esterly: eucalanus. 31 



cases where a nerve fibril cannot be followed continuously throughout 

 the cell, the enlargements at the ends may be seen around the margins 

 of the spaces (vacuoles) surrounding the interior bodies (Plate 5, Fig. 

 50). For a preparation stained in vom Rath's mixture, Figure 17 

 (Plate 2) is extremely suggestive of a continuity between nerve fibres 

 and interior bodies, though such a continuity can not be directly seen. 



The arrangement of the interior bodies with regard to the long axes 

 of the cells corresponds, in general, to the course of the nerve fibrils 

 in the cells. This is shown in Figures 49 and 50 (Plate 5). Further- 

 more, the nerves which in Figures 2 and 5 (Plate 1) are shown continu- 

 ing between the lateral eyes would, as fibrils within the cells, have the 

 general direction of the isolated interior bodies, or the long rows of 

 them (cf. Plate 5, Figs. 46 and 48 with Plate 1, Figs. 2 and 5). 



It seems to me that these facts all point strongly toward an intimate 

 structural relationship between the interior bodies and the nerves of 

 the eye, and certainly suggest that the former are functional parts of 

 the visual cells as such. 



j. Parts of the Eye in their Relation to the Hypodermis. — So far 

 we have considered the median eye merely in its anatomical relations 

 without making a comj)arison between it as a type and other charac- 

 teristic eyes of Crustacea. Parker ('91, p. 47) states that in Crustacea 

 "at least three types of retinal structure can be distinguished," de- 

 pending upon the final form assumed by the retina in its development 

 from a simple "thickening in the superficial ectoderm." The first 

 type is found in Decapoda, Schizopoda, Stomatopoda, Isopoda, 

 Nebalia and the Branchiopodidae, and is merely a thickening of the 

 hypodermis, which retains a superficial position permanently. The 

 retina is directly continuous at its edges with the h^qiodermis. 



The second type is found in the Apusidae, Estheridae and Clado- 

 cera. This type is distinguished from the first by the fact that the 

 retina comes to lie beneath a fold of integument, instead of remaining 

 permanently at the surface of the body. In the Estheridae, as repre- 

 sented h\ Limnadia, the eye lies in a pocket which communicates 

 with the exterior by means of a pore. In the Cladocera this pocket 

 becomes closed and partly obliterated, so that the retina is then not 

 continuous with the ectoderm. The right and left retinas may re- 

 main separate (Apusidae), lie close together (Estheridae), or fuse 

 (Cladocera). The three groups in which the second t^'pe of retinal 

 structure is foimd represented form a natural series, beginning with the 

 Apusidae and extending through the Cladocera. 



The third type of retinal structure is found in Amphipods and 



