MUSEUM OF COMPARATIVE ZOOLOGY. 89 



The cells which characterize the ommatidia in Isopods (except Serolis) 

 are as follows : cells of the corneal hypodermis, two ; cone cells, two ; 

 retinular cells, seven, six, or possibly five. Undifferentiated hypodermal 

 cells are sometimes present, and hyaline cells occur in a few genera. 



The structural peculiarities of the ommatidia in Serolis were first de- 

 scribed by Beddard ('84, pp. 339-341) about seven years ago. Recently 

 Beddard's observations have for the most part been confirmed by Watase 

 ('90), and it -must now be admitted without question that the ommatidia 

 in Serolis differ in several important respects from those of many other 

 Isopods. 



The material which I used in studying the eyes in this Crustacean 

 consisted of advanced embryos and matured individuals of Serolis 

 Schythei, Liitken. This material was collected in Patagonia by the 

 Hassler Expedition, and was preserved in strong alcohol. Fortunately, 

 it was in good histological condition, and sections prepared from it 

 showed very clearly the finer structure of the eyes. My observations, 

 as the following account will show, differ in no very important respects 

 from those of Beddard and Watase. 



Although Patten's generalization, that a corneal hypodermis was to be 

 found in the compound eyes of all Crustaceans, led Beddard ('88, p. 447) 

 to look for it in Serolis, he was not able to identify it. Watase ('90, 

 pp. 290 and 293) was more fortunate, and succeeded in finding under 

 each facet two cells in the corneal hypodermis. I have not been as 

 successful as Watase was in determining the exact number of hypo- 

 dermal cells in an ommatidium, but I have seen enough to convince me 

 that such cells are present. In sections approximately tangential to the 

 external face of the adult retina, one occasionally finds nuclei (Plate 

 VI. Fig. 60, nl. crn.) between the distal ends of the cone cells and the 

 corneal cuticula. These represent unquestionably the cells of the cor- 

 neal hypodermis, and are not to lie confused with the nuclei of the cone 

 cells, which lie in a deeper plane. In making sections, the corneal 

 cuticula splintered so irregularly that the tissue immediately below it 

 was completely disarranged. It was therefore possible to get only ir- 

 regular fragments of the tissue in this region, such as Figure 00 shows, 

 and these fragments were always too small to admit of an accurate 

 determination of the number of hypodermal cells under a single facet. 

 I have also been equally unsuccessful in my attempts either to isolate 

 these cells or to study them in situ on the corneal cuticula. 



The eyes in the adinu, owing to the thickness of the cuticula, are 

 unfavorable for the study of the corneal hypodermis ; but in embryos of 



