58 BRITISH TTJNICATA. 



In this interesting form,, in which the embryonic characters 

 have become permanent, we see the oral lamina still adhering 

 to the wall of the respiratory cavity, as well as the eiidostyle 

 and anterior collar or ciliated band. All these parts, then, 

 appear to be equally developments of the lining membrane ; 

 and the gill, which in the higher forms has been described 

 to originate in the oral band, or rather in the great ventral 

 channel, which always accompanies it, must likewise be con- 

 sidered a production of this same membrane, with which, too, 

 we have seen that it is connected throughout its development. 



THE RELATIONSHIP OF THE TUNICATA WITH THE POLYZOA AND 



LAMELLIBRANCHIATA. 



It is not my intention, on the present occasion, to enter at 

 any length on the relation that subsists between the Tunicata 

 and the Polyzoa on the one hand, and the Tunicata and the 

 Lamellibranchiata on the other. Nevertheless it seems de- 

 sirable to say a few words on this important branch of the 

 subject before concluding, with the view merely of indicating 

 what appears to be the result, in this respect, of my recent 

 investigations. 



That the Polyzoa are very closely allied to the Tunicata is 

 now generally admitted ; opinion, however, is divided respect- 

 ing the homology of the tentacular crown some naturalists 

 maintaining that it is represented by the branchial sac, while 

 others believe that it is homologous with the tentacles of the 

 respiratory tube, and that the branchial sac is really the 

 dilated pharynx of the polyzoon. These two views have been 

 ably advocated respectively by Prof. Allman and Prof. 

 Huxley. 



In my paper on the " Freshwater Bryozoa/' before cited, 

 the opinion that the branchial sac is homologous with the ten- 

 tacular crown was maintained ; but my belief in this view has 

 been of late much shaken, and even Prof. Allman's ingenious 

 explanation of his theory seems to me 110 longer satisfactory. 

 The peculiar idea entertained by this able physiologist is, that 

 the lophophore of a Hippocrepian Polyzoon is the homologue 

 of the ventral branchial channel of the Ascidian, and that the 

 tentacles of the former correspond to the transverse or primary 

 vessels of the branchial sac. But the lophophore is an appen- 

 dage of the mouth, and is developed from the margin of the 

 oral orifice, and therefore can scarcely be considered to be 

 the true representative of the branchial channel, which does 



