1342 THE STORY OF THE UNIVERSE 



knows, the fish chiefly swims, is always single, but 

 may be divided into halves. Most of our common 

 fishes have the halves of the tail-fin of equal size; 

 others, such as the sharks, sturgeons, etc., having the 

 upper half greatly exceeding the lower half of the 

 tail-fin in size. In one species of shark, named 

 the Thresher or Fox-shark, the upper half of the 

 tail-fin appears enormously developed as compared 

 with the lower half; and the names of this species 

 have been derived from the use the fish makes of its 

 tail in lashing the water, and from the long-tailed 

 appearance suggesting a resemblance to the familiar 

 Reynard of the land. In fishes the tail-fin is always 

 placed vertically, or in the same line as the body, 

 and moves from side to side; while in the whales 

 which are not fishes, but Mammalia or quadrupeds 

 possessing fish-like bodies the tail-fin is placed 

 across the body. Some fishes may want arms or legs 

 that is, the pectoral or ventral fins; the eels, for 

 example, possessing no ventral fins. The flying- 

 fishes, on the contrary, possess a very large develop- 

 ment of the pectoral or breast fins, and support them- 

 selves temporarily in the air by their aid. 



Fishes are usually very well provided in the matter 

 of teeth. What would be thought of a quadruped 

 which had teeth not only in its jaws, but had its 

 tongue, its palate, the sides and floor of its mouth, 

 and other parts, also bearing rows of these struc- 

 tures? Yet such is the case with many fishes. Then, 

 also, where the teeth of one set in fishes are lost, or 

 'destroyed through the natural wear and tear to which 

 they are subjected, new teeth are developed to supply 



