MUSEUM OF COMPARATIVE ZOOLOGY. 53 



The method by which the separation is here accomplished is not by 

 the closure of an involution, as in the second type, but by a process 

 the nature of which will be described in the following pages. The 

 third type is represented by the eyes in Amphipods, and possibly in 

 Copepods. 



Amphipoda. — The peculiar relation which the retina bears to the 

 hypodermis in Amphipods can be easily seen in Gammarus. In this 

 genus, as Carriere ('85, pp. 156-160) has clearly demonstrated, the 

 retina lies immediately below the hypodermis, and is separated from the 

 latter by a well defined structure, the corneo-conal membrane (Fig. 1, 

 mb. crncon.). This membrane, although visible with perfect clearness, 

 is nevertheless extremely delicate, and has the appearance of a single 

 lamella. I believe, however, that its structure is more complex, and 

 that it is composed of two very intimately united membranes, one of 

 which is produced by the retina, the other by the corneal hypodermis. 

 This belief is based upon the fact that at the edge of the retina the 

 apparently single membrane separates into what may be considered its 

 two constituents. One of these becomes the basement membrane of 

 the general hypodermis, and the other, which I have called the cap- 

 sular membrane, passes over the edge and proximal face of the retina, 

 and is finally reflected over the optic nerve (Fig. 1, mb. n. opt.). In 

 addition to the capsular membrane, the eye in Gammarus possesses 

 still another membrane (Fig. 1, mb. ba.). This is a delicate lamella, 

 which is approximately parallel to the deep face of the eye at a level 

 between the rhabdomes and retinular nuclei (compare Fig. 2), and which 

 consequently divides the space within the capsular membrane into two 

 chambers, a larger distal and a smaller proximal one. At its periphery 

 this intercepting membrane unites with the capsular membrane. 



The corneo-conal and capsular membranes in Gammarus show no evi- 

 dence of being perforated, but together constitute a closed capsule, which 

 separates the retina from all adjoining tissues except the optic nerve. 

 Both membranes are composed apparently of a homogeneous substance, 

 in which I have never been able to distinguish any trace of cells. It 

 is therefore probable that these membranes are cuticular. 



The intercepting membrane, unlike either the capsular or the corneo- 

 conal membrane, is pierced by a great number of holes, through which 

 the pi-oximal ends of the retinular cells project. This membrane, there- 

 fore, has the form of a mesh work. According to Carriere ('So, p. 158) 

 it is composed of numerous connective-tissue cells, but this statement 

 is not confirmed by my own observations. In depigmented sections of 



