368 



COMPARATIVE ZOOLOGY 



the end of its tail, and propels itself backward by 

 a quick downward and forward stroke of the ab- 

 domen. 



But fishes, whose bodies offer the least resistance to 

 progression through water, are the most perfect swim- 

 mers. Thus, the salmon can go twenty miles an hour, 

 and even ascend cataracts. They have fins of two 

 kinds : those set obliquely to the body, and in pairs ; 

 and those which are vertical, and single. The former, 

 called pectoral and ventral fins, rep- 

 resent 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 homo cereal. If the 

 vertebrae extend into the upper 

 lobe, making it longer than the lower one, as in the 

 shark, the tail is called heterocercal. The latter is the 

 more effective 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 finlike 

 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. 



FIG. 321. 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. 



