84 BULLETIN OF THE 
The cells in the ommatidium of Argulus are as follows: cells of the 
corneal hypodermis, not arranged in definite groups ; cone cells, four; 
retinular cells, probably five; accessory pigment cells probably present. 
Isopoda. 
The material which I used in studying the eyes in Isopods came from 
several sources. I collected specimens of Asellus and Porcellio in the 
neighborhood of Cambridge, and the two species of Idotea which I 
studied were obtained at Newport. Specimens of Serolis Schythei, 
Liitken, and of an undetermined species of Spheroma, were kindly fur- 
nished me from the collections in the Museum. 
The ommatidia in Isopods present two types of structure: one of 
these is characteristic of the eyes in a majority of the members of this 
group ; the other, so far as is known, is represented only in the genus Se- 
rolis. These two types will be considered separately, and the one which 
is common to the greater number of Isopods will be described first. 
The corneal hypodermis in the more common of these two ommatidial 
types was first identified by Grenacher. In Porcellio, according to this 
author (’79, p. 107), the proximal surface of each facet is covered with 
two comparatively thin cells. These are the cells of the corneal hypo- 
dermis. Bellonci (’81%, p. 98, Tav. II. Fig. 11 n.) figures similar cells 
in the ommatidium of Spheroma, and Beddard (90, p. 368) concludes 
justly, I believe, that, of the four nuclei found near the distal end 
of the cone in Arcturus, two represent cone cells and two cells in the 
corneal hypodermis. In Idotea irrorata I have identified two cells in 
the corneal hypodermis for each ommatidium. The nuclei of these cells 
lie very near the nuclei of the cone cells (compare n/. con. and ni. crn. in 
Figs. 50 and 51, Plate V.).. In an ommatidium of Porcellio, Grenacher 
(79, pp. 107, 108) observed that the plane which separates the two 
cone cells also separates the two cells in the corneal hpyodermis. In 
Idotea, also, both kinds of cells are separated by a single plane. 
The facetted condition of the corneal cuticula of Isopods was observed 
as early as 1816 by G. R. Treviranus -(’16, p. 64), in wood-lice, and 
subsequently in the same animals by Lereboullet (43, p. 107, 753, 
p. 119). The shape of the facets in different Isopods has given rise to 
some difference of opinion. According to Miiller (29, p. 42), in Cymo- 
thoa each has the form of a biconvex lens. Leydig (’64*, p. 40) states, 
however, that in Oniscus the facets are concavo-convex with their hollow 
faces innermost. In Asellus, according to the figure given by Sars 
(67, Planche VIII. Fig. 14), they are plano-convex with their flat faces 
