ORGANS OF FLOTATION 



489 



come this tendency many animals are supplied with 

 definite locomotor organs that enable them not only to 

 keep afloat but to move rapidly through the water. The 

 sharks, mackerels and herrings, all active swimmers, fall 

 in this class. Many species of shrimp-like animals, cer- 

 tain snails and worms, and the young or larval stages of 

 many animals are likewise provided with fins, paddles, 

 cilia, etc., that enable them to maintain favorable po- 

 sitions. 



^^L;^^^:^ 



Fig. 141. — Marine Copepods. A, cold-water form; B, tropical. 



In numerous other animals, with or without loco- 

 motor organs, the body is made lighter by the presence 

 of oil or fats. In certain fishes air bladders, filled by 

 gulping air at the surface or by secretory activity of the 

 enclosing walls are also of service in keeping the body 

 afloat. Then again there are feather-like appendages, 

 bristles, plates, etc., that, while they do not actually pre- 

 vent sinking, nevertheless prevent it taking place rap- 

 idly in the case of large numbers of mostly small-sized 

 organisms. Just as a lead shot sinks more rapidly than 

 when beaten into a thin sheet, so these structures that 

 greatly increase the surface of the body enable the organ- 



