ANATOMY 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 
