202 



BRANCH CHORDATA 



The brain is of the vertebral type, but small, and occupies but 

 a small portion of the cranium. The cerebrum is comparatively 

 small. The cerebellum is sometimes large. The optic and ol- 

 factory lobes are conspicuous (Fig. 161). The medulla is also 

 present, all the parts being distinct and visible from above. 

 The brain sends off at least ten pairs of 

 nerves. 



The Senses. Of all the sense organs, 

 the most noticeable are those along the 

 lateral line. The lateral line on either 

 side of the fish from tail to head is 

 "marked by a groove in the scales which 

 opens to the exterior by numerous canals 

 through the scales." (Examine several 

 scales along the lateral line.) The func- 

 tion of the lateral line is possibly to as- 

 certain the water pressure at different 

 depths. 



The skin and especially the lips are 

 the seat of the sense of touch. 



The eye has several peculiarities. The 

 lens is very convex, owing to the slight re- 

 fraction from the light in passing from the 

 water into the cornea. The eye is short sighted, since light is 

 so absorbed by water as to render objects a short distance away 

 invisible. Lids are wanting or very poorly developed. Only 



Fig. 161. Brain of 

 a cod: og, Olfactory 

 ganglia; ch, cerebral 

 hemispheres; ol, optic 

 lobes; c, cerebellum; 

 wo, medulla oblongata. 

 (Tenney.) 



Fig. 162. Lucifu'ga. A blind fish containing unborn young with well- 

 developed eyes. (Eigenmann, Bulletin 526, U. S. F. C., 1902.) 



a few fishes have a nictitating membrane. There are no tears. 

 Through disuse for generations the cave fishes have lost their 

 sight (Fig. 162). 



