44? THE XERVOl'S SYSTEM. 



supra-cesophageal centres in the embryo Blow- fly, as according 

 to Gaskell's view no such vesicles should exist. Nevertheless 

 the existence of these vesicles and the general arrangement 

 and characters of the nerve centres of Arthropods indicate 

 analogies and, perhaps, homologies with the central nervous 

 system of a Vertebrate, which cannot be lightly disregarded. 

 It lias long been known that three pairs of great ganglia exist 

 in the supra-cesophageal centres of insects ; the antennal 

 ganglia ; the central ganglia, of which the optic ganglia have 

 been regarded as mere lateral off-shoots ; and the ganglia of the 

 crura, developed in relation with the cesophageal connectives, 

 between the supra-cesophageal and the metameral infra- 

 cesophageal ganglia. Yiallanes [185] includes the central and 

 optic ganglia under the term protocerebron, whilst he terms 

 the antennal ganglia the deutero- and the crural ganglia the 

 tritocerebron. 



There are good reasons, I think, for discarding this nomen- 

 clature. The antennal ganglia and the ganglia of the crura 

 are merely special structures connected with the roots of the 

 antennal and pharyngeal nerves, whilst Viallanes' protocerebron 

 includes not only the optic ganglia, which are, perhaps, com- 

 parable with the antennal ganglia, in the same sense that the 

 olfactory bulb and the ganglionic retina of Vertebrates may 

 be compared ; but also all the complex structures of the brain 

 proper, which I shall show exhibits three distinct paired groups 

 of nerve centres. 



In order to understand the insect's brain it is necessary to 

 compare it with the brain of the more generalised crustacean 

 type. This has been attempted by Viallanes, but, although 

 in many points his work is accurate, his knowledge of the 

 Crustacean brain is so imperfect that he has entirely over- 

 looked many important facts, and has used his knowledge 

 rather for the purpose of establishing his theory that the insect's 

 brain is composed of a proto-, deutero- and tritocerebron, than 

 for the purpose of attempting a real elucidation of the struc- 

 ture of the Arthropod brain. 



E. Yung, [179J, whilst he recognised a general similarity 



