Morphological Studies. 747 



that particular structure to some other structure in the mouth of an 

 ordinary fish. 



Position of tlie Marsipobranchü. 



The question of the position of this group with regard to other 

 fishes is one which is not easy to settle definitely. Doubtless the 

 embryology of the Myxinoids would throw a good deal of light upon 

 the matter; for, while they are in some particulars more degenerate 

 than the Petromyzontidae , I think one must support Huxley as 

 against Dohrn in his conclusion that the former group are on the 

 whole the more primitive. The organs, which in the Myxinoids are 

 more degenerate than in the Petromyzontidae, are the eyes and the 

 branchial apparatus. It is, of course, easy to understand why the 

 eyes have degenerated: a life passed within other fishes, and the 

 development of powerful muscles in the region of the visual organs 

 must exercise a degenerating influence in these sense organs. I may 

 remind the reader that Wiedersheim (24) has demonstrated that in 

 certain coral-eating fishes, Tetrodonts, a tremendous development of 

 biting muscles has sufficed to put the nose, wholly or partially, out of 

 existence. While accepting some degeneration in the Marsipobranchs 

 along with Dohrn, the truth of Balfour's view that they are very 

 primitive forms must also be allowed. 



The persistence of the hypophysis-gut passage is a very import- 

 ant factor in this conclusion: for it is not unlikely (2, p. 22) that 

 we have in this structure and its special nervous system a very old 

 organ: in fact, the old Annelidan mouth of the ancestors of Verte- 

 brates, 



The parietal eye of the Petromyzontidae is another point in which 

 the group shew more primitive conditions. In the segmentally arrang- t 

 ed slime sacs of Myxine we also have very ancient organs, which 

 again point towards the Annelida. Other primitive characters have 

 been enumerated by Balfour. - As I have already given his views and 

 the reasons for them, I need not repeat them. 



The group of the Marsipobranchii must undoubtedly be added to 

 the gnathostomatous Vertebrata. And the name Cyclostomata may 

 very fitly give place to that proposed by Buonaparte, and adopted 

 by Profs. Huxley and Parker: the Marsipobranchii. 



The Marsipobranchii stand between the Selachians and 



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