SPIDERS AND MITES. 209 



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 sides of the lens 

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

 humour, 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 

 various colours, as red, green, or brown, in those species 

 which are active by daylight, while it is black at the back 

 of the eye. The nocturnal species have no dark pigments, 

 but are furnished with a curtain (tapetum), which reflects 

 a brilliant metallic lustre, and makes the eves of these 

 Spiders glare in the twilight, like those of cats. 



It will be interesting to compare with this range cf 

 eyes, the same organs in a kindred animal, the common 

 Harvestman (Phalanghnn cornutum). Here in the centre 

 of the cephalo-thorax rises a short pillar, which is 

 crowned with two rows of conical points, with polished 

 black tips. On each side of the pillar is a large black eye, 

 hemispherical in form and brilliantly glossy, exactly re- 

 sembling, indeed, those which we have just examined. 

 There are, however, only this single pair which thus look 

 out laterally, exactly like the eyes of Birds. There is, 

 indeed, a speck on each side of the thorax, considerably 

 removed from the eye-pillar, just above the origin of the 

 first pair of legs, which has been mistaken for an eye ; 

 'but it is truly a spiracle, or breathing-hole. 



There are many other points of interest about this Har- 

 vestman, such as the conical spines which stud the head, 



p 



