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INTEODUCTION TO ZOOLOGY. 



bladder, seems, in its habits, to be similar to fishes which are 

 thus_ provided. Nay, of two species of Mackerel found on the 

 British coasts, both of which swim near the sui-face, and with 

 apparently the same ease and swiftness, one has a swim-bladder 

 and the other has not.* 



The external organs of motion act in a manner more easily 

 understood. They consist of the tail and fins. We use the 

 word " tail " as expressing not only the lower extremity of 

 the body, but also the fin by which the body is terminated, 

 appropriately called the " caudal fin " (Latin, cauda, a tail). 

 This is the most efficient organ in progression. It acts upon 

 the water somewhat like the oar of the boatman, when he 

 propels his little craft by that alternate movement of the oar 

 which is called "sculling." The tail— placed vertically in 

 fishes, but horizontally in whales — is a very powerful instru- 

 ment of motion. To its movement a great part of the mus- 

 cular power of the fish can be applied ; and the great flexibihty 

 of the skeleton largely adds to the effect. The fins on the upper 

 and lower portions of the body bear their part in the exertion, 

 or unite with those nearer the head in retarding, stopping, or 

 changmg the direction of the movement. The annexed figure 

 of the Perch (Fig. 181) exhibits the fins, and also the spiny 

 processes by which they are supported. 



Fig. 181.— Skeleton of the Perch. 



The fins upon the back of the fish are naturally termed the 

 "dorsal" fins (dorsurn, the back), and if there be more than 



* YarrelVs British Fishes, vol. i. p. 39. 



