Freshwater Bryozoa, with descriptions of new Species. 197 
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 Ascidi@, which is also ex- 
ternal to the lamine. 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 Ascidie: in the 
former it is a cerebral ganglion resting on the cesophagus imme- 
diately behind the mouth ; in the latter, if it has relationship 
to any of the nervous centres of the Mollusca, it is apparently 
analogous 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 Ascidia ; 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. 
They are not, however, so closely related as might be supposed. 
At first sight an Ascidian undoubtedly seems very closely to ap- 
proximate to a bivalve shell; but this similarity on careful 
investigation would appear to be more that of analogy than 
homology—a mere resemblance rather than a true relationship. 
The branchial sac of the Ascidian is frequently assumed to be 
the same organ as the gill-plates of the Lamellibranchiata some- 
what modified ;—in function there is no difference; but anato- 
mically they are distinct. The former is a development from the 
alimentary canal; the latter, according to Professor Owen, “ are 
essentially internal folds of the pallial membrane.” The breathing 
organs then of these animals are not homologous. To turn 
therefore an Ascidian into a Lamellibranchiate mollusk, a new 
branchial organ must be developed. The vascular system, too, 
if not anatomically different in the Ascidian, is in a remarkable 
manner functionally so. In this the heart is at once systemic 
and pulmonic. And it is worthy of remark, that thus, on the 
first appearance of the vascular apparatus in this type of animals, 
it should shadow forth the peculiarities of both the molluscan 
and the piscine heart ; and this, too, in connexion with a pha- 
ryngeal gill. The test or outer sac, and the inner sac or tunic 
of the Tunicata are not related to each other in the same manner 
as the shell and mantle of the mollusk. In this the shell is 
extra-vascular, and is secreted by the mantle; in the former the 
test is vascular, and its growth is therefore not dependent on that 
of the inner sac or tunic. The reproductive system of the La- 
mellibranchiata is likewise very different from that of the Tunicata. 
In these it is formed on the type of the Radiata; another and 
very striking proof of the relationship that exists between the 
former and the Bryozoa. These and other points of difference 
