial 
sac of the other is altogether a new development, which is not by 
any means likely. 
In confirmation that the former is the fact, we have only to 
look at the growing bud of the polype. which so closely resembles 
an Ascidian, particularly when young, that it might at first sight 
be taken to be one. The tentacles at this time, all lying parallel 
to each other within the membranous sheath, have quite the ap- 
pearance of a branchial sac; and when we take in connexion 
with it the alimentary canal, the resemblance is almost complete. 
Indeed, all that is wanting to turn the polype into an Ascidian, 
so far as the alimentary and respiratory organs are concerned, is 
the union of the tentacles by a vascular membrane. And we 
have already seen that such an union has commenced in Frede- 
ricella, Plumatella, and Alcyonella. We have seen that in all 
these genera the tentacles are united at the base by a delicate 
membrane; and in the former this membrane is so extensive as 
to suggest the idea of a rudimentary form of the branchial sac of 
the Z'unicata. 
Taking this view of the homology of the parts, the longitudinal 
lamin in the interior of the branchial sac of the Ascidic will 
represent the tentacles of the Bryozoa; and the membrane at the 
base of the tentacles, being external, corresponds exactly in posi- 
tion to the vascular membrane of the Ascidicw, which is also ex- 
ternal to the lamin. ‘The position of the nervous ganglion in 
the two forms might at first sight appear to favour the contrary 
opinion; but, on closer inquiry, it is evident that the ganglion of 
the Bryozoa is not homologous with that of the Ascidiew: in the 
former it is a cerebral ganglion resting on the esophagis imme- 
diately behind the mouth; in the latter, if it has relationship to 
any of the nervous centres of the JJollusca, it is apparently ana- 
logous to the branchial ganglion of the Lamellibranchiata; but 
its position in the mantle is anomalous. 
We thus see how very intimate is the connexion between the 
Bryozoa and the Ascidie; and as the latter are generally sup- 
posed to be as closely connected with the Lamellibranchiata, no 
great distance would appear to divide them from the former. 
