MUSEUM OF COMPARATIVE ZOOLOGY. 197 



scorpions is unquestionable, yet whether the lateral eyes, like the me- 

 dian, have been formed by an involution vs^ith inversion, or whether 

 their formation is accompanied simply by a thickening and more or 

 less extensive depression in the hypodermis, is still an open question. 

 Graber's figure ('79, PI. V. fig. 4), with its preretinal membrane and 

 lentigen, would indicate that the eye arose by involution. Lankester and 

 Bourne's figures ('83, PI. X. figs. 2, 3, and 4), in which these structures 

 are absent, would favoi* ^he explanation that the eye is only a hypodermal 

 thickening. 



The position of the lateral eyes in scorpions has already been described. 

 In the adult Centrums each group consists of four eyes, three of which 

 are large and are designated by systematists as " principal " eyes, and 

 the fourth is small and known as an " accessory " eye. The larger eyes 

 are arranged in a horizontal line at the antero-lateral angle of the shield ; 

 the small eye is above a point midwav between the posterior and middle 

 larger eyes. 



A vertical section through the axis of one of the larger eyes (PI. III. 

 fig. 18) shows at the surface a strongly convex lens (Ins.) beneath which 

 a relatively small retina (r.) appears. The outline of the latter is marked 

 by the basement membrane (mb.), and on its dorsal and ventral edges it 

 is seen to be continuous with the hypodermis [hd.). In an eye from 

 which the pigment has not yet been removed, the whole retina is 

 intensely black. The pigment extends up to the margin of the lens, as 

 figured by Lankester and Bourne ('83, PI. X. fig, 1), and spreads out 

 above and below into the adjacent hypodermis. It is hr more abundant 

 in the dorsal hypodermis than in the ventral. 



The lens in the adult eye consists of essentially the same parts as in 

 the median eye, and contains no pore-canals. Its substance except the 

 front hyaline layer is stained throughout by alcoholic borax-carmine. In 

 young individuals (PI. III. fig. 21) the lenses of the lateral eyes, even 

 better than those of the median eyes, show a formation of stainable 

 cuticula (II"') under the hyaline layer (Jl) before a similar secretion has 

 taken place from the general hypodermis. 



In the adult eye not the least appearance of a lentigen or preretinal 

 membrane is to be found, even after careful depigmentation. Tlie fact 

 that the pigmentiferous tissue extends up to the lens is of itself sug- 

 gestive of the absence of a lentigen, for in ocelli generally this layer is 

 remarkable for its transparency. When to this is added the fact, that no 

 nuclei exist in the front part of the eye, and that in no place does the 

 basement membrane extend as a preretinal membrane across the front of 



