82 BULLETIN OF THE 



domeres belong, the retina contains a number of smaller nuclei (Fig. 21, 

 nl. Ii dnii.). 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 

 seme 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 V) 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 x, in Grenadier'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 x 

 bv 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, ccrd.). 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, eta.), -which, as 

 Muller ('31, p. 97) observed, is without facets. Proximally they are 

 separated from the- blood space by the delicate corneal membrane (Fig. 

 12, mb. crn.). 



