VERTEBRATES FROM A CRUSTAOEAN-LIKE ANCESTOR. 379 



On the Origin of Vertebrates from a Crustacean- 

 like Ancestor. 



By 

 "W, H. Gaskell, M.I>., F.R.S. 



With Plates XXV, XXVI, XXVII, XXVIII. 



Chap. I. — The Evidence given by the Central Nervous 

 System and Pineal Eyes of the Ammococtes of Petro- 



MYZON PlANERI. 



In a former paper (1) I have described how ray investiga- 

 tions into the Vertebrate nervous system have led me to the 

 conclusion that that system is composed of nervous material 

 grouped around a central tube which was originally the alimen- 

 tary canal of the Invertebrate from which the Vertebrate arose ; 

 and in a second paper (2) I have suggested that the physiology 

 and anatomy of the Vertebrate nervous system both fit in best 

 with the assumption that the Invertebrate ancestor was of the 

 Crustacean^ type. In both papers I have promised to point 

 out the confirmation of this theory which is afforded by the 

 study of the lowest Vertebrate nervous system^ viz. the 

 Ammocoetes form of Petromyzon. This promise I propose 

 now to redeem, and in order to bring out as prominently as 

 possible the theory which 1 hold to be true I will discuss the 

 nervous system of the Ammocoetes in terms of the Crustacean; 



' By Crustacean I include such a form as Limulus, without intending to 

 pronounce any opinion on the question whether Limulus is an Arachnid or a 

 Crustacean ; it would, perhaps, be better to use the term proto-Crustacean, 

 meaning thereby a form from which both Crustaceans and Arachnids may 

 have been derived. 



