76 BULLETIN OF THE 



The reticular cells in Limnadia cover the greater part of the sides of 

 the cones, and completely hide the rhabdome (Plate IV. Fig. 3G). 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 Estheridae 

 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 Evadue 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. jji'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, Clans, '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' 1 :) that the cone in Leptodora is com- 

 posed of four segments is disproved by Carriere's later observations, and 



