CHORD ATA 357 



tive layer. Light falling directly on this from all directions 

 would produce no image, just as the photographic plate exposed 

 outside the camera would present a general blur. We find then 

 the necessity of the same optical devices as are found in the 

 camera: (i) a sensitive surface, the retina; (2) a box for sup- 

 port and for keeping out the light except from 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 to- 

 gether again beyond the lens at a corresponding point on the 

 sensitive surface. The elementary relations of these parts 

 as found in the eyes of vertebrates may be gathered from a 

 study of Fig. 175. 



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. 



375. 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. 45.) 



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. 



376. Classification. The principal divisions of the sub- 

 phylum vertebrata are : 



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

 Page 3 58. 



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



375- 



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

 Page 384. 



Class IV. Aves (birds). Page 398. 



Class V. Mammalia (mammals). Page 437. 



