THE NERVOUS SYSTEM. 



181 



Going higher, we find a lens introduced forming a dis- 

 tinct image. The Snail, for example, has two simple eyes, 

 called ocellij mounted on the tip of its long tentacles, con- 

 sisting of a globular lens, 94 

 with a transparent skin 

 (cornea) in front, and a 

 colored 

 membrane 

 (ch oroid) 

 and a ner- 

 vous n et- 



FIG. 153. Eye of work (reti- 

 Pecten,muchen- 



larged: w.mouth; na) behind. 



1. lens; r, retina rrn o n 

 and choroid;n, The Scallop 



organs are the only eyes 

 possessed by Myriapods, 

 Spiders, Scorpions, and 

 Caterpillars. Adult In- 



such eyes in the edge of _ 



J . FIG. 154. Head of a Snail bisected, showing 



its mailtle (Fig. 153). Sucll structure of tentacles: a, right inferior ten- 



tacle retracted within the body ; b, right su- 

 perior tentacle fully protruded ; c, left supe- 

 rior tentacle partially inverted ; d, left inferi- 

 or tentacle ; /, optic nerve ; g, retractor mus- 

 cle ; h, optic nerve in loose folds ; i, retractor 

 nmscle of head ; k, nerve and muscle of left 

 inferior tentacle ; I, m, nervous collar. 



sects usually have three ocelli on the top of the head. 

 But the proper visual organs of Lobsters, Crabs, and In- 

 sects are two compound eyes, perched 

 on pedestals, or fixed on the sides of 

 the head. They consist of an immense 

 number of ocelli pressed together so 

 that they take an angular form four- 

 sided in Crustacea, six-sided in Insects. 

 They form two rounded protuberances 

 variously colored white, yellow, red, FIG. 155. Head of the Bee, 

 green, purple, brown, or black. Under theThriTeiTo^tem- 

 the microscope, the surface is seen to mata ' and the anteuuffi - 

 be divided into a host of facets, 95 each being an ocellus 

 complete in itself. Each cornea is convex on one side, 



