354 ZOOLOGY. 



one direction, the opaque layers of the ball of the eye; (3) 

 an aperture for the passage of the light into the interior of 

 the box, the pupil and the transparent cornea overlying it ; and 

 especially (4) a lens or series of refracting surfaces which 

 cause all the rays of light coming to the eye from each external 

 point to be brought together again beyond the lens at a corre- 

 sponding point on the sensitive surface. The elementary rela- 

 tions of these parts as found in the eyes of vertebrates may be 

 gathered from a study of Fig. 172. 



Accommodation of the eye to objects at different distances 

 is effected by means of changes in the shape of the lens through 

 the action of appropriate muscles. 



368. Library Exercise. What portions of the vertebrate 

 eye are derived directly from the ectoderm? Which from the 

 brain (i. e., indirectly from ectoderm) ? Which from meso- 

 derm? (See Fig. 43.) 



What variation occurs among vertebrates as to the condi- 

 tion of the bones in the middle ear? Whence are they con- 

 sidered to be derived? What variation in the cochlea? Study 

 from figures the structure of the cochlea. 



369. Classification. The principal divisions of the sub- 

 phylum vertebrata are : 



Class I. Pisces (e. g. } sharks, lung-fishes, bony fishes). 

 Page 355. 



Class II. Amphibia (frogs, toads, salamanders, etc.). 

 Page 372. 



Class III. Reptilia (crocodiles, lizards, snakes, turtles). 

 Page 380. 



Class IV. Aves (birds). Page 394. 



Class V. Mammalia (mammals). Page 427. 



