760 JourNAL oF CoMPARATIVE NEUROLOGY. 
sures are homologous in insects and Crustacea, I have retained 
his name although, in reality in the crayfish the lower optic 
commissure is, for the most part, higher than the upper optic 
commissure. In the bee brain Kenyon found this commissure 
to be a much more compact structure than it is in Cambarus. 
This is in keeping with the fact that the insect brain is much 
more compact than the crustacean brain. In the insects the 
optic lobes are fused with the procerebrum, but in the crayfish, 
the optic lobes are apart from the brain, being connected to it 
by the optic nerve. This difference accounts for the compact 
arrangement of the lower optic commissure in the crayfish. 
Thus it appears that the lower optic commissure exists in 
the lower as well as the higher Crustacea and that it is homolo- 
gous to the lower optic commissure of the insects. 
Optic Chiasm or Upper Optic Commissure. This commis- 
sure has long been known. To the best of my knowledge D1eti 
(76) and BERGEN (’78) were the first to describe it. VIALLANES 
(87) found it in Oedipoda, Cuccati (’88) in Somomya, KRIEGER 
(88) in Astacus and KENyon ('96) in the bee. Although all of 
these authorities are describing the same organ, yet there is a 
diversity of opinion concerning it which is, to say the least, 
confusing. 
Diet and Bercer think there is a partial decussation of 
fibers in this commissure, while VIALLANES, KRIEGER and KEN- ~ 
YON assert that there is a complete decussation. 
In Cambarus this commissure is quite conspicuous. A 
careful study of several series shows that each of these investi- 
gators has stated a truth, but that no one of them has stated 
the whole truth. Associated with this chiasma in the crayfish 
there are three sets of fibers (fig. 2, O.C.). One set is purely 
commissural (fig. 2,-7,--O.C.). >This set occupies’ the 
upper portion of the chiasm. It is a concave band with 
its concavity directed upwards (dorsad). A second set decus- 
sates to the other side. A third band passes by ina con- 
vex.curve without sending any fibers to the other side (fig. 7, 
O.C.). It will be seen that this commissure resembles very 
eis 
