54 BULLETIN OF THE 



the retina the intercepting membrane had the appearance of a delicate 

 hunella, in which I was unable to lind anj' trace of cells. Kot iinfre- 

 quently the nuclei of certain accessory pigment cells (Fig. 2, nl. Jcdrm.) 

 appear to touch the membrane, and even at times to lie with their long 

 axes parallel to it, but in no case could these nuclei be said to be in the 

 membrane. In sections of the retina from which the natural pigment 

 had not been removed, it was often difficult to decide whether a given 

 nucleus was in the membrane or only next to it. Possibly appearances 

 such as these have led Carriere to believe that the membrane was cel- 

 lular. My own opinion is, that the intercepting membrane, like the 

 other two membranes, is a cuticula, and does not contain cells. 



From the foregoing account, it will be seen that in an adult Gammarus 

 the retina lies immediately under an undifferentiated corneal hjpoder- 

 mis, and is enclosed, excepting where the optic nerve emerges from it, 

 by a non-perforated cuticular capsule. The space within this capsule 

 is divided by a perforated cuticular membrane into a large distal and a 

 small proximal chamber. 



In Hyperia, judging from the figure given by Carriere ('85, p. 161 

 Fig. 123), the retina has essentially the same structure as in Gammarus. 

 The intercepting membrane is in a position proximal to the rhabdomes 

 and distal to th^ retinular nuclei. The layer of pigment cells, which 

 Carriere QSo, p. 161, Fig. 12-4) apparently considers the intercellular 

 membrane itself, in my opinion marks only approximately the position 

 of that membrane. Probal)ly in Hyperia, as in Gammarus, these cells 

 rest on the distal face of the intercepting membrane. 



In Phronima eacli side of the head is occupied by two eyes, instead of 

 one, contrary to the condition in the more typical Aniphipods. Of the 

 two eyes, one is dorsal, the other lateral. This difference in position 

 affords a convenient means of distinguishing them. The lateral eye pre- 

 sents all the essential structural features of the single eye in Gammarus 

 (compare Carriere, '85, Figs. 125 and 121). The dorsal eye, although 

 differing considerably in shape from the lateral one, is nevertheless con- 

 structed upon the same morphological plan. Its most important pecu- 

 liarity is the shape of its intercepting membrane and the adjoining 

 structures. In the dorsal eye the intercepting membrane, instead of 

 lying in a plane nearly parallel with the external surface of the retina, 

 as in the lateral eye, is cone-shaped. The axis of this cone corresponds 

 to the axis of the eye ; its apex is near the brain, and its base faces the 

 external surface of the eye (compare Clans, '79, Taf. III. Fig. 20, and 

 Taf. VII. Fig. 58). The oramatidia are arranged approximately parallel 



