THE HIGHER ORGANISMS 



159 



so simple that it is difficult to account for all that is 

 accomplished, it already has discriminative powers. 

 Useful objects when touched are apprehended by the 

 tentacles of the ccelenterates, indifferent objects are 

 neglected, harmful objects may be avoided. 



So soon as a central nervous system appears, nerve 





FIG. 60. Diagrams showing some of the stages in the increasing complexity 

 of the simple eye in invertebrates. A, Simple pigment spot in epithelium have 

 nerve-endings associated with pigment cells (as in some medusae); B, pigment 

 cells in a pit-like depression (as in Patella) ; C, with pin-hole opening and vitreous 

 humor in cavity (as in Trochus) ; D, completely closed pit, with lens and cornea 

 (as in Triton and many other Mollusks) ; E, pigment area elevated instead of 

 depressed lens of thickened cuticula (as in the medusa, Lizzia); F, retinal cells 

 more highly magnified, ep, Epidermis; /, nerve fibre; Z, lens; op, optic nerve; 

 2?, pigment cells; r t retina; v.h, vitreous humor. (Galloway.) 



fibres connect the sensitive cells upon the surface with 

 controlling centres, and more exact conduction and 

 distribution of impressions become possible. The per- 

 ipheral apparatus continues to specialize until the sense 

 of contact is capable of differentiation from the sense 

 of harmful contact or pain, and eventually into a great 

 variety of impressions, pleasurable, painful, thermic, 



