ORGANS FOR FLOATING. 205 



swimming organs, the latter do not. In the former the specific 

 gravity, compared with that of the stratum of water in which 

 they live, must be a little in excess. In the latter it may 

 be the same as that of the water, or even less. This comparison, 

 however, is not, and cannot be, absolute ; for since the strength 

 of the current varies \x ith that of the wind, with the fall and 

 the mass of water, <tc., most of the actively swimming animals 

 even, must often be transported passively, as, for instance, when 

 their strength is insufficient to contend with the current ; nay, 

 even the strongest swimmers, as the Porpoises, Whales, Sharks, 

 <fcc., must often voluntarily allow themselves to be borne along 

 by it. Those organs which serve to enable swimming or floating 

 creatures to maintain a position at the surface or at a certain 

 level, are known in zoology as hydrostatic organs ; to these 

 belong, for instance, the swim-bladders of many fishes, the air- 

 bladders which keep floating colonies of polyps (Siphonophora) 

 at the surface of the sea, 06 also those air-vacuoles which are 

 sometimes found in the protoplasm of testaceous Khizopoda 

 'la). Even creeping animals, such as the fresh-water snails 

 (Lymncea, P/iysa, Planorbis, tc.), can become floaters, for they 

 fill their lungs so full of air that they become specifically 

 lighter than the water, and consequently rise to the surface. 97 

 This enables them to creep by the motion of the cilia or by the 

 action of the foot on the stratum of air in contact with the 

 surface of the water. It is easy to see from these examples 

 that nature employs an infinite variety of means for solving so 

 simple a problem. Tn Arcdla it is a temporary bubble, which 

 disappears when the creature desires to sink to the bottom ; in 

 the Siphonophora the organ appears to serve no purpose but that 

 of keeping the colony at the surface ; in Fishes, as we have seen 

 above, the hydrostatic organ also subserves the purpose of an air 

 reservoir, and in some cases even of a lung in the physiological 

 sense ; in the pulmonate water-snails, again, the lung itself 

 can, at the will of the animal, become a hydrostatic organ. 



Far more various and important are the actual swimming 

 organs of the true swimmers, by which they are enabled to ex- 

 change an unfavourable spot for one more suitable, to escape 

 the pursuit of their enemies, or to extend their own hunting- 



