196 Mr. A. Hancock on the Anatomy of the 
which in C/avelina is frequently of great length. Why then 
should the entrance to it be considered to correspond to the 
cardia in Bryozoa? These as well as the Ascidie have a well- 
marked stomach with cardiac and pyloric orifices ; in both, too, 
there is a distinct cesophagus ; then should not the orifice leading 
to it be assumed to be the mouth, or analogous to the mouth in 
both? In the polype a series of respiratory tentacles, in the 
Ascidie the branchial sac, surrounds this mouth ; should not these 
then be considered homologous? The affirmative of this would 
appear to be the natural inference in the first instance. But we 
are referred to the tentacular filaments at the entrance of the 
respiratory sac as the true representatives of the tentacles of the 
polype. With the view to ascertain how far this is correct, I 
examined with much care Ascidia sordida and Molgula arenosa, 
and found that these tentacular filaments are not anatomically 
connected with the branchial sac, but are developments from the 
tunic. The sac terminates a little way below these filaments, and 
they fringe the inner circumference of the belt of sphincter 
muscles which guard the respiratory orifice. These tentacular 
filaments, then, originating in the tunic, cannot possibly be the 
homologue of the tentacles of the polype, as these undoubtedly 
belong to the alimentary canal; but are in fact a new develop- 
ment in connexion with the sphineter of the tunic, and share its 
function. The tentacles then of the polype and the branchial 
sac of the Ascidian would appear to be homologous ;—unless 
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 respiratary 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 lave 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 Tunicata. 
Taking this view of the homology of the parts, the longitudinal 
lamin in the interior of the branchial sac of the Ascidie will 
