392 



COMPARATIVE ZOOLOGY 



the edge of its mantle (Fig. 350). Such organs are the 

 only eyes possessed by myriapods, spiders, scorpions, 



and caterpillars. Adult 

 insects usually have three 

 ocelli on the top of the 

 head. But the proper 

 visual organs of lobsters, 

 crabs, and insects are two 

 compound eyes, perched 

 on pedestals, or fixed on 

 the sides of the head. 

 They consist of an im- 

 mense number of ocelli 

 pressed together so that 

 they take an angular 



FIG. 351. Head of a Snail bisected, showing form foUT-Sided in 



structure of tentacles: a, right inferior tentacle . . , , . 



retracted within the body; b, right superior CrUStaCCa, SIX-Slded in 



tentacle fully protruded ; c, left superior ten- .-j-%1 r 



tacle partially inverted; rf, left inferior ten- IHSCCtS. They form tWO 



tacle;/, optic nerve ; g retractor muscle; roim( J e( J protuberances 

 ft, optic nerve in loose folds; /, retractor muscle 



of head ; k, nerve and muscle of left inferior Variously Colored 



tentacle; /, m, nervous collar. . . 



white, yellow, red, green, 



purple, brown, or black. Under the microscope, the 

 surface is seen to be divided into a host of facets, 149 

 each being an ocellus complete in itself. Each cornea 

 is convex on one side, and either convex or flat on the 

 other, so that it produces a focus like 

 a lens. Behind the cornea, or lens, is 

 the pigment, having a minute aperture 

 or " pupil." Next is a conical tube 

 one for each facet with sides and 

 bottom lined with pigment. These 

 tubes converge to the optic ganglion, 

 the fibers of which pass through the FIG. 352. -Head of the Bee. 



_ showing compound eyes, 



tubes tO the COrnea. 150 Vision by the three ocelli, or stem- 

 , , . mata, and the antennae 



such a compound eye is not a mosaic ; (magnified). 



