E}-es of Molluscs and Arthropods. 595 



form auy couclusiou, as to what this appareutly iuteutioual exposure of 

 the red pigmeut to the light, signifies. 



The optic vesicle, with its eight layers, is coutained in the o mina- 

 te al sac. The anterior wall, the septum ofKiiOiiN, forms a stout and 

 elastic eushion or spring, upon which the lens rests. The septum is 

 formed of a double membrane ; the thickened and struetureless central 

 part of the outer layer, a little to one side of the optic axis, is perforated 

 by the ganglionic nerve brauch (fig. 19) ; the peripheral part of the sep- 

 tum, gradually diminishing in thickness toward the edge of the omma- 

 teal sac. consists of numerous connective tissue cells modiiied into 

 circular fibres. The nuclei of the cells are extremely difficult to find in 

 the adult condition, on account of their minute size, and flattened shape. 

 They are most easily seen in surface preparations of the membrane, 

 which may be isolated without special difficulty. In young eyes, the 

 nuclei of the septal membrane may be easily seen (fig. 10). The sheath 

 of the ganglionic brauch — according to Hensen — fuses with the septal 

 membrane; but my own observations indicate that it terminates when 

 the nerve leaves the connective tissue and enters the blood Spaces sur- 

 rounding the lens. The inner membrane of the septum is uniformly 

 thin aud struetureless. The ganglionic^branch forms a disc-like expan- 

 sion upon its outer surface , over which the nerve fibres radiate in all 

 directions. Toward the periphery, the nerve disc is so thin that the two 

 membranes become closely united, and finally fuse with each other. 



One may easily isolate this inner membrane, together with the disc- 

 like end of the ganglionic nerve. It may then be seen that the outer 

 «urface of the membrane is smooth, but that the inner one is covered 

 with the ends of the nerves which have penetrated the membrane. 

 Carrière has attempted to show that the tapetum is coutinuous with the 

 septum, to which he ascribes a cellular nature. The real nuclei of the 

 septum, he did not see, for it is evident, from his figure 80, that what he 

 considers to be the septum is composed of the septal membrane together 

 with the single layer of cells formed by the continuation toward the 

 periphery of the outer ganglionic layer ! 



The sclerotica, or inner wall of the ommateal sac, consists of 

 a tough. hyaline, connective tissue membrane, thickest in the median 

 parts opposite the pupil , whence it is continued , gradually decreasing 

 in thickness, to the periphery of the retina, there becoming continuous 

 with the septal membrane. The sclerotica has always been des- 

 cribed as a single layer. According to my own observations it is formed 

 ■of two layers, the inner of which is marked with short parallel cross 



40* 



