AND PHYSIOLOGY OF THE TUNICATA. 343 



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

 Lamellibranchiata on the other. Nevertheless it seems desira- 

 ble 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 respecting 

 the homology of the tentacular crown — some naturalists main- 

 taining 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 re- 

 spectively 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 tentacular 

 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 no 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 appendage of the mouth, 

 and is developed from the margin of the oral orifice, and there- 

 fore can scarcely be considered to be the true representative of 

 the branchial channel, which does not seem to be so related, but 

 appears rather to be developed in connexion with the lining mem- 

 brane coating the pallial cavity, and has all the appearance of a 

 true vessel in direct communication with the heai't. And there 

 are other difficulties in the details of this view, to which it is 

 imnecessary, at this moment, to make further allusion. 



The view so forcibly advocated by Prof. Huxley seems to rest 

 more upon a wide and philosophical generalization of Molluscau 

 organization than on anatomical and embryological data, and- is 

 therefore difficult to discuss from a standpoint of the details of 

 such matters. It must, however, be stated that the anatomical 

 facts, so far as I have been able to examine them, do not seem 

 to contradict this hypothetical view ; indeed, in many respects, 

 they appear rather to support it. The anatomical data, never- 

 theless, will, I think, bear another interpretation, which, perhaps, 

 it will be well to consider, merely premising that I have no wish 

 to support it further than as a suggestion which has a few cor- 



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