TEB VERRATES FROM A CRUSTACEAN- LIKE ANCESTOR. 439 



appearance of this peculiar Invertebrate tissue. I will not, 

 however, dwell further here upon the question of the olfactory 

 nerves, as I shall deal with them and the rest of the cranial 

 nerves in my next paper. 



Although I have dealt with the nervous system, I have 

 purposely omitted the consideration of the pituitary body 

 because it does not in reality belong to the central nervous 

 system; it represents, in my opinion, the Crustacean green 

 glands, and will be considered when I come to deal with the 

 formation of the Vertebrate skeletal tissues, and of the 

 excretory organs. 



In the next chapter I propose to point out how the present 

 alimentary canal arose; and as it is perhaps advisable to make 

 a preliminary communication in this paper, I will say now 

 that the view, the truth of which I intend to discuss, is as 

 follows : 



1. The alimentary canal is formed by the prolongation of a 

 respiratory chamber. 



2. The respiratory chamber contains the gill-bearing legs of 

 the Crustacean-like ancestor, which are still present in the 

 Ammoccetes as the bars bearing branchiae, and still retain their 

 Invertebrate type of muscles. 



3. The first pair of such appendages do not bear branchise ; 

 they form the so-called velum, and are simply the foremost 

 pair of legs of the Crustacean -like ancestor. 



4. The segmental cranial nerves are the nerves which arise 

 from the infra-cesophageal and thoracic ganglia, and supply the 

 foremost appendages up to and inclusive of the gill-bearing legs. 



5. The first two cranial nerves are the nerves of special 

 sense arising from the supra-oesophageal ganglia. 



Literature referred to. 



1. Gaskell. — " On the Relation between the Structure, Function, Distribu- 

 tion, and Origin of the Cranial Nerves ; together witii a Theory 

 of the Origin of the Nervous System of Vertebrata," ' Journ. of 

 Physiol.,' vol. x, p. 153. 



