86 BULLETIN OF THE 



These observations naturally lead to the conclusion that in all Isopods 

 each cone is composed of two segments. To this general statement, 

 however, there are two noteworthy exceptions, one recorded by Sars, the 

 other by Beddard. Sars ('G7, p. 110) has shown that, of the four om- 

 matidia in each eye of Asellus aquaticus, three have cones composed 

 each of two segments; in the fourth, however, the cone is divided into 

 three nails. This observation has been confirmed by Carriere ('85, 

 p. 155). It is important to observe that in the figure given by Sars 

 ('67, Planche VIII. Fig. 12) the three parts of the cone are not of 

 equal size; one is about as large as a single segment in the cones of 

 the other three ommatidia, whereas the remaining two are each about 

 half as large. In the eyes of the species of Asellus found about Cam- 

 bridge, the ommatidia are usually twice as numerous as in the European 

 species, A. aquaticus, and, so far as I could observe, the cones in the 

 American species were always composed of only two segments. In 

 Arcturus, according to the figures given by Beddard ("JO, Plate XXXI. 

 Figs. 1 and 4), cones of three segments are occasionally met with. 



The cellular composition of the retinula in Isopods was first made out 

 by Grenadier ('74, p. 653), who found that in Porcellio this structure 

 consisted of seven cells. Distally these cells surround the cone; proxi- 

 mally they are continuous with the optic-nerve fibres. A retinula con- 

 sist 'iiil: of seven cells has also been demonstrated by Puller ('70, p. 513) 

 in Cymothoa, and by Beddard ('88, p. 413) in .Ega and Ligia. As 

 Beddard ('88, Plate XXX. Fig. 13) has shown, the seven cells in the 

 retinula of .F-a pass through the basement membrane and become con- 

 tinuous with the nerve fibres. In Porcelho, as I have observed, the 

 fibrous ends of the seven retinular cells not onty can be identified as nerve 

 fibres below the basement membrane, but each cell contains a well de- 

 veloped fibrillar axis (Plate V. Fig. 40, ax. n.), and I therefore conclude 

 that in Porcellio all seven cells are functional as nervous elements. 



In Idotea robusta, transverse sections of the retinula in the region 

 where the rhabdome is thickest present the outlines of what seem to be 

 i retinular cells (Plate V. Fig. 48). In positions either distal or 

 proximal to this, however, only six cells appear. These six cells pass 

 through the basement membrane and taper into nerve fibres, and their 

 nuclei, unlike the corresponding nuclei in other Isopods, occur in that 

 part of the cell which is proximal to the basement membrane (Figs. 49 

 and 50, nl. rtu'.). The seventh body (Fig. 48, cl. rud.), in those sections 

 in which it occurs, has in all essential respects the same appearance as 

 any one of the adjoining six cells. It differs from these, however, in that 



