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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 ccelenterates 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, Epidermh; /, nerve fibre; I, lens; op, optic nerve; 

 p, pigment cells; r, 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 



