TYPE ARACHNIDA. 



439 



cup-shaped structures, the cavity of the cup being filled by 

 the cuticular cornea and its wall transformed into the retina, 

 which is continuous at the margins of the cup with the gen- 

 eral hypodermis. The retinal cells are of two kinds, viz., 

 large sensory cells and smaller interstitial cells. Each sen- 

 sory cell is surrounded by pigment and bears upon its lateral 

 walls a chitinous secretion which, with the corresponding 

 secretion of contiguous cells, forms a rhabdom. The nuclei 

 of the cells are situated nearer their inner than their outer 

 ends, and behind them in Euscorpius highly refractive spheres 

 occur imbedded in the cells, constituting what have been 

 termed the phaospheres. Upon its inner wall the retina is 

 lined by a basement-membrane continuous with that lying 

 below the general hypodermis and perforated by the fibres of 

 the optic nerves which come into contact with the inner ends 

 of the sensory cells. 



In the Spiders, in which there are six or eight eyes arranged 

 upon the dorsum and sides of the cephalothorax, the ante- 

 rior dorsal pair differs in structure from the remaining ones. 

 In both forms of eyes (Fig. 200) the cuticular cornea (c) rests 



_V 



B 



FIG. 200. EYES OF SPIDEK. A, ANTERIOR, AND B, POSTERIOR EYE (after 



BKRTKAU from KORSCHELT and HEIDER). 

 b = rods. r = retina. 



I = lens. t = tapetum lucidum. 



v = vitreous layer. 



upon a corneal hypodermis, (the vitreous cells, v\ but the 

 arrangement of the retina differs greatly. In the anterior 

 dorsal pair (A] it is composed of a layer of elongated cells (r) 

 whose nuclei are situated towards their inner ends, while near 



