Turner, Mushroom Bodtes of the Crayfish. 351 
enter fibers which will be described in connection with the fiber 
tracts of the brain. 
FIBER TRACTS AND COMMISSURES. 
Fibers Related to the Optic Nerve. The fibers related to the 
optic nerve may be classified as commissures and tracts. The 
commissures are two in-number; the upper and lower optic 
comimissures. 
Lower Optic Commissure (fig. 8, 9, la. [b. Lc. Id.) In 
Cambarus the presence of the lower optic commissure can be 
demonstrated by either the methylen-blue or the copper-sulphate 
haematoxylin method. In methylen-blue preparations a few 
fibers may be seen connecting the optic nerves; but in the 
copper-sulphate haematoxylin sections four bands of fibers con- 
nect the optic nerves (figs. 8, 9). All of these fibers lie in the 
same plane and are sub-parallel, forming downwardly directed 
’ convex curves. For convenience in reference, beginning at the 
upper end of the brain these tracts are designated ‘‘a,”’ ‘‘b,”’ 
‘«c,” “d.”. Band ‘‘a,’”’ which lies immediately below the dorsal 
nidus, connects the dorsal portion of the optic nerve. Band 
“b,” which lies about half way between band ‘‘a’’ and the 
central body, and ‘‘d,” which lies immediately below the 
central body, connect the lower portions of the optic tracts. 
Bands ‘‘a,”’ ‘‘b,”’ ‘‘d’’ seem to be purely commissural, but into 
band ‘‘c’’ enter fibers which originate in its dorsal nidus, From 
band ‘‘c’ fibers pass into the central body and in some cases it 
can be demonstrated that some of the fibers that enter the 
central body come from fibers which pass ‘rom the dorsal 
nidus to band ‘‘c” of this lower optic commissure. As has 
already been stated, some of the fibers of the central body are 
derived from band ‘‘d” of the lower optic commissure. 
This lower optic commissure is found not only in the 
higher but also in the lower Crustacea, for I have found it in 
Branchipus, where it consists of three instead of four bands. In 
that case bands ‘‘a’ and ‘‘b” seem to be fused. 
KEnyon (’96) recognized this in the bee brain and called it 
the lower optic commissure. Feeling sure that these commis- 
