THE NERVOUS SYSTEM 



391 



external. Birds have, besides, an auditory passage, open- 

 ing on a level with the surface of the head, and sur- 

 rounded by a circle of feathers. Most mammals have 

 an external ear. 



Sight is the perception of light. 147 In all animals it 

 depends upon the peculiar sensitiveness of the optic 

 organ to the luminous vibrations. In vertebrates the 

 optic nerve comes from the middle mass of the brain, 

 in invertebrates it is derived from a ganglion. Many 

 animals are utterly destitute of visual organs, as the 

 Protozoa, and the lower radiates and mollusks, besides 

 intestinal worms and the blind fishes and many cave- 

 animals ; but the protozoan Euglena has a red pigment 

 spot which is probably affected by light waves in a 

 manner different from that in which the rest of the body 

 is influenced. The eyes of the starfish are at the tips 

 of its arms or rays. Those of the sea urchin form a ring 

 at the dorsal pole of the body. Around the margin of 

 the jellyfish are colored spots, supposed to be rudimen- 

 tary eyes ; but, as a lens is wanting, there is no image ; 

 so that the creature can merely distinguish light from 

 darkness and color without form. Such an eye is 

 nothing but a collection of pigment granules on the 

 expansion of a nervous thread, and the 

 perception of light is probably the sensa- 

 tion of warmth, the pigment absorbing 

 the rays and converting them into heat. 



Going higher, we find a lens introduced, 

 forming a distinct image. The snail, for FIG. 35 o. E ye of 



, , . , 11 i Pecten, much en- 



example, has two simple eyes, called i arge d : m > mantle; 

 ocelli, mounted on the tip of its long !J ens jJjf 

 tentacles, each consisting of a globular nerve - 

 lens, 148 with a transparent skin (cornea) in front, and 

 a colored membrane (choroid) and a nervous network 

 (retina) behind. The scallop (Pecten) has such eyes in 



