220
THE SEAS
round to supply the various parts of our bodies.
But in
the sea we never see sh coming to the surface to take in a
,
gulp of air as the marine mammals, such as whales, do.
They have no need to do so, for oxygen is everywhere
present dissolved in the surrounding water and sh have
a
special apparatus, the gills, for extracting it.
All the
larger marine animals have gillsor some
such specialized
structure for this purpose.
But in the case of the very
small animals, in which the area of the body surface is
very large compared with the actual volume of the body
itself. oxygen can be absorbed anywhere over the surface
.
the body.
There is probably no part inthe open sea where oxygen
is not
in solution in sufcient quantity to support '
a
large number of animals.
Isolated cases are known
'
however where there is no oxygen. Such conditions are
'
to be found in the deeper waters of the Black‘Sea.
Here
we have a layer of light surface water down to about a
hundred fathoms, below which is heavy water.
The upper
layers are of low salinity owing to the fresh water brought
down by the Danube and other rivers, but the deeper
waters have a high salinity ; hence the differenCe in weight
of the waters.
Thus there is at a certain depth a layer
where the light surface water andthe deep heavy water
meet.
This forms a kind of boundary layer between the
-
two;and the waters above and-below this' depth do not
'
mix one with another.
Thereforè although. the oxygen
supply in the upper layers may be very high, there is no …
_
means by which this gas can be transported into the deeper—
layers.
As a result there is no oxygen. below about a
'
hundred fathoms, the only gas present
the stinking
_
'
sulphuretted hydrogen.
No living animals are therefore
.
.
present below this depth.
The only organisms that can
'
live are certain bacteria that do not require oxygen, and
THE SEAS
round to supply the various parts of our bodies.
But in
the sea we never see sh coming to the surface to take in a
,
gulp of air as the marine mammals, such as whales, do.
They have no need to do so, for oxygen is everywhere
present dissolved in the surrounding water and sh have
a
special apparatus, the gills, for extracting it.
All the
larger marine animals have gillsor some
such specialized
structure for this purpose.
But in the case of the very
small animals, in which the area of the body surface is
very large compared with the actual volume of the body
itself. oxygen can be absorbed anywhere over the surface
.
the body.
There is probably no part inthe open sea where oxygen
is not
in solution in sufcient quantity to support '
a
large number of animals.
Isolated cases are known
'
however where there is no oxygen. Such conditions are
'
to be found in the deeper waters of the Black‘Sea.
Here
we have a layer of light surface water down to about a
hundred fathoms, below which is heavy water.
The upper
layers are of low salinity owing to the fresh water brought
down by the Danube and other rivers, but the deeper
waters have a high salinity ; hence the differenCe in weight
of the waters.
Thus there is at a certain depth a layer
where the light surface water andthe deep heavy water
meet.
This forms a kind of boundary layer between the
-
two;and the waters above and-below this' depth do not
'
mix one with another.
Thereforè although. the oxygen
supply in the upper layers may be very high, there is no …
_
means by which this gas can be transported into the deeper—
layers.
As a result there is no oxygen. below about a
'
hundred fathoms, the only gas present
the stinking
_
'
sulphuretted hydrogen.
No living animals are therefore
.
.
present below this depth.
The only organisms that can
'
live are certain bacteria that do not require oxygen, and