76 BULLETIN OF THE 
The retinular cells in Limnadia cover the greater part of the sides of 
the cones, and completely hide the rhabdome (Plate IV. Fig. 36). Their 
number can be determined in transverse sections in the region of the 
rhabdome. In such sections each rhabdome is surrounded by five retin- 
ular cells (Fig. 39, cl. rtn.'). Occasionally nuclei can be distinguished 
in the pigment about the base of the cone. These are probably the 
nuclei of the retinular cells. 
Besides the elements thus far enumerated, the retina in the Estheride 
is not known to contain other kinds of cells. The cells in the omma- 
tidia of this family are, therefore, as follows: cells of the corneal hypo- 
dermis, not regularly arranged ; cone cells, usually five, sometimes four ; 
retinular cells, five. 
Cladocera, — The extreme minuteness of the ommatidia in the eyes of 
the Cladocera renders their study especially difficult. In an undeter- 
mined species of Evadne which I have studied, the ommatidia are 
comparatively large, and in this respect are especially favorable for in- 
vestigation. In the particular specimens which I used, however, I was 
entirely unsuccessful in all attempts to differentiate the nuclei. Al- 
though I tried a number of dyes and reagents, I was never able to make 
these structures visible. In consequence of this, there are several impor- 
tant questions concerning the eyes in the Cladocera which I have not 
been able to answer. 
It is reasonable to believe that.a corneal hypodermis much like that 
in Limnadia is present in Evadne, but, probably on account of my inabil- 
ity to stain the nuclei, I have seen no traces of it. 
The cones in Evadne are very clearly composed of five segments (Plate 
IV. Figs. 41, 42). At their distal ends the cone cells are expanded so 
that their peripheral membranes (Fig. 41, mb. po’ph.) are in contact with 
one another. At this level, however, the substance of the cone proper is 
collected about the axis of the ommatidium. Proximally the peripheral 
membranes of each cone contract, and under these circumstances the 
cavity of each cone cell is apparently filled completely with the differen- 
tiated material of the cone itself (Fig. 42). 
A cone composed of five segments has been observed in a considerable 
number of Cladocera. Thus it is known to occur in Bythotrephys 
(Leydig, ’60, p. 245, Claus, ’77, p. 144), Daphnia (Spangenberg, ’76, 
p- 522, Grenacher, ’79, p. 117), Polyphemus, Evadne (Claus, ’77, p. 144), 
Podon (Grenacher, ’79, p. 117), and Leptodora (Carriere, ’84, p. 678). 
Weismann’s assertion (’74, p. 36) that the cone in Leptodora is com- 
posed of four segments is disproved by Carriere’s later observations, and 
a 
