342 



THE SENSORIAL FUNCTIONS. 



a caterpillar, which has eight of these eyes, are shown in Fig. 

 419, connected together by a circular choroid membrane 



o 



o o 



e o o 



o o o o 



o o e o o 



O o o e o o 



O o o o o O o 



(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 facilities for dissection, 

 which are not met with among proper insects. Their num- 

 ber 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 dis- 

 play its internal parts; in which are seen the cornea (c,) de- 

 rived from an extension of the integument (i;) the dense 

 spherical crystalline lens (l;) the choroid coat with its pig- 

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

 mour (v,) covered behind 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 Myriapo- 

 da: 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 terrestriSi or common 

 Millepede, these eyes, amounting to 28, form a triangle, be- 



• Marcel dc Serres states, that some of the stemmata of the insects which 

 he examined contain a thin choroid, having a silvery lustre, as if intended as 

 a reflector of the light which falls on it. 



