60
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THE SEAS
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‘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 “ bipolarity,—” which states that the animals inhabiting these cold
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seas are more :closely related to one another than they are “
inhabiting the warm waters which stretch
Î
between them.
It is thought that they maintain connection
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:
by way of the cold deeper waters of the,tropics.
It may,
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however, be that the animals from the two polar regions
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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
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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 conne our attention
to giving a general account of vertical distribution with
examples taken from our own seas.
It is convenient to
divide the seabottom into various areas ; the rst, called
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the Littoral zone, extends sfr-om the shore to about twenty
Ï
iathoms ;. the Sub—littoral or Shallow Water zoneextends to
‘
100 fathoms-; from the continental edge down the slope to a
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depth of some 500 fathoms there is a ContinentalDeep-sea
zone ;
while, nally, there is the Abyssal zone.
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LITTORAL ZONE
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In the previous chapter we discussed the animals oi.
,
the shore
Strictly speaking, part of the littoral
faunaÿ '
There are, however, ve areas coming immediately
below
low—tide mark—the .fringes of
are usually
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exposed during spring tides—each of Which has a distinct
population,;
namely; the
Zostera belt,
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hard bottom, sandy bottom and muddy bottom.
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In the rst of these, characterized by veritable forests