132
THE bî—:AS
'
-
The eectis brought about either by greatly increasing ,
the surface area by means of long spines or feather—liké “
projections, or by extreme attening so that the creature
,
is like a leaf and when lying horizontally in the water is
prevented from sinking except at a very low speed.
The
spiny and hair—like processes on many of the plankton
diatoms depicted in Plate 88 serve this purpose in nature.
.
The diatoms themselves are so very minute that the many
spines must increase their surface area comparatively to_ an
"
'enormous degree, and they become literally suspended in
the water so slow is their sinking speed. The rate at which a
diatom will
the water has been measured, and,
of course, varies for the different species according ”to their
shape.
Let us
speciesof Chætoceros
_
(Plate 88, g. 3) ; this takes on an average about four
'
three—quarter hours to sink three feet, or eight minutes au
_
inch.
This was in still water, but in nature, near the sea
surface, owing to wave action there are continual little
swirls and eddies which will bring it up or down at a much
faster rate than it sinks.
Now, the animals, _of- course, onaccount of their larger
'
size and weightÏwill sinkhonsiderably faster than plants, —
but they also possess the power of locomotion, by which they“
can regain their level. The presence of spines and feathery
processes on the “animals’ bodies, by slowing down the rate
at which they sink, preverits them from having to be
continually on the move to keep up in the water.
If a ‘
living planktôn animal is watched it will be seen to
make a
.
rapid .upward movement
swimming and will then rest '
'
while it sinks only _slowly through the distance that it so
rapidly moved up through.
‘
An extreme example of attening in an animal is to be
,
found in the ',case— of the larva of the common crawsh or
rock lobster.
This is, of course, a different species from the
THE bî—:AS
'
-
The eectis brought about either by greatly increasing ,
the surface area by means of long spines or feather—liké “
projections, or by extreme attening so that the creature
,
is like a leaf and when lying horizontally in the water is
prevented from sinking except at a very low speed.
The
spiny and hair—like processes on many of the plankton
diatoms depicted in Plate 88 serve this purpose in nature.
.
The diatoms themselves are so very minute that the many
spines must increase their surface area comparatively to_ an
"
'enormous degree, and they become literally suspended in
the water so slow is their sinking speed. The rate at which a
diatom will
the water has been measured, and,
of course, varies for the different species according ”to their
shape.
Let us
speciesof Chætoceros
_
(Plate 88, g. 3) ; this takes on an average about four
'
three—quarter hours to sink three feet, or eight minutes au
_
inch.
This was in still water, but in nature, near the sea
surface, owing to wave action there are continual little
swirls and eddies which will bring it up or down at a much
faster rate than it sinks.
Now, the animals, _of- course, onaccount of their larger
'
size and weightÏwill sinkhonsiderably faster than plants, —
but they also possess the power of locomotion, by which they“
can regain their level. The presence of spines and feathery
processes on the “animals’ bodies, by slowing down the rate
at which they sink, preverits them from having to be
continually on the move to keep up in the water.
If a ‘
living planktôn animal is watched it will be seen to
make a
.
rapid .upward movement
swimming and will then rest '
'
while it sinks only _slowly through the distance that it so
rapidly moved up through.
‘
An extreme example of attening in an animal is to be
,
found in the ',case— of the larva of the common crawsh or
rock lobster.
This is, of course, a different species from the