70 



ARACHNIDA. 



the form of a pit, filled by the lens ; the floor of this pit (the retina) 

 being directly continuous with the hypodermis. Although this eye 

 possesses, in the above features, the principal characters of a simple 

 eye, we are not able to regard it as such, on the one hand, because of 

 its detailed structure, and, on the other, because of the striking 

 agreement existing between the eye of Scorpio and that of Limulus. 

 The lateral eyes of the latter are unilaminar, as are those of Scorpio r 

 while the median eyes in both forms consist of two or three layers. 

 The lateral eyes of the Scorpiones, as well as those of Limulus, 

 develop in the form of simple depressions ; the median eyes, on 

 the contrary, have a complicated method of development, which. 

 apparently is very similar in the two forms. 



TOO 



Fig. 37.— Three ommatidia of the lateral eye of Limulus (after Watase). A is a me'ian 

 longitudinal section of the retinula, B and C show the retinulae in surface-view, c, central 

 ganglion-cell ; ch, chitinous cuticle ; hyp, hypodermis ; I, lenticular cone ; mes, mesodermal 

 tissue ; n, nerve ; rh, rhabdom ; rt, retinula. 



The lateral eyes of Limulus consist of a number of retinulae, each 

 provided with a corneal lens (Fig. 37). The retinulae are continuous 

 with the hypodermis, and each group might be regarded as a single 

 eye, which has arisen in the way described above through the sim- 

 plification of an ocellusdike eye. The lenses of the single eyes are 

 only distinct from one another internally (Fig. 37, /) ; externally, 

 they appear fused (ch). "We might suppose that this process of 



