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 

 tunensis, 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 

 terrestris, 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. 42].t 



* 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. 



