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 retiuular 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, coel.). This peculiar condi- 

 tion was first clearly described by Leydig ('50, p. 331), although as early 

 as 1806 Jurine ('06, p. 447) l'emarked that the eye in this genus was 

 contained in a transparent membranous sac, which apparently contained 

 a fluid, and Midler ('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 Clans ('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). 



