342 



THE DOGFISH. THYLUM CHORDATA 



and obliquus inferior, and behind these are four, arranged as a 

 cross : the rectus intcrnus, or anterior ; the rectus externus, or 

 posterior ; the rectus superior, and the rectus inferior. The lens 

 is spherical, and as it has a muscle which can move it forwards 

 it may be assumed that accommodation is carried out in this 

 way. The retina contains only rods. The internal ear has semi- 

 circular canals, sacculus and utriculus, but lacks the other parts 

 present in higher vertebrates. It is undoubtedly an organ of 



TABLE IV. 



The Cranial Nerves of the Dogfish 



