No. FS} EYES OF ARTHROPODS. 165 
doubt as to whether the retina arose from the hypodermis or 
from the brain, or from neither; or while that organ sometimes 
called the optic ganglion, in the absence of all evidence, might 
be regarded as a retina, a retinal ganglion, the brain, an out- 
growth of the brain, or an optic nerve, or whatever else might 
commend itself to the imagination of the investigator. 
There were the one-layered, the two-layered, and the three- 
layered eyes, one with upright, another with inverted, another 
with horizontal rods. The structure of one eye was as intel- 
ligible as that of another; no more, no less. There was no unit 
of measure. 
It was inferred in “Eyes of Molluscs and Arthropods”’ 
that the ground plan in all these confusing variations of 
structure was a three-layered eye, an invaginated optic vesi- 
cle, the inner wall of which became the retina, and an over- 
lying layer of hypodermis the corneagen. Such an eye would 
be much like that of Peripatus; flatten the vesicle vertically, 
reduce the outer wall to a thin membrane, and you have 
the ocelli of some Insects and Spiders. The observations re- 
corded in this paper confirm the supposition mentioned above. 
We may now go still farther and say that a lateral flattening 
would produce the larval Insect, and Myriapod eyes, with hori- 
zontal rods; that the outer wall of the vesicle may, in some 
cases, develop inverted retinal cells side by side with the up- 
right ones of the inner layer; and that the rods of these 
inverted cells may be converted into a lens inside of the optic 
vesicle, as in Chauliodes and perhaps Peripatus, or they may 
take the place altogether of the upright ones, as in the tapetal 
eyes of Spiders. 
It can no longer be affirmed that there is a wide difference 
between the so-called Molluscan and Arthropod eye; both belong 
to the same type, as I formerly maintained. This difference it 
has been urged was due to the presence in the former of an optic 
cavity filled with an inspissated, refractive substance. But there 
is just such a cavity in the early stages of eyes V. and VI. of 
Acilius and one in Chauliodes which is filled with a lens like 
that in Peripatus and some Molluscs and Worms. 
Professor Mark maintains that “none of Locy’s predecessors 
have in the least foreseen the true course of events’’ concerning 
the origin and method of formation of the retina of Arthropods. 
