VISION. 43;j 



row, or form a triangle. Their structure lias been 

 minutely examined by Professor Muller, who found 

 them to contain a hard and spherical crystalline 

 lens, a vitreous humour, and a choroid coat, with its 

 accompanying black pigment; the whole being 

 covered externally by a convex cornea. The stem- 

 mata of a caterpillar, which has eight of these 

 eyes, are shown in Fig. 41.9, connected together by 



a circular choroid membrane (x x) common to the 

 whole ; together with the separate branches (o o) of 

 the optic nerve (n), belonging to each. 



All the Arachnida possess eyes of this latter 

 description ; and from their greater size afford faci- 

 lities for dissection, which are not met with among 

 proper insects. Their number in Spiders is gene- 

 rally eight ; and they are disposed with great sym- 

 metry 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, dis- 

 sected 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 crystal- 

 line lens (l) ; the choroid coat with its pigment 

 (x),* forming a wide opening, or pupil ; the vitreous 



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

 sects which he examined contain a thin choroid, having a silvery 

 lustre, as if intended as a reflector of the light which falls on it. 



VOL. II. F F 



