4 POSSIBLE MODES OF ORIGIN. 



cells. Such a body is known as an otolithe. On the 

 other hand, it might serve as a lens, and by condensing 



Fig. 3.— Diagram of origin of a sense-organ, c, Cuticle ; h, hypoderm ; * n, nerve. 



the light would act like a burning-glass, and increase 

 its effect on the cells below. A further stage would be 



Fig. 4.— Diagram of further stage in tlie origin of a sense-organ. 



that the immediately subjacent cells, acted on by the 

 increased stimulus, might (Figs. 3 and 4) develop into 

 special nerve-tissue. 



* I.e. the cellular layer below the cuticle. 



