86 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



of perfection. In the simplest cases they are known as eye-spots, and 

 consist of irritable protoplasm, i.e., nervous substance, containing pig- 

 ment granules ; and in this form they are perhaps scarcely capable 

 of distinguishing light from darkness, but are only susceptible to the 

 warm rays. It is hardly possible to conceive that pigment is indis- 

 pensable for the sensation of light, because there are many eyes of 

 complicated structure from which pigment may be altogether absent. 

 The view, however, according to which the pigment itself is sensitive 

 to light, i.e., is chemically changed by the light waves and transmits 

 the excitation produced by these movements to the protoplasm or 



fz 



Ot 



Wz 



FIG. 83 Auditory vesicle of a Heteropod (Pterotrachea). Jf, acoustic nerve ; Of, otolith 

 the fluid of the vesicle ; Wz, ciliated cells on the inner wall of the vesicle ; Hz, auditory 

 cells ; Cz, central cell. 



the adjacent nervous substance cannot in itself be contradicted, but 

 it is by no means clear that such changes are produced by the light 

 rays as opposed to the heat rays. Of greater importance in this 

 relation appears the special nature of the nerve endings, through 

 which certain movements, progressing in regular waves, the so-called 

 ether waves, are transmitted to the nerve fibres and give rise to a 

 stimulus which travels to the central organ and is by it perceived 

 as light. In all cases in which in the lower animals specific nerve 

 endings cannot be made out, we have probably only to do with a 

 forerunner of the eye, consisting merely of the pigmented termina- 



