16 ARACHNIDA. 



retinal cells (Mark, Parker). If the above is really the case, these 

 processes are of great histological interest, but these changes need 

 further investigation before they can be regarded as fully established. 

 From a theoretical standpoint, the account just given of the optic 

 nerves is highly suggestive. The rudiment of the eye appears as 

 an invagination, and we should expect that, by the closing of this 

 invagination, the lens and the vitreous body would develop from the 

 outer, and the retina from the inner, wall of the optic pit. The 

 nerve would then join the posterior wall of the eye. This latter is 

 actually the case, but the lens and the vitreous body are formed 

 from a part of the hypodermis lying outside the invaginated area 

 (Fig. 10, A-C). A striking modification in the formation of the 

 eye has thus come about, the cause of which is as yet unknown. 

 The result of this modification is that the surface of the retina 

 which, at an early stage, was directed inwards, is now approximated 

 to the lentigen hypodermis (Fig. 10, B). With this portion of the 

 optic pit the nerve retains its primary connection. In order that 

 the eye may form according to this new method, however, the nerve 

 must shift from the original convex, lower surface to the primary 

 concave surface of the invagination (Fig. 10, B and 0). A portion 

 of the wall of the pit becomes, during this process, the post-retinal 

 layer, losing its sensory character. This layer must necessarily be 

 traversed by the nerve, as is actually the case in the adult animal 

 (Ray Lankester and Bourne, jS t o. 20). 



The lateral eyes arise much more simply than the median eyes. 

 At the time when these latter arise, two long pigmented areas of the 

 integument appear laterally and somewhat posteriorly to them ; 

 these are the rudiments of the lateral eyes. The hypodermis is 

 much thickened at these parts, and a number of depressions appear ; 

 there may be as many as five, but the number varies in the different 

 species of Scorpio (Fig. 1 1 A, II- V). Each of these pits gives rise 

 to an eye, which develops very simply in keeping with the simple 

 structure of the adult lateral eye (Parker). The greater part of the 

 invagination becomes the retina. When the lens is formed, the more 

 peripheral cells grow inward over the central portion (retina) and 

 secrete the lens, which now lies over the slightly depressed central 

 part (Fig. 11 B). In this latter, we now find differentiated retinal 

 cells and the intercalated pigment cells, while, laterally, this single 

 layer of cells passes gradually into the peripheral (lentigen) cells, 

 which, in their turn, are continued direct into the hypodermis 

 (Fig. 11 B). This continuous celldayer secretes posteriorly a cuti- 



