380 W. H. GASKELL. 



in other words, I will take separately the prominent features 

 of the alimentary canal and central nervous system of a Crus- 

 tacean-like animal, and point out how each one exists in the 

 nervous system of the Ammocoetes. 



Before commencing the study of the special nervous system 

 treated of in this paper I will briefly recapitulate the general 

 arguments which I have put forward in my previous papers. 



The central nervous system of all Vertebrates consists of two 

 parts — (1) a nervous portion, and (2) a non-nervous epithe- 

 lial portion, which is in part free from admixture with nervous 

 elements, and partly helps to form the supporting tissue of the 

 nervous elements. This non-nervous part of the central ner- 

 vous system forms a canal around which the nervous material 

 is grouped, in the same manner as the nervous system of the 

 Crustacean is grouped around the alimentary canal. This 

 similarity of grouping is not merely anatomical, but is also 

 physiological; the functions of the supra-oesophageal ganglia 

 of the infra-oesophageal and of the ventral chain correspond to 

 the functions of those parts of the Vertebrate central nervous 

 system which are situated in the same anatomical position, 

 with respect to the non-nervous tube, as the corresponding 

 ganglia of the Crustacean with respect to the alimentary 

 canal. 



The natural conclusion to draw from this striking coincidence 

 is that this nou-nervous tube of the Vertebrate central nervous 

 system is the altered alimentary canal of the Crustacean ancestor 

 of the Vertebrate. 



If this conclusion is right, then not only should the evidence 

 upon which it is based come out more clearly when the lowest 

 Vertebrate nervous system is examined, but also fresh evidence 

 of Crustacean characteristics may reasonably be expected to be 

 found in such nervous system. We should expect, in fact, to 

 find a nearer approach to the Crustacean type than in the 

 higher Vertebrates. 



We should expect, therefore, to find that the vestiges of the 

 mouth, oesophagus, and cephalic stomach were more conspicuous 

 than in the higher forms ; that such an ancestral characteristic 



