4 POSSIBLE MODES OF ORIGIN. 



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

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



c 



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



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

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



Fig. i — Diagram of furllier stage in the 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. tlie cellular Iiijer below tlie cuticle. 



