SPIDERS AND MITES 239 



underground 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 the middle of the 

 forehead, as in the Bird-spider, the Clotho, etc. The Spi- 

 ders which inhabit short tubes, terminated by a large web 

 exposed to the open air, have the eyes separated, and more 

 spread upon the front of the cephalo- thorax. Those Spi- 

 ders 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 structure is well marked in the genus 

 Thomisa, the species of which lie in ambuscade in flowers. 

 Lastly, the spiders called Errantes, or wanderers, have 

 their eyes still more scattered, the lateral ones being 

 placed at the margins of the cephalo-thorax. " 1 



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 off its outer coats 

 successively in the process of moulting, like that of the 

 rest of the body. The centre of its inner surface is deeply 

 excavated for the reception of a crystalline lens, which is 

 globular in form, and which rests behind on the front sur- 

 face of a hemispherical vitreous body, without sinking into 

 it. The space between this body and the side of the lens 

 forms a ring-like channel which is filled with an aqueous 

 humor, and into this projects a circular process of the 

 thick pigment-coat, which corresponds to the choroid, thus 

 defining the pupil of the eye, and , at the same time con- 

 fining the lens to its proper situation. The margin of this 

 pigment-ring may be considered as an iris, and is of vari- 



1 "Comp. Anat." (Ed. 2), p. 451. 



