Morplioluf^ical Studies. 745 



These points stand now in a very different position to that in 

 which they were when Balfour died. In the fifth of his „Studien" (5, 

 p. 52 et seq.) Dohrn has shewn that in Petromyzon each branchial bar 

 arises as one piece and never, as in Selachians, becomes segmented 

 into several pieces, and he concludes that the latter condition is secon- 

 dary, and that there can be no doubt that the single bars of Petromyzon 

 are homologous with the inner branchial skeleton of Selachians. 



With his demonstration of the true nature of the branchial skele- 

 ton of Petromyzon, Dohrn has removed one of the main supports of 

 Balfour's conclusions, for Balfour's view of the nature of the bran- 

 chial skeleton — orginally enunciated by Rathke and supported by 

 Gegenbaur — was employed as an additional argument as to the 

 character of the Cyclostome mouth. 



We can here leave the presence or absence of limbs out of 

 question, remembering, none the less, that Dohrn has produced some 

 evidence for their former existence in the Cyclostomata in the presence 

 of the „Kuppel-Zellen" of Ammocoetes (6, p. 405). After all the main 

 point is the nature of the mouth. 



Balfour's argument in this matter has been disposed of by Dohrn, 

 so far as the a priori question is involved, but of course the problem 

 is not settled by that refutation. 



Balfour was of opinion that a sucking mouth was a very primi- 

 tive character. To this view he inclined because a sucking mouth 

 (more correctly a mouth with an intrinsic or extrinsic sucking appa- 

 ratus) is to be found in Lepidosteus (larvae), Cyclostomata, and some 

 larval Amphibians. 



On the other hand, Dohrn has pointed out that the sucking appa- 

 ratus is derived from very différent structures in the three cases, and 

 has promised a special Study in which the suckers shall be investigated, 

 and referred to their proper origin in the différent cases. 



Now, it appears almost certain that the suckers of Lepidosteus i 

 and the larval Amphibians are homologous structures, but derived from ; 5 

 the mucous sacs of very different segments. — The one of a segment ' ; 

 or segments in front of the mouth, the other of a segment behind the 

 mouth. I mean here mucous sacs homologous with the segmental ; j 

 sacs of Myxine. 



Eisig, very rightly, sees in these latter structures very primitive 

 structures, corresponding to the spinning glands of Annelids. He hints | 

 that he and Dohrn are of opinion that in other fishes the mucous ^ 

 organs are bound up with the lateral sense organs (7, p. 420). 



