198 MARVELS OF THE ANIMAL WORLD 



which they are able to alter their degree of buoyancy 

 so that they can float or swim at any desired ele- 

 vation. Some species, however, such as the flat- 

 fish, the sharks, and the lampreys do not possess 

 that organ. 



The majority of people are under the impression 

 that a fish propels itself through the water by the 

 use of its fins, but, with few exceptions, such is not 

 the case. The taU — that is to say, the posterior 

 portion of the creature's anatomy, and not its tail- 

 fin — is the member employed for propulsion, the 

 fins playing only a secondary part and serving as 

 balancing and steering organs. Of those fish which 

 rely entirely upon their fins for the purpose of pro- 

 gression, two species, namely the pipe-fish and the 

 sea-horse, are to be found in British waters. Both 

 swim by rapidly vibrating their dorsal or back fins, 

 and when so doing usually hold their bodies in a 

 vertical or slightly inclined plane, instead of, as in 

 the case of most fish, in a horizontal position. The 

 sea-horse is further peculiar in the fact that it pos- 

 sesses a prehensile tail. 



The needle-fish and the globe-fish are even more 

 remarkable than the foregoing in the position they 

 assume when swimming, the former standing upon 

 its head when thus engaged, while the latter wfll 

 often float belly uppermost. Under normal con- 

 ditions, however, the glob^-fish progresses in the 

 usual fish-Uke manner, but when in danger from the 

 attack of an enemy it inflates itself with air and 

 distends the skin of its abdomen to such an extent 

 as to cause itself to turn over and float with its 



