> • VISION. 485 



facilities for dissection, which are not met with 

 among proper insects. Their number in Spiders 

 is generally eight; and they are disposed with 

 great symmetry on the upper side of the head. 

 Fig. 420 represents, on a magnified scale, one of 

 the large stemmata, on the head of the Scorpio 

 tuiiensis, dissected so as to display its internal 

 parts; in which are seen the cornea (c), derived 

 from an extension of the integument (i) ; the 

 dense spherical crystalline lens (l) ; the choroid 

 coat, with its pigment (x),* forming a wide open- 

 ing, or pupil ; the vitreous humour (v), covered 

 behind by the retina (r), which is closely ap- 

 plied to it ; and the optic nerve (o), with which 

 the retina is continuous. 



Examples of the conglomerate eye occur in 

 the Myriapoda : in the Scolopendra, for instance, 

 they consist of about twenty contiguous circular 

 pellucid lenses, arranged in five lines, with one 

 larger eye behind the rest, which Kirby com- 

 pares to a sentinel, or scout, placed at some little 

 distance from the main body. In the Julus 

 terrestriSf or common Millepede, these eyes, 

 amounting to 28, form a triangle, being disposed 

 in seven rows ; the number in each regularly 

 diminishing from the base to the apex ; an 

 arrangement which is shown in Fig. 421. -f 



* Marcel de Serres states, that some of the stemmata of the 

 insects which he examined contain a thin choroid, having a sil- 

 very lustre, as if intended as a reflector of the light which falls 

 on it. 



t Kirby and Spence's Introduction, &c., iii. 494. 



