Freshwater Bryozoa, with descriptions of new Species. 1 97 



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 Ascidia, which is also ex- 

 ternal to the laminae. 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 Ascidice : in the 

 former it is a cerebral ganglion resting on the oesophagus 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 Ascidice ; and as the latter are generally sup- 

 posed to be as closely connected with the Lamellibranchiataj 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 



