158 



BIOLOGY: GENERAL AND MEDICAL 



labor begins, and it becomes necessary for a more elabo- 

 rate response to follow certain stimuli than could be 

 effected by cells acting individually. Moreover, certain 

 cells are so situated as to be, above their fellows, suscep- 

 tible to external agents, so that we need only ascend to 

 the coelenterates to find the ectodermal cells more sensi- 

 tive than others, and to find a mechanism by which the 

 external impressions are communicated to groups of 

 cells, by which they are to be utilized, through inter- 

 mediate nerve cells. Though the sensory apparatus is 



FIG. 59._ 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; I, lens; op, optic nerve; 

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



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 



