METHODS OF
263
,
By the means described above all sorts of animals _may
be captured for examination, but the instruments used
cannot be said to be quantitative; they Will not supply
accurate information as to the abundance of the animals
on the sea oor.
,
For this latter purpose an instrument known as the
“
grab
”
has been made.
.
This is essentially the same as
a coal—grab and is constructed so that when it strikes the
bottom it literally bites out a piece of the sea bed.
The
instrument is very heavy, and consists of two hinged jaws
which are open as it sinks to the bottom ; on striking
gravel or mud the jaws sink in under their weight, and as
the grab is hauled up they are pulled tOgether so that a
solid sample of the “sea oor is retained (Plate 95).
The
"
sample is then sifted through a series of graded sieves and
the sand and mud washed away.
All the animals
‘
ing in the ‘sieve can be picked out and counted and so a
knowledge of the population on a denite area of the sea
‘
oor is gained.
Recently it has been shown that this
,
grab or bottom—sampler is —not completely efcient for
studyihg life on hard sandy bottoms as owing to“ the rm
consistency of the sand the grab does not sink deep enough
and so fails to catch many of the creatures that
live in their
-'burrow$ a few inches below the surface,
TO meet this
difculty a new instrument has now been invented which
possesses a pump which is put into action just when hauling—
in commences.
Until the pump has nished
its worlç_
however the whole machine cannot be
bottom.
The pump is operated —by a wire running over a pulley
wheel, and until all. the wire is pulled o'no strain c0mes
on the instrument itself.
With the suction set up by the
pump the sampler sucks its way into the sand down to
.
& depth of just over a foot, and so a sample of known area -
and. depth is procured.— On a similar principle is 8_little
.
'
263
,
By the means described above all sorts of animals _may
be captured for examination, but the instruments used
cannot be said to be quantitative; they Will not supply
accurate information as to the abundance of the animals
on the sea oor.
,
For this latter purpose an instrument known as the
“
grab
”
has been made.
.
This is essentially the same as
a coal—grab and is constructed so that when it strikes the
bottom it literally bites out a piece of the sea bed.
The
instrument is very heavy, and consists of two hinged jaws
which are open as it sinks to the bottom ; on striking
gravel or mud the jaws sink in under their weight, and as
the grab is hauled up they are pulled tOgether so that a
solid sample of the “sea oor is retained (Plate 95).
The
"
sample is then sifted through a series of graded sieves and
the sand and mud washed away.
All the animals
‘
ing in the ‘sieve can be picked out and counted and so a
knowledge of the population on a denite area of the sea
‘
oor is gained.
Recently it has been shown that this
,
grab or bottom—sampler is —not completely efcient for
studyihg life on hard sandy bottoms as owing to“ the rm
consistency of the sand the grab does not sink deep enough
and so fails to catch many of the creatures that
live in their
-'burrow$ a few inches below the surface,
TO meet this
difculty a new instrument has now been invented which
possesses a pump which is put into action just when hauling—
in commences.
Until the pump has nished
its worlç_
however the whole machine cannot be
bottom.
The pump is operated —by a wire running over a pulley
wheel, and until all. the wire is pulled o'no strain c0mes
on the instrument itself.
With the suction set up by the
pump the sampler sucks its way into the sand down to
.
& depth of just over a foot, and so a sample of known area -
and. depth is procured.— On a similar principle is 8_little
.
'