BRANCHIAL SENSE ORGANS OP ICHTHYOPSIDA. 185 



of at least four segmental nerves, probably of more than four, viz. 

 vagus 2, 3, 4, 5. The fifth root is the rudiment of the nerve of the 

 rudimentary cleft mentioned before. 



We have seen that the facial, which is probably a compound nerve, 

 has a large forked supra-branchial branch, and we shall find that the 

 fifth and ciliary also, as already well known, have each a very long 

 supra-branchial nerve, extending over the snout (fig. 51, op.s. and 

 oph. 2^ro.), and hence we need not be much surprised that a supra- 

 branchial nerve, which is made up of the elements of at least four 

 supra-branchial branches, should grow right away to the tail, and 

 supply a very long series of branchial sense organs. 



In a former note 1 I put forward certain hypotheses concerning the 

 posterior roots of spinal nerves to account for the apparently abnormal 

 innervation by the vagus, that is by a cranial nerve complex, of a 

 region extending right to the tail. These hypotheses I now see reason 

 to reject, and after a study of the actual facts of development in Elas- 

 mobranchs, as now recorded, I can only conclude that the so-called 

 lateral line only differs in length and direction of growth from the 

 other branchial sense organs. Its length is sufficiently accounted for 

 by its containing the elements of at least four supra-branchial nerves, 

 and its direction offers in itself nothing really remarkable, for the 

 direction of growth of the other supra-branchial branches is not always 

 the same. Those of the fifth, seventh, and ciliary grow forwards ; those 

 of the glossopharyngeal and vagus I. grow dorso-anfceriorly, and that 

 of the rest of the vagus grows backwards (figs. 46 and 51). 



In fact, the direction of growth of sense organs and nerves would 

 seem to be determined by the usefulness or need of having branchial 

 sense organs in regions of the body other than the region just above 

 the gill-clefts where they primitively occur. 



Judging by the great variations one meets with in the arrangement 

 of these branchial sense organs in Ichthyopsida it would seem as 

 though different families of fishes and Amphibians had independently 

 solved the matter for themselves. The great morphological point to be 

 noticed, and I shall lay great stress on it later, is that at first there is 

 the rudiment of one branchial sense organ with its associated ganglion 

 over each gill-cleft or over the site of a potential gill-cleft. 



With reference to the hypotheses about spinal nerves mentioned 

 above, I may here state that I see no reason now for assuming that 

 1 Beard, •' On Segmental Sense Organs, &c." ' Zool. Anz.,' 161, 162, 1881. 



