72 BULLETIN OF THE 



proximal five. In Carriere's opinion, these rays indicate the five rhab- 

 domeres. In Gammarus ornatus, the species which I have studied, the- 

 rhabdome (Plate I. Fig. 6, 7-hb.) is cross-shaped in transverse section 

 throughout its length. Each rhabdomere has the form of an elon- 

 gated plate, which is folded on its longest axis, so that its halves are 

 at right angles to each other. In the rhabdome, the four rhabdomeres 

 lie so that their folded edges occupy the axis of the ommatidium. 

 Each of the four large retinular cells rests in the furrow produced 

 by the folding of a rhabdomere (compare Fig. 6). The fifth retinular 

 cell always, lies at the end of one arm of the cross-shaped rhabdome. 

 The two rhabdomeric constituents of that arm usually separate slightly, 

 so as to allow the small retinular cell to slip in between them. Possi- 

 bly this cell produces a small rhabdomere, as the corresponding cell in 

 G. pulex does; but if such is the case, the rhabdomere must be a very 

 small one, for I have not been able to discover it. A rhabdome of 

 essentially this structure occurs in Talorchestia. 



As the preceding account shows, the rhabdome in Amphipods always 

 presents some indication of the number of rhabdomeres of which it is 

 composed. This number is usually five, although it is possible that in 

 Gammarus it may be only four. 



In addition to the cells which have thus far been described as entering 

 into the composition of the retina in Amphipods, certain other cells may 

 be present. These may be embraced under the one head of accessory 

 pigment cells. 



In Gammarus, as Carriere ('85, p. 159) has shown, the space between 

 the ommatidia is filled with rather large cells, the nuclei of which are 

 usually visible with ease (Fig. 2, nl. h'dnn.). These cells extend from 

 the basement membrane very nearly, if not quite, to the corneal hypo- 

 dermis. In the fresh condition they contain a whitish opaque pigment. 

 On account of their having no definite arrangement, it is difficult to esti- 

 mate their nimiber, but there are probably two or three for each omma- 

 tidium. Cells similar in position to these have been described by Watase 

 ('90, p. 29G) in Talorchestia. 



In Hyperia there are apparently three kinds of accessory pigment 

 cells. One kind occurs in tlie region of the basement membrane (Car- 

 riere, '85, p. 161, Fig, 124, m.) ; another kind surrounds the proximal por- 

 tion of the cones (Carriere, '85, p. IGl) ; a third kind is applied to the 

 retinulae, and, according to Carriere, exactly equals in number the cells 

 of the retinula itself. Possibly the cells which Grenacher ('79, p. 112) 

 described as lying at the distal end of the retinula in Hyperia belong 



