308 ZOOLOGY 



Musculature. — The muscles may be classified, like the 

 general parts of the body, into muscles of the trunk (Fig. 283) 

 and tail and muscles of the head, including those of the jaws, 

 tongue-bones, operculum, and gill-arches. 



The nervous system consists of the brain and spinal cord 

 and of the nerves supplying the skin, sense-organs, -viscera, 

 and limbs. The brain (Fig. 287, crh, etc.) is by far the largest 

 mass of the nervous system because it receives nerves of general 

 sense and probably has a general control over the behavior 

 of the animal. It is divided into three parts, sometimes called 

 fore, mid, and hind brain. In the higher vertebrate classes 

 these parts develop in different proportions. 



Sense-organs. — The organs of chemical sense reside in the 

 olfactory sac on the snout. The li]3S of the sac have closed 

 near the middle, leaving a front and rear opening through 

 which the water rushes. Other organs of chemical sense are 

 the organs of the lateral line, which at its anterior end di- 

 vides into many branches running over the head. Originally 

 the sense-organs of this line were distributed along the bot- 

 tom of a sort of trench. Now, however, the trench is cov- 

 ered over by the skin and scales forming a canal. The scales 

 are perforated at intervals to connect the outside vnih the 

 interior of the canal. There is good reason for thinking that 

 the ear is only a specialized part of the lateral line organ, and 

 that is where it got its semicircular canals. There is also an 

 otohth in each ear which probably assists in the equilibrium 

 of the fish. The eye of the fish has to be used under the 

 water. Our eyes are not fitted for seeing under water as 

 our lens is too flat. In the case of the trout the lens is 

 nearly spherical and is forced back towards the retina. Rays 

 from the water will not be bent on entering the lens, which 



