TuRNER, Mushroom Bodies of the Crayfish. 339 
METHODS. 
Several methods were tried but the best results were ob- 
tained with Mallory’s haematoxylin and the methylen-blue 
method. In the first case the brains were fixed in 10% formal- 
dehyde. Before staining the sections were mordanted in about 
a 5% aqueous solution of copper sulphate for from 12 to 24 
hrs. Inthe second case the fresh brains were stained ina 
weak solution of methyien-blue. The brains were kept on ice 
while being stained, washed and hardened. They were fixed 
in the usual way and mounted whole or sectioned. After many 
failures a few good specimens were secured. Unfortunately 
all my notes on the method have been lost and details cannot 
be given. 
GENERAL TOPOGRAPHY. 
There are sufficient physiological reasons to justify KENYON 
(96) in including under the general term brain ‘‘the whole of 
the neural mass included within the head, excepting only the 
small ganglia known as the stomatogastric ganglia; but 
throughout this paper the word is used in quite a different 
sense, In these studies the word brain is restricted to that 
nervous mass which is composed of the protocerebrum, deuto- 
cerebrum and tritocerebrum. It will be seen at once that what 
is here called brain has been:termed by Kenyon, the dorso- 
cerebrum, and by others the supra-oesophageal ganglion. 
Viewed from its cephalic aspect the brain (supra-oesopha- 
geal ganglion) of Cambarus is sub-rectangular in outline. The 
dorsal and ventral sides of this rectangle are slightly concave, 
but the lateral boundaries are strongly convex (fig. 1). From 
right to left the brain is about twice as long as it is from above 
downwards (dorso-ventrally); but from before backwarks (ceph- 
alo-caudal) the diameter is shorter still. 
The decapod brain is quite compact and resembles in many 
respects the supra-oesophageal ganglia of insects. There are, 
however, two points of contrast that are quite conspicuous. In 
the first place, in the Insecta, a pair of optic nerves are fused 
with the brain; but in the Decapoda the optic ganglia are lo- 
