DEVELOPMENT OF ELASMOBKANCH FISHES. 215 



From this enumeration of the nerves the optic nerve is 

 excluded for obvious reasons, and although it has been shewn 

 above that the olfactory nerve developes like the other nerves 

 as an outgrowth from the brain, yet its very late appearance 

 and peculiar relations are, at least for the present, to my mind 

 sufficient grounds for excluding it from the category of seg- 

 mental cranial nerves. 



The nerves then give us indications of seven cranial seg- 

 ments, or, if the nerves to the eye-muscles be included, of at the 

 least eight segments, but to these must be added a number 

 of segments now lost, but -which once existed behind the last 

 of those at present remaining. 



The branchial clefts have been regarded as guides to seg- 

 mentation by Gegenbaur, Huxley, Semper, etc., and this view 

 cannot I think be controverted. In Scyllium there are six 

 clefts which give indications of seven segments, viz. the seg- 

 ments of the mandibular arch, hyoid arch, and of the five 

 branchial arches. If, following the views of Dr Dohrn^, we 

 regard the mouth as representing a cleft, we shall have seven 

 clefts and eight segments ; and it is possible, as pointed out in 

 Dr Dohrn's very suggestive pamphlet, that remnants of a still 

 greater number of praeoral clefts may still be in existence. 

 Whatever may be the value of these speculations, such forms 

 as Hexanchus and Heptanchus and Amphioxus make it all 

 but certain that the ancestors of Vertebrates had a number of 

 clefts behind those now developed. 



The last group of organs to be dealt with for our present 

 question is that of the Head-Cavities. 



The walls of the spaces formed by cephalic prolongations of 

 the body-cavity develope into muscles and resemble the muscle- 

 plates of the trunk, and with these they must be identified, 

 as has been already stated. As equivalent to the muscle-plates, 

 they clearly are capable of serving as very valuable guides for 

 determining the segmentation of the head. There are then a 

 pair of these in front of the mandibular arch, a pair in the 

 mandibular arch, and a pair in each succeeding arch. In all 

 there are eight pairs of these cavities representing eight seg- 

 ments, the first of them prseoral. As was mentioned above, 

 each of the sections of the head-cavity (except perhaps the 



1 Ursprung d. Wirbelthicre. 



