MUSEUM OF COMPARATIVE ZOOLOGY. 51 



exterior by means of a small opening (po. brs.), the optic pore. The 

 free surface of the stalked portion of the eye is covered with a delicate 

 cuticula, which, after being reflected from the base of the stalk over the 

 inner surftice of the wall of the pocket, becomes continuous at the pore 

 of the pocket with the superficial cuticula. The retina (Fig. 35, r.) 

 occupies the greater portion of the optic stalk. Its distal face is bounded 

 by the delicate cuticula already mentioned, and its proximal face is lim- 

 ited by a basement membrane (mb. ba.). This membrane becomes indis- 

 tinct as the base of the stalk is approached, but the retina itself is 

 apparently continuous in this region with the layer of cells which rests 

 on the cuticular wall of the optic pocket, and which finally unites at the 

 pore of the pocket with the superficial hypodermis. Thus the retina 

 may be said to be continuous with the hypodermis. 



The sti-ucture of the eyes in Limnadia Agassizii is such that they 

 can be described as stalked eyes which have been surrounded by a fold 

 of the integument, so as to become enclosed within a space, the optic 

 pocket, which communicates with the exterior only by means of the 

 optic pore. 



An eye of essentially this structure has been described by Grobben 

 ('79, p. 255) in Limnadia Herraanni, Limnetis brachyurus, and Estheria 

 ticineusis, and in the last genus enough of the development of the eye 

 was observed to indicate that the optic pocket was foi'med by the growth 

 of a fold of integument over the optic stalk. 



Apnsidoe, — In*Apus, according to Grobben (79, p. 256), the plan of 

 the eye is essentially similar to that in the Estheridae. The eyes pro- 

 ject into an open pocket, the cavity of which permanently communi- 

 cates with the exterior. Judging from the figure given by Clans {'d>Q, 

 Taf. VIL Fig. 11, compare p. 366), the right and left retinas in Apus 

 are not so close to one another as in the EstheridcE (compare Plate IV. 

 Fig. 34). 



Cladocem. — The structure and development of the retina in the 

 Cladocera has been carefully studied by Grobben. My own observa- 

 tions on this group have been limited to a single genus, Evadne, and 

 as this genus is not very favorable for the determination of the general 

 relations of the retina I must rely almost entirely upon Grobben's 

 descriptions. 



In the development of Moina, according to Grobben (79, p. 253), the 

 retinal thickening is covered by a fold of the integument in such a 

 manner that an open optic pocket is produced, as in Limnadia. By the 

 closure of what corresponds to the optic pore, this pocket eventually 



