220 



INTRODUCTION TO ZOOLOGY. 



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

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

 the British coasts, both of which swim near the surface, and 

 with apparently the same ease and swiftness, the 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 

 instrument of motion. To its movement a great part of the 

 muscular power of the fish can be applied; and the great 

 flexibility of the skeleton largely adds to the effect. The fins 

 on the upper and lower sides of the body bear their part in 

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

 stopping, or changing 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 (dorsum, the back), and if there be more than 



* Yarrell's British Fishes, vol. i. p. 39. 



