BRYOZOA, WITH DESCRIPTIONS OF NEW SPECIES. 397 
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 ho» 
mology—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 breath- 
ing 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 Zunicata 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 
Lamellibranchiata is likewise very different from that of the 7’u- 
nicata. In these it is formed on the type of the Radiata ; ano- 
ther and very striking proof of the relationship that exists be- 
tween the former and the Bryozoa. These, and other points of 
difference, led Professor Milne-Edwards, in his valuable memoir 
on the ‘ Ascidiens Composées,’ to propose the separation of the 
Tunicata from the JZollusca, and the formation of them into a 
distinct group to be placed between the bivalves and the polypes. 
We then cannot find a passage from the Bryozoa through the 
Ascidice into the Lamellibranchiata. There are however two 
distinct branches of the Ascidian polypes,—one with the tentacles 
