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 Sphaeroma, 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 Sphaeroma, and Beddard ('90, p. 3G8) 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 nl. con. and nl. ci-n. 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, '53, 

 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 YIII. Fig. 14), they are plano-convex with their flat faces 



