The Eetina and Optic Ganglia in Decapods, especially in Astacus. 55 
fibres between the first and second ganglia. In Branchipus^ liowevei\ 
as figure 28 shows, there is no indication of such a crossing; the 
fibres that arise from one end of the first ganglion extend to the 
corresponding end .of the second one, not to the opposite end as in 
Astacus. Claus (86, pag. 311, PI. 30 Fig. 4), in his account of 
Branchipus^ describes and figures a crossing of fibres (Faserkreuzung) 
between the first and second ganglia, but does not commit himself 
to the statement that this crossing represents the first decussation 
in the higher crustaceans. A criticai comparison of his figure with 
one showing the decussation as in Astacus ^ will demonstrate at once 
that the crossing figured by Claus lacks the essential features of a 
true decussation. Moreover, in spite of careful search I bave faiied 
to detect in Branchipus even the crossing described by Claus.. The 
fibres that enter the »Punktsubstanz« of the second ganglion often 
cross those lying parallel to the surface of this structure, but such 
crossings are so limited that they could not give rise to the general 
appearance figured by Claus. I cannot, therefore, confirm his Obser- 
vation, and, being thrown back upon my own, I conclude that, between 
the first and second ganglion in Branchipus^ there is, in contrast to 
Astacus^ no real decussation and only a limited crossing of fibres. 
Another difference between Astacus and Branchipus is in the 
sequence of the materials of the second ganglion. In passing trans- 
versely through the ganglion in Astacus (PI. 1 Fig. 35), the materials 
are met with in the following order: ganglionic cells [cl.gn]^ nerve 
fibres [cx.l]^ »Punktsubstanz« (II); in Branchipus (PI. 1 Fig. 28) the 
sequence is ganglionic cells [cl.gn)^ »Punktsubstanz« (//), nerve fibres 
(fbr,n)^ the last two constituents being inverted in their relations to 
each other. 
The structure of the optic ganglia in Branchipus^ as this account 
shows, is obviously much simpler than that in Astacus. Of the four 
optic ganglia in the crayfish, only the first and second are repre- 
sented in Branchipus. In the first, a single nuclear layer represents 
the two nuclear layers and the fibrous layer in Astacus. The decus- 
sation present between the first and second ganglia in Astacus is 
lacking in Branchipus. A further point of difference between the 
two forms is that the nerve fibres which enter the second ganglion 
in Branchipus are external to its »Punktsubstanz« instead of being 
between this and the ganglionic cells as in Astacus. 
Of the other crustaceans the structure of whose optic ganglia 
I am acquainted with either from Observation or from the work of 
