438 DEVELOPMENT OF ELASMOBRANCH FISHES. 



complex nerve resulting from the coalescence of two or three 

 distinct nerves. 'My observations do not embrace the origin or 

 history of the third, fourth, and sixth nerves, but it is hardly 

 possible to help suspecting that in these we have the nerve of 

 one or more segments in front of that supplied by the fifth 

 nerve ; a view which well accords with the most recent morpho- 

 logical speculations of Professor Huxley 1 . 



From this enumeration of the nerves the optic nerve is ex- 

 cluded 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 2 , 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 prseoral 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 the cephalic prolongations 



1 Preliminary note upon the brain and skull of Amphioxus, Proc. of the Royal 

 Society, Vol. XXII. 



2 Ur sprung d. Wirbelthiere. 



