BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 211 



neural parapodia. That they had nothing to do with the gill-clefts 

 themselves is pretty certain. 



Nature of the Mouth. 



A few words may be here said on the bearing of these researches on 

 the nature of the mouth. 



Dohrn 1 first suggested that the mouth was primitively a pair of gill- 

 clefts, which have coalesced and come to open mesially. He after- 

 wards showed 2 that it arises in Teleostei as two lateral depressions just 

 like gill-clefts. In the preceding pages I have shown that in Elasmo- 

 branchs there is a primitive branchial sense organ over the angle of the 

 mouth, and with this sense organ an associated ganglion, the Gasserian ; 

 and also that, just as in the nerves of other gill-clefts, a supra-branchial 

 nerve is afterwards developed from this gauglion in connection with 

 the extension of the branchial sense organs of the mouth cleft. I need 

 hardly say that I see in these facts a strong additional support for 

 Dohrn's view. 



Segmentation of the Head. 



Admittedly this is one of the most difficult problems in Vertebrate 

 morphology, and I cannot flatter myself that I am nearer a solution of 

 it than other zoologists. But it may be remarked that the tendency 

 of recent researches has been to increase the number of segments 

 recognisable in the Vertebrate head. In ordinary sharks with five 

 true gill-clefts, Marshall and Van Wijhe recognised nine segments, but 

 Van Wijhe rejected Marshall's olfactory segment, and Marshall did not 

 regard the hyoid as composed of two segments. I should increase the 

 number to at least eleven in sharks, with four roots to the vagus, and 

 apparently Dohrn would agree with this number, but his segments 

 might not be quite the same. 



Indeed, at present it is impossible to solve the problem with any 

 degree of probability, and it is a question whether it ever will be solved. 

 Hence the following table is only a tentative one, and is only meant 

 to give a general view of the results of the researches recorded here. 

 In passing I may remark that Dohrn's recent criticism of Ahlborn's 3 



1 Dohrn, * Ursprnng der Wirbelthiere.' 



2 Dohrn, "Studien, &c," 'Mittheil. a. d. Zool. Station zu Neapel.' £d. iii I. "Der 

 Mund der Knochenfische." 



3 Ahlborn, " Ueher die Segmentation des Wirbelthiei>KSrpers," * Zeit. fiir wiss. Zool 

 Bd. xl., p. 309. 



