138 THE BOOK OF NATURE STUDY 



is curved towards that side ; the muscles on the opposite side 

 contract still more rapidly, and the tail is straightened. It is 

 the second stroke which jerks the fish forwards. But while the 

 posterior part of the body is curved to one side, the anterior part 

 of the body is curved to the other. The efficiency of the swim- 

 ming depends on the complementary action of the anterior and 

 the posterior parts of the body. In a long slender fish, like an 

 eel, the body is thrown into several curves. Another important 

 point is that in the bending stroke which is relatively slower 

 the surface striking the water is reduced to a minimum, while in 

 the straightening stroke which is relatively more rapid the 

 surface striking the water is increased to a maximum. This may 

 be illustrated by reference to " feathering " the oar in rowing. 



The teacher may be able to follow up the matter in books, 1 

 but the important point is to get the pupils to observe that the 

 tail is the chief swimming organ in an ordinary fish, and that its 

 action is like that of the stern-oar in sculling a boat. 



The fish's tail really describes a complex curve, like a figure 

 8, and produces a suction-current behind it. " There is a striking 

 analogy between the movements of a fish's tail in swimming and 

 the action of the screw of a steamer, but as a propelling organ 

 the former is far superior to the latter. As we have seen, the 

 tail of a living fish can so adjust its shape and surface that it 

 alternately eludes and grips the water in accordance with the 

 needs of particular strokes/' 2 This self-adjusting, self-regulating 

 power is one of the most striking characteristics of living creatures 

 as compared with mere machines. 



(2) The centre of gravity in a fish's body is high up, and a 

 dead fish floats upside down. The chief use of the paired fins 

 seems to be in balancing and in keeping the fish back up. The 

 fore-fins sometimes help in steering, as in sharks, but it must again 

 be noted that the chief steering organ is the tail. The fore-fins 

 occasionally help in locomotion, as may be well seen in the 15- 

 spined stickleback Gastrosteus spinosus. In skates and rays 

 the enormous triangular fore-fins (or pectoral fins) are the only 

 swimming organs, but this is very exceptional. Still more un- 



1 Pettigrew, see p. 150. 2 Bridge, see p. 150. 



