52 
G. H. Parker 
seem to me to argue stroDgly for the accuracy of the total number 
of optic tibres as estimated in the way described, and, further, they 
show that the number of fibres in the dorsal part of the nerve is 
approximately 700. The one actual enumeration of this region that 
I have made was carried out on specimen B and gave as a result 
506 fibres. Where it is difficult to distinguish fibres, it is more 
probable that an actual enumeration would fall short of the real 
number than that it would exceed it, and the discrepancy between 
the estimated number and the number obtained by actual enumeration 
is to be explained, I believe, in this way. 
The following" table gives by way of summary the results of the 
preceding enumerations. 
Length 
Ommatidia 
Retinal Fibres 
Optic Nerve 
Fibres 
Specimen A 
j 3 cm. 
1155 
8085 
2021 
Specimen B 
7.9 cm. 
2375 
16 625 
4156 
Eatio of fibres to ommatidia . . . 
1 
7 
1.75 
As this table indicates, seven retinal fibres extend from each omma- 
tidium to the first optic ganglion, and fibres to the proportion of 
1.75 for each ommatidium connect the fourth optic ganglion with the 
brain. Granting that each ommatidium originates a single visual 
impulse, the 1155 Impulses produced in the retina of specimen to 
take a specific example, are transmitted, probably, over all the 8085 
retinal fibres to the first optic gauglion, and are transfefred from the 
fourth optic ganglion to the brain over the 2021 optic nerve fibres. 
This reduction in the number of fibres concerned with the transmis- 
sion of the impulses, as they pass through the optic tracts from the 
retina to the brain, is the chief observable anatomical change effected 
in the optic ganglia. It is of interest to notice that in no place in 
the course of the optic tracts is there a smaller number of fibres 
than the assumed number of retinal impulses. 
6. Comparison of Optic Ganglia in Crustaceans. 
In attempting to gain some understanding of the complicated 
structure of the optic ganglia in the crayfish, a comparative study 
of the corresponding organs in other crustaceans naturally suggests 
