The Eetina and Optic Ganglia in Decapods, especially in Astacus. 49 
The optic nerve of the crayfìsh must, therefore, be regarded as 
mix e din character; for, in addition to its multitude of optic fibres^ 
it coutains eight or ten motor fìbres and a few special sensory fibres 
resembling nervi nervorum. 
d. Systems of Neurons. 
So far as the succession and course of the neurons are concerned, 
the diagrammatic figure 59 (PI. 3) may be taken as a convenient 
summary of the preceeding description. The four masses of »Punkt- 
substanz« represent as many interruptions in the course of the nerve 
fìbres from the retina to the brain; in other words, each mass marks 
the end of one system of neurons and the beginning of another. 
An impulse in passing from the retina to the brain would, therefore, 
ordinarily travel over five neurons beginning with one of the first 
order and ending with one of the fifth. The only exception to this 
statement is to be found in the case of those peculiar fibres that, 
starting from the anterior end of the second ganglion, pass over the 
third and fourth and make their way directly to the brain. In this 
case the impulse would necessarily pass over only three neurons. 
Whether in any part of the eye there is a series of four neurons, in- 
termediate between this series of three and the usuai one of five, or 
not, is a question on which I bave no conclusive evidence. If there 
were such a series, one might expect the distai three of its members 
to extend, as the ordinary neurons do, from the retina to the third 
ganglion, from which the fourth one would make its way directly to 
the brain. I bave never seen any conclusive evidence on this ques- 
tion, but among my earlier observations is recorded a fibre that 
extended directly from the optic nerve to the third ganglion, where 
it separated into fibrillae, precisely what would be expected if a 
series of four neurons were present. The fact that this Observation 
is a single one and that it was made before I fully appreciated the 
significance it might bave, leads me to refrain from concluding that 
a series of four neurons does exist, though it seems to me highly 
probable that such may be the case. 
Finally, the connections established by the different series of 
neurons may be thus briefly summarised. The series beginning at 
the posterior side of the retina (PI. 3 Fig. 59, green) passes first to 
the corresponding side of the first ganglion, then to the anterior side 
of the second ganglion, to the posterior side of the third ganglion, 
Mitthe Hungen a. d. Zoolog. Station zu Neapel. Bd. 12. 4 
