HOW ANIMALS MOVE. 



159 



vertical, and single. The former, called pectoral and ven- 



tral fins, represent the fore and hind limbs of Quadrupeds. 



The vertical fins, which are only expansions of the skin, 



vary in number; but in most Fishes there are at least 



three : the caudal, or tail-fin ; the dorsal, or back-fin ; and 



the anal, situated on the abdomen, 



near the tail. The chief locomotive 



agent is the tail, which sculls like a 



stern-oar; the other fins are mainly 



used to balance and raise the body. 



"When the two lobes of the tail are 



equal, and the vertebral column stops 



near its base, as in the Trout, it is said 



to be homocercal. If the vertebrae 



extend into the upper lobe, making 



it longer than the lower one, as in 



the Shark, the tail is called hetero- 



cereal. The latter is the more effec- 



tive for varying the course; the 



Shark, e. a., will accompany and resultant of the tw 



' y ' , . . ., Pulses is the straight line 



gambol around a ship in lull sail m front. 

 across the Atlantic. The Whale swims by striking the 

 water up and down, instead of laterally, with a fin-like 

 horizontal tail. Many air-breathing animals swim with 

 facility on the surface, as the Water-birds, having webbed 

 toes, and most of the Reptiles and Quadrupeds. 



(2) Locomotion in Air. The power of flight requires a 

 special modification of structure and an extraordinary 

 muscular effort, for air is 800 times lighter than water. 

 Nevertheless, the velocity attainable by certain Birds is 

 greater than that of any Fish or Quadruped ; the Hawk 

 being able to go at the rate of 150 miles an hour. The 

 bodies of Insects and Birds are made as light as possible 

 by the distribution of air-sacs or air-cavities. 82 



The wings of Insects are generally four in number; 



