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 tins, which are only expansions of the skin, 

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

 three : the caudal, or tail-tin ; 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 tins 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- 

 cercal. The latter is the more effec- 

 tive for varying the course ; the 

 Shark, e. g., will accompany and 



FIG. 124. Diagram illustrat- 

 ing the locomotion of a 

 Fish. The tail describes 

 the arc of au ellipse ; the 

 resultant of the two im- 

 pulses is the straight line 

 in front 



gambol around a ship in full sail 

 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; 



