The Internal Anatomy of Spiders 



nu. 



The cornea is moulted and renewed in the same manner as 

 other portions of the cuticula of the body-wall, and at the same 

 time. 



The corneal hypodermis. — Immediately beneath the cornea 

 is a layer of cells (Fig. 167, c. hy) which is continuous with the 

 hypodermis of the body-wall, and which is merely a more or less 

 modified portion of the hypodermis. 



Several different names have been applied to this part. It 

 has been termed the vitreous layer, or the vitreum, a term suggested 



by its analogy with the vitreous hu- 

 mour of the eye of vertebrates; the 

 lentigen, because it produces the lens; 

 and the corneal hypodermis, a self- 

 explanatory term. I prefer the last 

 name notwithstanding its greater length. 

 The retina. — Lying immediately be- 

 hind the corneal hypodermis, is a layer 

 composed of highly specialized visual 

 cells; this is the retina (Fig. 167, r). 



Each visual cell consists of several 

 parts; there can be distinguished a cell- 

 body, in which there is a nucleus 

 (Fig. 168, nu); the cell-body is con- 

 nected with the central nervous system 

 by means of a nerve-fibre (Fig. 168, nf); 

 and there is also a hard structure, which is known as the 

 optic rod (Fig. 168, rd). 



The optic rod is the distinctively characteristic feature of 

 a visual cell. The form of this part varies in different eyes, 

 and, sometimes in different parts of the same eye; it is commonly 

 double, occupying two faces of one end of the cell with a thin 

 layer of the cell-body extending between the two elements, but 

 sometimes the two parts are united. 



As the optic rods of adjacent visual cells are placed side 

 by side, there is a well-marked layer of the retina occupied by the 

 rods; but the position of this layer is not always the same. Two 

 types of eyes are recognized which are distinguished by the 

 position of the nuclei of the retina with reference to the position 

 of the layer of optic rods. In one type the nuclei lie in front of 

 the optic rods or bacilli, as the rods are often termed (Fig. 169); 



162 



/. 2. 



Fig. 168. DIAGRAMS OF TWO 



VISUAL CELLS 



1, postbacillar 2, prebacillar 



nu nucleus 

 nf, nerve fibre rd, optic rod 



