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- 
cercal. The latter is the more effec¬ 
tive for varying the course; the 
Shark, e. g ., will accompany and 
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; 
Fig. 124.— Diagram illustrat¬ 
ing the locomotion of a 
Fish. The tail describes 
the arc of an ellipse; the 
resultant of the two im¬ 
pulses is the straight line 
in front. 
