THE NERVOUS SYSTEM. 



181 



Fio. 153. — Kye of 

 Pecteii, mttchen- 

 lurged: m^mouth; 

 If leps; r, i-etina 

 and choL-oid; n, 

 nerve. 



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

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

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

 sisting of a globular lens," 

 with a transparent skin 

 (cornea) in front, and a 

 colored 

 membrane 

 (choroid) 

 and a ner- 

 vous n e t- 

 work (reti- 

 na) behind. 

 The Scallop 

 (Pecten) has 

 such eyes in the edge of 

 its mantle (Fig. 153). Sucli 

 orgiins are the only eyes 

 possessed by Myriapods, 

 Spiders, Scorpions, and 

 Caterpillars. Adult In- 

 sects usually have three ocelli on the top of tlie 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 

 nifmber of ocelli pressed together so 

 that they take an angular form — four- 

 sided in Crustacea, six-sided in Insects. 

 Tiiey form two rounded protuberances 

 variously colored — white, yellow, red, Fig.i.w.— Hena,oftheBee„ 



11 1 1 1 TT J shdwiugcompouiideTes,, 



green, purple, brown, or black. Under the three oceiii, or stem- 

 the microscope, the surface is seen to MSifled."""""""*' 

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

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



Pig. IM Ueiid of a Snail bisected, showing 



structure of teutacles: a, riglit inferior ten- 

 tacle retracted within the body ; b, right su- 

 perior tentacle fully protruded ; c, left supe- 

 rior tentacle parti.illy inverted ; d, left inferi- 

 or tentacle ; /, optic nerve ; g^ retractt)r muiS-- 

 cle; A, optic nerve in loose folds; 2, retractor' 

 njnscle of head ; fc, nerve and muscle of left 

 inferior tentacle ; i, w, nervous collar. 



