MUSEUM OF COMPARATIVE ZOOLOGY. 77 



Claus's statement ('76, p. 37*2) that the same number of segments oc- 

 curs in the cone of Sida is probably erroneous. There is, therefore, 

 reason to believe that the cones in the Cladocera are always composed 

 of five segments. 



The composition of the retinula in Cladocera, so far as I am aware, has 

 never been fully worked out. In Evadne, on account of tbe relatively 

 large size of the ommatidia, the number of cells in the retinula can be 

 determined. At the proximal end of the cone, this structure is sur- 

 rounded by four distinct masses (Fig. 43). The regularity with which 

 these masses occur leaves no doubt as to their number. Each one prob- 

 ably represents a retinular cell. In transverse sections made through 

 the rhabdome (Plate IV. Fig. 45), this structure is surrounded by five 

 bodies, each one of which I take to be a retinular cell. It is therefore 

 probable that the retinula of Evadne is composed of five cells, four of 

 ■which approach nearer the surface of the eye than the fifth. 



In Evadne I have seen no evidence of the existence of other cells than 

 those belonging to the cone and retinula. According to Carriere ('84, 

 p. 678). the interommatidial space in Leptodora contains a number of 

 cells which envelop the cones more or less completely. These are proba- 

 bly to be regarded as accessory pigment cells. 



From the foregoing account the following general statement can be 

 made for the ommatidia in the Cladocera : corneal hypodermis, not 

 observed ; cone cells, five ; retinular cells, five (in Evadne) ; accessory 

 pigment cells present (in Leptodora). 



Copepoda. 



I have studied the lateral eyes' in Pontella and Argulus, as representa- 

 tives of the Copepods. As is well known, the eyes in these two genera 

 differ greatly in structure, and I shall therefore describe them separately, 

 beginning with the eyes in Pontella. 



Eucopepoda. — The species of Pontella which I. studied was extremely 

 abundant at Newport in August, 1890. This animal was so transparent 

 when living, that the general structure of its eyes could be ascertained 

 by a simple microscopic inspection of it. In addition to its median eye, 

 which occupies a ventral position, it possesses a pair of lateral eyes 

 (compare Clans, '63, Taf. III. Fig. 5) situated one on either side of the 

 sagittal plane at the antero-dorsal angle of the head. 



Each lateral eye in Pontella, as Clans ('G3, p. 47) has already stated, 

 is provided with a spherical lens (Plate II. Fig. 18, Ins.), which is usu- 

 ally firmly attached to the superficial cuticula. Immediately behind 



