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 (’67, 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 parts. 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 (’90, 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 Grenacher (’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- 
sisting of seven cells has also been demonstrated by Buller (’79, p. 513) 
in Cymothoa, and by Beddard (’88, p. 443) in /Zga and Ligia. As 
Beddard (’88, Plate XXX. Fig. 13) has shown, the seven cells in the 
retinula of Alga pass through the basement membrane and become con- 
tinuous with the nerve fibres. In Porcellio, as I have observed, the 
fibrous ends of the seven retinular cells not only can be identified as nerve 
fibres below the basement membrane, but each cell contains a well de- 
veloped fibrillar axis (Plate V. Fig. 46, 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 
seven retinular cells (Plate V. Fig. 48). In positions either distal or 
proximal to this, however, only sez 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. rtn!.). 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 
