No. 1.] EVES OF ARTHROPODS. 169 
of Coelenterates and Molluscs; for, while many ganglion-cells 
arise from the invagination on the side of the eye, others wander 
inward from the optic thickening itself. The details of the lat- 
ter process are best studied in the great cells that are the last 
to form. While these cells are still in the optic thickening 
they are distinctly tripolar, one prolongation being directed out- 
ward to form the rudiment of a nerve end. Such cells would 
correspond to the tripolar neuroepithelial cells of Coelenterates, 
or those tripolar ones already mentioned in Haliotis and Acilius. 
In Acilius there is no reason to suppose that a ganglion-cell 
once established in the above manner ever loses its direct con- 
nection with the ectoderm. According to Hertwig, those con- 
nected by an outward prolongation with the ectoderm are 
intermediate between fully developed ganglion-cells and sen- 
sory ones, and we are to infer that they are intermediate 
because of this outward prolongation. I fail to see the neces- 
sity for this conclusion, for there is reason to suppose that 
most, if not all, ganglion-cells, have some connection with the 
outer world, consequently the existence of such a connection 
would not indicate the age of the ganglion-cells. 
The history of the giant ganglion-cells of Acilius, together 
with a consideration of the neuroepithelial cells of other groups, 
it seems to me, warrant the conclusion that the primitive gang- 
lion-cells were tripolar, and were derived from tripolar neuroept- 
thelial cells. The outer extremities of these neuroepithelial cells 
were reduced to intercellular nerve ends, the bases of which, in 
Actlius, became the protoplasmic prolongations of the ganglion- 
cells, and are probably homologous with the axts-cylinders of 
Vertebrates. 
SUMMARY. 
The more important results of the foregoing study are as follows : — 
(1) The larval optic ganglion is composed of three segments, each 
of which is united on the one hand with a segment of the brain, and on 
the other, with a segment of the optic plate. 
(2) Each segment of the optic plate bears a pair of eyes. 
(3) The ocelli are composed of four or more sensory spots, or pits, 
each pit being supplied with a separate cuticular thickening and nerve. 
