-TuRNER, Mushroom Bodies of the Crayfish. 341 
CEREBRAL NERVES. 
The crayfish brain, as here defined, gives rise to the follow- 
ing five pairs of nerves: optic nerves, oculo-motor nerves, an- 
tennulary nerves, tegumentary nerves and antennary nerves. 
Optic nerves (fig. 1, O.N.). These constitute a pair of 
large nerves which pass from the brain to the optic ganglia. 
One arises from each dorso-lateral corner of the brain and passes 
upwards and outwards (dorso-laterad) to the optic lobe. 
The nerve does not exist in the insects, but its homologue 
is found within the hexapod brain. 
This nerve is composed of the extra-ganglionic portions of 
the fiber tracts that lead from several ganglionic centers to the 
optic ganglia. In the Hexapoda the optic ganglia are a part of 
the brain, hence there is no extra-ganglionic portion of these op- 
tic tracts. In those insects that have eyes situated ata greater or 
less distance from the brain there is a nerve, known as the optic 
nerve, which passes from the optic ganglion to the eye. But 
this is quite a different structure from the optic nerve of those 
~ Decapoda whose optic ganglia are situated within the eyestalks. 
It must not be hastily concluded that in all Crustacea there 
is an extra-ganglionic portion of the optic tracts, known as the 
optic nerve, which passes from the brain to the optic ganglion, 
which is situated either in the eyestalk or near the eye. In 
Gammarus, an amphipod, the eye is sessile on the brain and 
this nerve is absent. Nor would one be warranted in stating 
that in the higher C7wstacea we have invariably an optic nerve 
of the type found in Cambarus, while in the lower forms the 
nerve is always absent; for in the phyllopod Bvanchzpus, a 
form much lower in the scale than Gammarus my preparations 
show quite a long optic nerve connecting the brain to the eyes. 
Even in closely allied forms the relation of brain to optic gan- 
glion is not the same; for according to Samassa (’g1), in Sida 
crystallina, a cladoceran, the optic ganglion is fused with the 
brain, while in Daphnia sima, O. F. MULLER, another clado- 
ceran, the optic ganglion is united to the brain by two stalks. 
Among the fresh water Ostracoda, the optic nerve is, ex- 
