THE HEAD CAPSULE. 109 



head capsule, our investigation must not be limited to a mere 

 inspection of the sclerites of which it is composed, or to a 

 study of the appendages which arise from it. Just as the 

 parts of the skull of a vertebrate have a definite relation to the 

 enclosed nerve centres, so the parts of the head capsule have 

 similar relations to the nerve centres of the insect. 



It was formerly supposed, and the idea originated in the 

 mind of Dohrn,* that the neural surface of a vertebrate and an 

 insect correspond ; in other words, that the ventral surface of 

 an insect represents the dorsal surface of a vertebrate. Such 

 an hypothesis is, however, clearly untenable in the light of 

 modern discoveries, and may be said to be practically defunct. 



With regard to the nature of the pre-oral nerve centres, they 

 were, and still are, regarded by many, as similar to the post- 

 oral neuromeres, or paired ventral ganglia a view which I 

 regard as equally untenable, and reject with Dohrn's hypothesis 

 and the theory that the head capsule consists of several pre-oral 

 metameres. 



The older writers on insect anatomy recognised strong 

 relationships between the arthropod and the vertebrate brain. 

 Even as lately as 1864, Leydig [42, p. 185] pointed out that 

 in both classes of animals the brain consists of special paired 

 ganglia, arranged in three groups the fore-brain, the mid-brain, 

 and the hind-brain. To the fore-brain he attributed the function 

 of volition, to the mid-brain visual, and to the hind-brain 

 co-ordinating functions. Faivre stated that in Dytiscus these 

 co-ordinating functions are located in the infra-cesophageal 

 ganglia. Leydig included the infra-ossophageal ganglia, and 

 regarded them as the representatives of the hind-brain. 



In my Presidential address, delivered at the Annual General 

 Meeting of the Quekett Microscopical Club, February 22, 1889, 

 I made the following statements : 



' The theory of segmentation was formerly applied to the 

 vertebrate skull, and originated in the brains of Oken and 

 Goethe. Professor Huxley, in his lectures on the vertebrate 

 skull, published in London in 1864, disposed for ever, I believe, 



* ' Ursprung der Wirbelthiere,' Leipzig, 1875. 



