DRlFTING LIFE
123
hatehing, the young crab remains in this stage for some
‘
time during which it undergoes several moults.
F inally, it
suddenlÿ moults into a stage quite unlike the zoea and more
like the adult crab ; _
albeit when rst found it .was not
'
recognized as such and was labelled “
megalopa
"
(Plate 44).
From this stage it moults into a perfect littlecrab.
80 we
see that in the lite—history of one animal, the crab, the
successive stages are so different that no less than three
animals have been thought to exist, the zoea, the megalopa
and the crab, which were in reality all crab.
.
,
.
.
These animals which appear for only a short period in the
drifting community form the temporary -membersbf the
'
plankton, and their appearance in the upper water layers
-
et course, on the times'at which the parents
spawn, and this gives rise to the seasonal changes in the
plankton mentioned on page 244.
It is chiey in the coastal
.
regions that these temporary drifterS are found because
'
of thegreat wealth of bottom life in the .shallower regions.
,
DISTRIBUTION on PLANKT_ON
_
'
__
fHaving given the reader some idea of what composeS this
‘
driftin‘g life or
let usn0w examine what is known
of the distribution of the plants and animals that form it in
.
the sea._:_
‘
.
-
_
-
To begin with .it must be-fealiz‘6d that besides the immense
horizontal area of alllthe Oceans and seas of'the world, all
of which are inhabitablè, the waters also have‘ a considerable
depth, the average of which is as much as two miles.
Seeing
then that organisms that live in the water'layers'
themselves are free to move up and down—, as well “as in a
horizontal direction,_ it is evident that we have“ two
of
.
distribution to deal with, namely, «horizontal or ge‘0— .
graphical distribution, andvertiCal or depth distribution.
‘
Much work has been done on the geographical distribution
—
_
123
hatehing, the young crab remains in this stage for some
‘
time during which it undergoes several moults.
F inally, it
suddenlÿ moults into a stage quite unlike the zoea and more
like the adult crab ; _
albeit when rst found it .was not
'
recognized as such and was labelled “
megalopa
"
(Plate 44).
From this stage it moults into a perfect littlecrab.
80 we
see that in the lite—history of one animal, the crab, the
successive stages are so different that no less than three
animals have been thought to exist, the zoea, the megalopa
and the crab, which were in reality all crab.
.
,
.
.
These animals which appear for only a short period in the
drifting community form the temporary -membersbf the
'
plankton, and their appearance in the upper water layers
-
et course, on the times'at which the parents
spawn, and this gives rise to the seasonal changes in the
plankton mentioned on page 244.
It is chiey in the coastal
.
regions that these temporary drifterS are found because
'
of thegreat wealth of bottom life in the .shallower regions.
,
DISTRIBUTION on PLANKT_ON
_
'
__
fHaving given the reader some idea of what composeS this
‘
driftin‘g life or
let usn0w examine what is known
of the distribution of the plants and animals that form it in
.
the sea._:_
‘
.
-
_
-
To begin with .it must be-fealiz‘6d that besides the immense
horizontal area of alllthe Oceans and seas of'the world, all
of which are inhabitablè, the waters also have‘ a considerable
depth, the average of which is as much as two miles.
Seeing
then that organisms that live in the water'layers'
themselves are free to move up and down—, as well “as in a
horizontal direction,_ it is evident that we have“ two
of
.
distribution to deal with, namely, «horizontal or ge‘0— .
graphical distribution, andvertiCal or depth distribution.
‘
Much work has been done on the geographical distribution
—
_