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 surface of the wall of the pocket, becomes continuous at the pore 
of the pocket with the superficial cuticula. The retina (Fig. 35, 7.) 
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 structure 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 Hermanni, Limnetis brachyurus, and Estheria 
ticinensis, and in the last genus enough of the development of the eye 
was observed to indicate that the optic pocket was formed by the growth 
of a fold of integument over the optic stalk. 
Apuside. —In Apus, according to Grobben (’79, p. 256), the plan of 
the eye is essentially similar to that in the Estheride. The eyes pro- 
ject into an open pocket, the cavity of which permanently communi- 
cates with the exterior. Judging from the figure given by Claus (86, 
Taf. VIL Fig. 11, compare p. 366), the right and left retinas in Apus 
are not so close to one another as in the Estheridz (compare Plate IV. 
Fig. 34). | 
Cladocera. —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 
