MUSEUM OF COMPARATIVE ZOOLOGY. 71 



longitudinal bands of pigment on it. Grenacher ('79, p. 110) took 

 this as an indication that there were at least four retinular cells in 

 the ommatidium of this genus, but he was unable to satisfy himself as to 

 whether there were a greater number or not. Carriere ('85, pp. 156, 

 157) easily identified the four cells first seen by Sars, and in favor- 

 able cases observed what he thought might be indications of a fifth cell. 

 In Gammarus ornatus, as the present observations show, the retinula 

 is certainly always composed of five cells, one of which, as Carriere 

 observed, is usually much smaller than the other four (compare d. rt/iJ, 

 Figs. 4-7). 



In Talorchestia, according to Watase ('90, p. 296), the retinula is 

 composed of only four cells. I have studied T. longicoruis with the 

 purpose of determining the number of retinular cells, and I find that, 

 although there are four large retinular cells, there is also one small one, 

 which is even more reduced than in Gammarus. Hence I conclude that 

 the total number of retinular cells in an ommatidium of Talorchestia 

 is five, not four. 



Claus's statement ('71, p. 151), that in Oxycephalus the retinula is 

 usually composed of four cells, is probably inaccurate, as Grenacher 

 ('79, p. 114) suggests; and the same is perhaps true of Delia Valle's 

 ('88, p. 94) observation, that in the Ampeliscidte the retinulae contain 

 only four cells each. It is therefore probable that the retinula in all 

 Amphipods is composed of five cells, although possibly in some excep- 

 tional cases the number may be four. 



The retinular cells in Gammarus envelop the sides of the cone, as 

 Carriere suspected, and extend distally as far as the corneal hypodermis 

 (Plate I. Fig. 2). In Hyperia and Phronima, according to the descrip- 

 tion and figures given by Carriere ('85, p. 161, and Fig. 128, p. 165), 

 these cells appear to be limited to the proximal part of the retina. 



The rhabdome in Amphipods, first described by Pagenstecher ('61, 

 p. 30) as the cylindrical element in the eye of Phronima, presents a 

 very simple structure. In Hyperia, according to Grenacher ('77, p. 31), 

 it is a simple rod-like body, composed of five rhabdomeres, one for each 

 retinular cell. In Phronima, as Claus ('79, p. 128) has shown, the 

 rhabdome is a tubular structure with five sides. Each side of the tube, 

 as can be seen in the figure given by Carriere ('85, p. 165, Fig. 128), 

 corresponds to a rhabdomere. In Gammarus locusta, Grenacher ('77, 

 p. Ill) has shown that, in transverse section, the distal end of the 

 rhabdome is cross-shaped. In G. pidex, according to Carriere ('85, 

 p. 157), the distal end of the rhabdome in section shows four rays, the 



