CHAPTER IV 

 Swimming Animals 



Over the sea bottom lie the waters of the ocean extending 

 for miles in all directions. Above the surface of the land 

 the air is the medium through which birds and insects, and 

 even man, can make their way with great speed. In the 

 same way the sea water forms a medium through which 

 certain animals can travel rapidly from place to place by 

 swimming. Although many minute animals can swim (in 

 the true meaning of the word), that is, make their way 

 through the water, it is usually agreed that the real swim- 

 ming animals of the sea are those only who can make head- 

 way with sufficient speed to move from place to place 

 against any current they may meet. Those smaller 

 creatures that can swim without sufficient strength to stem 

 the currents, and aie drifted to and fro by them, are usually 

 classed under the heading of drifting life, and will be dealt 

 with at length in the next chapter. 



The true swimmers are to be found in only a few groups 

 of the animal kingdom, and are the fishes, the whales and 

 seals, and the squids or cuttlefish. 



Fishes 



Most sea fishes can be divided into two main groups, the 

 cartilaginous fishes or Elasmobranchs, and the bony fishes 

 or Teleosts. In the group of cartilaginous fishes are to be 

 found all the shark family — dog-fish, tope, sharks, rays 

 and skates. These are characterized by the fact that no 



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