58
'
THE SRAS
one to exist in mud and the other in shell gravel.
It hai "
.
been f0und that the free swimming larvæ of certain marina
worms, although ready to metamorphose to the adult
structure and settle on the bottom, will delay this if a suit— ’
able substratum is lacking and continue to swim about and
ï
to grow until a suitable bottom is found.
In the second
place the bottom _material may contain important re—
serves of lime, silica and manurial salts, although modem
investigation
shows
that
actually
the
products
oi
the decay of animal and plant life are quickly returned
'
to the water.
LIFE ON THE SEA BOTTOM
_
The animals which inhabit the bottom of the ocean “ara
called the bentho\s,
to distinguish them from the driftmg
and actively swimming animals known as plankton and
nektbn respectively.
Extensive investigations by means
'
dredges and trawls have shown that the benthos
varies greatly in different regions both in quantity
quality.
Generally speaking animals are most numerous in
coaStal waters; and become less and less so the further we
pass from the land into deeper : water, though there is
animal
often of a unique and interesting kind,
even in the
regions with red clay deposits.
The
-
greatest number of different speCies is found in the tropics,
»
especiallyaround continents and in the neighbourhood of
-
coral islands,‘ but the tempetate and polar seas make up for
paucity of variety of species by
gre‘ater
,
numbers of individuals of those kindswhich occur there.
Differences in
chiey the result of differences in
depth, temperature, sanity and food, and of the interactions ' .
‘
'
of these factors”.
The inuence of the last is universally felt,
of salinity
is apt to be local, the result of currents or
,
_
great inuxes of
fresh Water from rivers, that of temperature
_
is especially effective in the shallower waters near the Coast