240 EVENINGS AT THE SnCEOSCOPE. 



pared with the compound eyes of insects ; the anterior 

 of these has nsiially a downward aspect, whilst the 

 posterior looks backwards ; tlie variety in the arrange- 

 ment of the ocelli of Spiders always bears a constant 

 relation to the general conformation and habits of the 

 species. Dujes has observed that those Spiders which 

 hide in tubes or lurk in obscure retreats, either under- 

 ground or in the holes or fissures of walls or rocks, 

 from which they only emerge to seize a passing prey, 

 have their eyes aggregated in a close group in tlie middle 

 of the forehead, as in the Bird-spider, the Clotho, &c. 

 The Spiders which inhabit short tubes terminated by a 

 large web exposed to the open air, have the eyes sep- 

 arated, and more spread upon the front of the cephalo- 

 thorax. Tliose Spiders which rest in the centre of a 

 free web, and along which they frequently traverse, 

 have the eyes supported on slight prominences which 

 permit a greater divergence of their axes ; this struc- 

 ture is well marked in the genus Tfiomisa, the species 

 of wliicli lie in ambuscade in flowers. Lastlv, the 

 spiders called Errantes, or wanderers, have their eyes 

 still more scattered, the lateral ones being placed at the 

 margins of the cephalo-thorax." * 



The shining hemisphere (or nearly a sphere) is in 

 each case covered with a thick cornea, a continuation 

 of the skin, perfectly transparent, and throwing ofl' its 

 outer coats successively in the process of moulting, 

 like that of the rest of the body. Tlie centre of its 

 inner surface is deeply excavated for tlie reception of a 

 crystalline lens, which is globular in form, and which 

 rests behind on the front surface of a hemispherical vit- 

 reous body, without sinking into it. The space between 



* Comp. Anat. (Ed. 2) p. 451. 



