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,' 4 

 with a transparent skin 

 (cornea) in front, and a 



colored 



membrane 



(choroid) 



and a ner- 



vous n e t- 

 IG. 153. Eye of work (reti- 



Pecteu,tn uchen- _ 



larged: m, month; Iia) behind. 



l t lens; r, retina m\ 



and choroid; n, 1 he 



such eyes in the edge of FIG 1M _ Head of a Snail b .^. ted showiug 



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



tacle retracted within the body; b, right su- 



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



nosspssed hv Mvriannfk nor tentacle partially inverted ; d, left inferi- 



Dy JilJ r 1 apoas, or tentac]e . ^ optic nerve . ^ re tractor mus- 



SpiderS, Scorpions, and cle; A, optic nerve in loose folds;*, retractor 



muscle of head ; A;, nerve and muscle of left 

 Caterpillars. Adult In- inferior tentacle ; Z, 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, 



T , ill TT j showinsrcompoundeves, 



green, purple, brown, or black. L nder the three ceiii, or st'em' 



the microscope, the surface is seen to Magnified. the anteun8e ' 

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

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



