82 BULLETIN OF THE 
domeres belong, the retina contains a number of smaller nuclei (Fig. 21, 
nl. h'drm.). These nuclei have been drawn in the figures of the various 
sections in which they occur, and probably represent undifferentiated cells. 
To what extent the retina of Pontella can be resolved into omma- 
tidia may be seen from the foregoing account. Evidently the two 
cone cells, the subjacent groups of five retinular cells, and probably 
scme of the undifferentiated cells, are the equivalent of one omma- 
tidium. The sixth cell, with its rod, is probably the representative of 
a second ommatidium, and the seventh and eighth cells are probably 
representatives of one, or perhaps two, more. 
If this interpretation be correct, the cells in the one complete omma- 
tidium in Pontella would be as follows: corneal hypodermis, undifferen- 
tiated ; cone cells, two; retinular cells, five; undifferentiated pigment 
cells (ectodermic?) present. 
Each retina in Sapphirina, according to Grenacher (’79, pp. 69, 70), 
contains one group of three rhabdomeres. These are accompanied, 
as in Pontella, by an equal number of large nuclei. The body desig- 
nated at y, and perhaps some of those marked a, in Grenacher’s figure of 
Sapphirina (Fig. 43), may also represent isolated rhabdomeres. In Co- 
pilia, Grenacher believes that the number of rhabdomeres in each retina 
is three. Possibly in this genus, as in Sapphirina, the body marked « 
by Grenacher (Taf. VI. Fig. 40) may represent an isolated rhabdomere. 
Grenacher’s observations, when coupled with what I have seen in Pon- 
tella, show that in Copepods the number of retinal elements is open to 
considerable variation, and that what would correspond to the retinula 
in Sapphirina, and perhaps in Copilia, consists of a cluster of only three 
cells, instead of five, as in Pontella. 
Branchiura, —The ommatidia in Argulus are rather small, and their 
structure is consequently imperfectly known. The specimens of this 
Crustacean which I studied were obtained from an aquarium in which 
the common Killifish, Fundulus heteroclitus, had been kept. I have not 
been able to determine the species to which these specimens belong. 
The corneal hypodermis in Argulus is separated from the retina proper 
by a space filled with blood (Plate II. Figs. 11, 12, cel.). The cells in 
this layer (Fig. 12, h’drm.), as in the corneal hypodermis of Amphipods, 
are not arranged in groups, but are irregularly scattered. On their 
distal faces they produce the corneal cuticula (Fig. 12, cta.), which, as 
Miller (31, p. 97) observed, is without facets. Proximally they are 
separated from the blood space by the delicate corneal membrane (Fig. 
12, mb. ern.). 
