MULTICELLULAR ANIMALS 



225 



in Man is much less than in many other vertebrates. The taste buds 

 are located chiefly on the tongue in the human adult but in the 

 very young they are also found elsewhere in the mouth. 



Light Receptors. The receptfon of a stimulus by light pre- 

 supposes the presence in the animal of some sort of substance that 

 is light-sensitive, affected chemically by light rays. In certain Proto- 

 zoa, for example euglena, the light-sensitive substance is localized 

 in a visible spot, the stigma, seen as a pink body near the reservoir 

 (Fig. 29). This spot must not be regarded as an eye similar to the 



PIGMENT 



NEftVE FIBERS 



COffNEA 



CRYSTALLINE 

 CONE 



TAPETUM CELL 



Fig. 153. — Diagram of the compound e^^e of an insect. (Partly after Kiihn: Grun- 

 driss der cdlgemeinen Zoologie. Georg Theime, Leipzig.) 



image-forming structures of higher animals, but merely as a region 

 of light sensitivity. In the eyes of invertebrates the light-sensitive 

 substances are located in association with the endings of nerves. In 

 some forms structures occur that have the properties of convex 

 lenses and converge light on these endings. In insects and other 

 Arthropoda the eyes are compound, consisting of a considerable 

 number of tubes shaped like inverted cones. In the apex of each 

 cone is a nerve ending, with associated light-sensitive structures; the 

 base of each cone contains a lens which serves to converge light 

 rays on the apex (Fig. 153). It is said that this type of eye, by 

 reason of the mosaic image which it forms, is peculiarly suited to 

 the detection of movement. 



