442 THE NERVOUS 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 has 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. Viallanes [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, [179], whilst he recognised a general similarity 
