60 THE SEAS 



fauna have much in common. The fact that a number 

 of animals from the Arctic and Antarctic regions are closely 

 akin has led to the development of the theory of " bipolar- 

 ity," which states that the animals inhabiting these cold 

 seas are more closely related to one another than they are 

 to the animals inhabiting the warm waters which stretch 

 between them. It is thought that they maintain connection 

 by way of the cold deeper waters of the tropics. It may, 

 however, be that the animals from the two polar regions 

 which resemble one another are both descended from the 

 same deep-water animals of intermediate regions which 

 found their way into shallower water at both ends of the 

 earth. 



Since it is impossible for us to describe all the mul- 

 titudinous bottom animals which make up the different 

 horizontal zones or faunas, we must confine our attention 

 to giving a general account of vertical distribution with 

 examples taken from our own seas. It is convenient to 

 divide the sea bottom into various areas ; the first, called 

 the Littoral zone, extends from the shore to about twenty 

 fathoms ; the Sub-littoral or Shallow Water zone extends to 

 ioo fathoms ; from the continental edge down the slope to a 

 depth of some 500 fathoms there is a Continental Deep-sea 

 zone ; while, finally, there is the Abyssal zone. 



Littoral Zone 



In the previous chapter we discussed the animals of 

 the shore which are, strictly speaking, part of the littoral 

 fauna. There are, however, five areas coming immediately 

 below low-tide mark — the fringes of which are usually 

 exposed during spring tides — each of which has a distinct 

 population ; namely, the Laminaria belt, Zostera belt, 

 hard bottom, sandy bottom and muddy bottom. 



In the first of these, characterized by veritable forests 



