VISUAL TISSUES 



239 



are well demonstrated by the ocellus of the orthopterous insect, Perla 

 bicaudata, which is represented in axial section by Figure 211. 



The eye of Limulus is a simple form which shows an approach to 

 the crustacean type. 

 It is formed, much as 

 was the ocellus of Dy- 

 tiscus, from a surface 

 area of some extent 

 whose epithelium has 

 been invaginated at 

 numerous, regularly 

 placed points into 

 cup-like depressions. 

 The lower surface of 

 the cuticle, which is 

 formed in Limulus as 

 in other Arthropoda, 

 all over the surface of 

 the body, fits into 



vis. c." 

 gn. c. 



ep 



rh. 



phemus. Two of the numerous ocelli are shown, one in me- 

 dian axial section (^4); the other by a lateral surface view 

 (B). cu., cuticle, which forms a partial lens over each ocellus; 

 ep., epidermis which secretes the cuticle and from some of whose 

 cells the visual cells (vis.c.) and the ganglion cells (gn.c.) were 

 formed; rh., rhabdome; nv.f., nerve fiber. (After WATASE.) 



these depressions, 

 while the upper sur- 

 face is even and con- 

 tinuous and shows 



Scarcely any effects of FlG 2I2 . _p ar t of an axial section of the eye of Limulus poly- 



the invaginations be- 

 neath. It thus forms 

 a lens. 



In the bottom of 

 each epithelial cup a 

 group of some fifteen to twenty cells of the layer are considerably 

 enlarged and grouped together to form a melon-shaped body, which 

 is the retinal portion of the organ. The cells are now differentiated 

 into visual nerve cells or retinulae by the development of a lateral 

 rhabdome edge and production of the proximal cell body' into an 

 efferent nerve fiber (Fig. 212). 



It is in the spider that one of the most interesting visual conditions 

 is found. This animal possesses many eyes on its dorsal surface, and 

 both direct and inverted eyes can be found among them. Figure 213 

 shows a picture of a section taken vertically through two of the eyes 

 of Epeira diadema. To the left is a direct eye called the principal eye, 

 while to the right is the accessory eye, which is inverted. Each of these 

 eyes was derived from an epidermal invagination which, later, was 

 differentiated from the other. Both of these eyes consist of a simple 



