_
'
.
D_RIFTINGÏLIÈE
129
_
down movements are—the changes in the strength of light
experienced by the animals ; this is further conrmed by the
fact that many animals-live deeper in the bright sunny days
of midsummer than they do earlier in the year, when the
‘
light.entering; the .…_water is .not_so;strong.then, '
we
see that most of the plankton animals tend to avoid
strong light andprefer the dimly lit conditions of--thedeeper
layers ; at the same time all the evidence goes to show that
'
_
each animalshows‘ a preference for a certain strength of
light to which it is
Towards the evening they
follow the
optimum
”
strength of light towards the
‘
surface as night draWs on, but in the dark there is no light
,
stimulus and they are free to move anywhere (see Plate 48) ;
,
at dawn they Once more pick up their optimum intensity
and move downwards as the daÿlight Strengthens. ' It is also
a curious fact that the older'an animàlbecomes the more it
shuns. the light, and itisgenerally the young‘erstages that
.
are found near the surface.
This is not, however, always
the case ; the early stages of ”theVelella ôr “
By- thé wind
sailor ”
are found at Very deep levels, while the adult: as
mentioned below, oats right on
Water,
where it is blown hither and thither by thewinds. '
-
.
_
ADAPTATIONS FOR SUSPENSION
.
»In considering this drifting life it may' have struck the
reader that it is a curious thing that
these organisms,
.
'
many
of Which, such as the diatoms; are practically in_—
capable of any independent movements, should remain
.
suspended in the water. Why do they
not sink rapidly
to the bottom of the sea?
‘The answer to this question
,
rests in the demonstration of some of the most remarkable
structural adaptations, which t most of the plankton
organisms to the conditions under which they live.
.
Their main requirement
is that bys‘ome‘ means or other
-