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MUSEUM OF COMPARATIVE ZOOLOGY. 57 
from Serolis to the Amphipods. It seems to me, therefore, that the 
objection suggested at the beginning of this paragraph is almost without 
weight. This conclusion, moreover, is supported by the fact that in 
Idotea (Plate V. Fig. 49) the retinular nuclei lie proximal to the base- 
ment membrane, whereas in the majority of other Isopods they are 
distal to that membrane. 
From the preceding discussion, I conclude that the retina in Amphi- 
pods originates as a simple thickening in the superficial ectoderm, and 
that this thickening subsequently becomes separated, probably by a pro- 
cess of delamination, into a deeper portion, the retina proper, and a 
more superficial portion, the corneal hypodermis. The latter alone re- 
tains its original connection with the adjacent hypodermis. Of the two 
membranes present in the basal portion of the eye in Amphipods, that 
which I have called the intercepting membrane is homologous with the 
basement membrane of the retina in other Crustaceans, and that which 
has been designated as. the capsular membrane is in large part the 
cuticular sheath of the optic nerve. 
Copepoda. — The retinas in the Branchiura and Eucopepoda, the two 
divisions of the Copepods, present such different structural conditions 
that for purposes of description it is better to consider them separately. 
Branchiura. — In adult specimens of Argulus, the retina is completely 
separated from all surrounding tissue, excepting the optic nerve, by an 
intervening blood space (Plate II. Fig. 11, cv/.). This peculiar condi- 
tion was first clearly described by Leydig (’50, p. 331), although as eariy 
as 1806 Jurine (’06, p. 447) remarked that the eye in this genus was 
contained in a transparent membranous sac, which apparently contained 
a fluid, and Miiller (31, p. 97) some twenty-five years later described the 
retina as separated from the “cornea” by an intervening space filled 
with fluid. It remained, however, for Leydig to determine the extent 
of this space, and to demonstrate that the fluid which it contained was 
blood. The more essential features of Leydig’s description have since 
been confirmed by Claus (’75, pp. 254-256). 
The development of the eye in Argulus has not been studied with 
sufficient fulness to allow one to determine the relation of its retina to 
the hypodermis. But from the strong resemblance which the eye in the 
adult bears to that in Amphipods, it is probable that the course of 
development in the two cases is not unlike. Probably the retina in 
Argulus originates as a thickening in the superficial ectoderm, and subse- 
quently not only suffers delamination, as in the Amphipods, but becomes 
actually withdrawn from the superficial layer (corneal hypodermis). 
