214 



THE CRAYFISH. PHYLUM ARTHROPODA 



joints of the antennules and provided with nerves. Each has a 

 cuticular lining beset with hairs, with which the nerve fibres 



'^-^''^- c2r/. 



B 



omm. 



f^P-g- 



-mus. 



op.n. 



cnc— 



P9— 



pr--- 



D 



)r,tt- 



iy^rh. 



n. f. 



Fig. 145. — The eye of the crayfish. 



lA, The left eye removed ; B, a portion of the cornea magnified to show the facets ; C, a longitudinal section 

 of the eye under low magnification ; D, a single omraatidium highly magnified. — D after Parker. 



cr.c, Outer refractive body or crystalline cone ; cu.f., cuticular facet ; epid., epidermis (hypodermis) ; 

 mus., muscles which move the eye ; n.f., nerve fibres ; omm., ommatidia ; op.g.. optic ganglion ; 

 op.n., optic nerve ; p.g., outer pigment cells ; p.g'., inner pigment cells ; ret., retinula cells (the sense 

 cells) — these cells contain pigment ; rh., inner refractive body or rhabdome secreted by the retinulae ; 

 vt., vitrellx or cells which secrete the crystaUine cone. 



are in communication. Within it are grains of sand, which are 



scattered over the opening of the sac by the pincers and fall into 



it. It is probable that the principal function of the organ is inform- 



the animal of its position by the movements of the sand grains 



