MUSEUM OF COMPARATIVE ZOOLOGY. i 
“Claus’s statement (’76, p. 372) 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 retinza in Cladocera, so far as I am aware, has 
never been fully worked out. In Evadne, on account of the 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 jive 
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 
' gells 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 
n@ade 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 Claus, ’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 Claus (’63, p. 47) has already stated, 
is provided with a spherical lens (Plate II. Fig. 18, /ns.), which is usu- 
ally firmly attached to the superficial cuticula. Immediately behind 
