The Ketina and Optic Ganglia in Decapods, especially in Astacus. 55 
fibres between tbe first and second ganglia. In Branchipus^ however^ 
as figure 28 shows, tbere is no indication of such a Crossing; tbe 
fibres that arise from one end of the first ganglion extend to tbe 
corresponding end .of tbe second one, not to tbe opposite end as in 
Astacus, Claus (86, pag. 311, PI. 30 Fig. 4), in bis account of 
Branchipus^ describes and .figures a. Crossing of fibres (Faserkreuzung) 
between tbe first and second ganglia, but does not commit bimself 
to tbe Statement Ibat this Crossing represents tbe first decussation 
in tbe bigber, crustaceans. A critical comparison of bis figure witb 
one sbowing the decussation as in Astacus,, will demonstrate at once 
tbat the Crossing figured by Claus lacks tbe essential features of a 
true decussation. Moreover, in spite of careful searcb I bave failed 
to detect in Branchipus even tbe Crossing described by Claus.. Tbe 
fibres tbat enter tbe »Punktsubstanz« of tbe second ganglion often 
cross those lying parallel to tbe surface of tbis structure, but such 
crossings are so limited tbat tbey could not give rise to tbe general 
appearance figured by Claus. I cannot, therefore, confirm bis Obser- 
vation, and, being thrown back upon my own, I conclude tbat, between 
tbe first and second ganglion in Branchipus, tbere is, in contrast to 
Astacus, no real decussation and only a limited Crossing of fibres. 
Anotber difference between Astacus and Branchipus is in the 
sequence of the materials of tbe second ganglion. In passing trans- 
versely througb tbe ganglion in Astacus (PI. 1 Fig. 35), tbe materials 
are met witb in tbe following Order: ganglionic cells [cl.gn), nerve 
fibres (cx,l), »Punktsubstanz« (II); in Branchipus (PI. 1 Fig. 28) tbe 
sequence is ganglionic cells [cl.gn], »Punktsubstanz« (//), nerve fibres 
[fbr.n], tbe last two constituents being inverted in tbeir relations to 
eacb other. 
Tbe structure of tbe optic ganglia in Branchipus, as this account 
sbows, is obviously much simpler tban that in Astacus. Of tbe four 
optic ganglia in the crayfisb, only tbe first and second are repre- 
sented in Branchipus. In tbe first, a single nuclear layer represents 
tbe two nuclear layers and tbe fibrous layer in Astacus. Tbe decus- 
sation present between tbe first and second ganglia in Astacus is 
lacking in Branchipus. A furtber point of difference between the 
two forms is that tbe nerve fibres wbich enter tbe second ganglion 
in Branchipus are external to its »Punktsubstanz« instead of being 
between tbis and the ganglionic cells as in Astacus. 
Of the other crustaceans tbe structure of whose optic ganglia 
I am acquainted witb eitber from observation or from the work of 
