ANATOMY AND PHYSIOLOGY. 59 



not seem to be so related, but appears rather to be developed 

 in connexion with the lining membrane coating the pallial 

 cavity, and has all the appearance of a true vessel in direct 

 communication with the heart. And there are other difficul- 

 ties in the details of this view, to which it is unnecessary, 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 

 Molluscan 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, nevertheless, 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 corroborative facts in its 

 favour : more information is still required before this matter 

 can be determined satisfactorily. The interpretation alluded 

 to is, that the branchial sac is a new and distinct develop- 

 ment, as the endostyle is, and as are the oral lamina, the 

 branchial tubercle, and the tentacular filaments of the inhalant 

 tube, and that all these organs have equally their origin in 

 the lining membrane or inner tunic of Huxley, and have no 

 homological representatives in the Polyzoa. 



And, further, this interpretation Of the facts leads to a belief 

 that the branchial sac is the rudiment of the Lamellibranchiate 

 gill, the structure of the two organs being essentially the 

 same. The principal blood-channels in the gills of the 

 Lamellibranch are simple transverse vessels ; and the most 

 persistent and essential parts in the structure of the branchial 

 sac of the Tunicates are the transverse or primary vessels. 

 Thus, fundamentally, these organs are similar. And when 

 the branchial sac is furnished with longitudinal folds, as 

 generally is the case in the Cynthiada?, the primary vessels as- 

 sume relatively the same position as their supposed homologues 

 do in the gill-plate. The folds, too, as the nature of the structure 

 implies, are formed of two laminte united at their distal mar- 

 gins, and have the space between them divided by septa into 

 transverse pouches, which only want to be elongated by the 

 further development of the fold to make them correspond in 

 every respect to the interbranchial water-tubes of the gill- 

 plate of the Lamellibranch. And already the pouches sub- 

 serve the function of water-tubes. 



