AX ATOMY OF THE NERVE CENTRES. 441 



a very limited modification occurs even in widely distinct types. 

 I should judge that the brains of Arthropods scarcely differ 

 more than those of Fishes and Amphibians, if the lower forms 

 of Crustacea are excepted, and these, perhaps, do not differ 

 more from the common Arthropod type than that of Amphioxus 

 does from those of other fishes. 



The connective cords between the supra- and infra-cesophageal 

 ganglia differ greatly in length ; and the proportionate develop- 

 ment of the several parts is very dissimilar in widely different 

 groups of Arthropods. The transverse commissures of insects 

 exhibit a complexity unknown in the Crustacea ; but otherwise 

 there is so close a conformity of structure that, until a definite 

 nomenclature is adopted, nothing but confusion can exist. No 

 real advance of knowledge can be expected until someone is 

 bold enough to pass from a special to a general description which 

 may serve to connect facts otherwise so special and technical 

 that few can feel any interest in them. 



If the brains of the Blow-fly and the Wasp were totally 

 unlike those of Arthropods generally, and if the brain of each 

 insect group must be studied ' monographically,' then I for 

 one do not think the study worth entering upon. My object 

 is not to give my readers a description of the brain of the 

 Blow-fly and nothing more, but to show that its study throws 

 light on the whole Arthropod sub-kingdom, and is important 

 because it enables the student of Nature to see further into 

 the arcana of organic life, and adds to our knowledge of the 

 relations of Arthropods and other groups of animals. 



Whatever the ultimate fate of Gaskell's bold hypothesis [45] 

 as to the relation of the alimentary canal in Arthropods and 

 the axial canal of the nervous system in Vertebrates, it is 

 indubitable Gaskell and Patten [48] have done a service in 

 reopening a question which has long lain dormant, as to the 

 relation of the supra-cesophageal ganglia and the ventral cord 

 of Arthropods to the central nervous system of Vertebrates. 



As I have already remarked, there are grave difficulties which 

 must be cleared away before Gaskell's hypothesis can be ac- 

 cepted, and not the least of these is the vesicular character of the 



