NO. 1.) EYES OF ARTHROPODS. 149 
one of which is continued half way across the optic ganglion to 
the fifth medulla. 
There are some minute fibres scattered about among the cells 
of the cortical layer, that I cannot trace to distinct cells. They 
are seen running in an irregular manner from one cell to an 
other, or from the cortical layer to the neurilemma. They 
often extend over the surface of the large cells branching in all 
directions (Pl. X., Fig. 48). They are not numerous enough to 
form a sheath or envelop about the cells, such as that so fre- 
quently described in the Annelids, although it is not improbable 
that they belong to the same category. They may possibly 
arise from the small, dark cells, sometimes called the inner neu- 
rilemma, that form an investment for the medulla, and which are 
specially well developed in the brain and ventral nerve chord. 
The fact that a not inconsiderable number of the ganglion- 
cells arise directly from the gigantic, tripolar ones might lead 
one to suppose that all the cells of the optic ganglion were tripo- 
lar and differed from the large ones only in size. All that one 
can actually observe points towards this conclusion. 
In my paper on the eyes of Vespa, page 198, attention was 
called to certain conical or pear-shaped ganglion-cells at the inner 
edge of the middle lobe of the optic ganglion, and it was stated 
that some of the conical ones were possibly tripolar, for indica- 
tions of two prolongations at the broad distal ends of such cells 
had occasionally been seen. A re-examination has led me to 
believe that all of them are tripolar, like the large ones in 
Acilius. Their deceptive, unipolar habitus is due to the great 
size of the inner prolongation as compared with the very delicate 
outer ones. 
As there is no reason to suppose that more than one kind of 
cell is present in the cortical layer of the optic ganglion, — we 
do not include the minute, dark cells surrounding the medulla, 
or those interspersed among the fibres of the optic nerves, con- 
cerning whose minute structure we are ignorant, —and in con- 
sideration of the facts we have presented concerning the origin 
of ganglion-cells, we have arrived at the following conclusion: 
The cortical layers of the optic ganglia are composed of large and 
small tripolar ganglion-cells. The large, inward prolongation of 
cach of these cells ts filled with granular protoplasm continuous 
with that in the cell body, and represents a part of the outer end 
