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, 84 

 with a transparent skin 

 (cornea) in front, and a 

 colored 

 membrane 

 ~ r J) (cboroid) 

 and a ner- 

 vous n e t- 



Fio.l53.-Eyeof wor k (reti- 

 Pecteu, much en- 

 larged: wi.mouth; Iia) behind. 



and n c 8 horoi r d e f ? The Scallop 

 nerve. (Pecten)l\&S 



such eyes in the edge of F[(j ]54 _ Head of a Snajl bisected showing 



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



. tacle retracted within the body; b, right sn- 



OrganS are the Only eyes perior tentacle fully protruded ; c, left supe- 



i^AQQ^eeprl hv Mvviannrls rior tentacle partially inverted ; d, left inferi- 



Dy iviy i lapoas, or tentacle . /t optic ne ,. ve . fft retractor mu8 . 

 Spiders, Scorpions, and cle ? * P" C u f ve t in lo 8e fo ' d8 ; ^ ^tractor 



r ' muscle of head ; k, nerve and muscle of left 



Caterpillars. Adult In- 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, 

 green, purple, brown, or black. Under 

 the microscope, the surface is seen to 

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

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



55. Head of the Bee, 

 showiiiirconipoundeyes, 

 the three ocelli, or stem- 

 mata, and the antennas. 



