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 ocelli, 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 net- 

 work (reti- 



Fig. 153. — Eye of 

 Pecten, much en- 

 larged: w,moutn; na) behind 



I, lens ; r, retina 

 and choroid ; n, 

 nerve. 



Fig. 154. — Head of a Snail bisected, showing 

 structure of tentacles : a, right inferior ten- 

 tacle retracted within the body; &, right su- 

 perior tentacle fully protruded ; c, left supe- 

 rior tentacle partially inverted ; left inferi- 

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

 cle; h, optic nerve in loose folds ; retractor 

 muscle of head ; k, nerve and muscle of left 

 inferior tentacle ; I, m, nervous collar. 



The Scallop 

 (Pecten) has 

 such eyes in the edge of 

 its mantle (Fig. 153). Such 

 organs are the only eyes 

 possessed by Myriapods, 

 Spiders, Scorpions, and 

 Caterpillars. Adult In- 

 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, 



-1 1 1 i i tt i showingcompoundeyes, 



green, purple, brown, or black. U nder the three ocelli, or stem- 



the micrOSCOpe, the Surface is Seen tO ^ta, and the antennas. 



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

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



