122
THE sms
»
,
5'
or are large and strong enough to seek for themselves their
natural home on the bottom.
_
'
:
Worms, starsh, crabe, lobsters, oysters, sea squirts and
even'the majority of shes in the sea, spend the rst days of
their lives drifting about in this manner (see Plate 91).
‘
One of the main advantages of this mode of life is that it
_
'
ensures
complete dispersal of the young.
Many marine
animals live xed to the rocks and bottom, such as hydroids
and mussels, or are at most very sluggish and do not move
.
far aeld.
It is obvious, therefore, that if their ospring
were
born and hatehed “
at home ” the parental abode
=
would soon become so crowded with young and half—grown ‘
children that there would be no room to turn.
But it the
children are sent out to tend for themselves in the upper
water layers they will be carried far and wide by the
currents, and those that survive will settle to the bottom
many miles irom.where their parents lived.
'
drifting young of many marine animals are very
different in appearance from their parents; indeed, it is
_
safe to say that if they were shown to a novice it would be
impossible f0'r him to say into what they would grow-.
Because of these extraordinary differences between adults
and young, the early stages of many marine animals, when'
'
rst discovered by naturalists in the plankton catches, were
regarded as newspecœs of animals and described and given
names of their own'.
It was only by 'rearing them, or
5
'piecing together successive stages in the life histories from
,
the catches, that the adult into which they were going to
;
develop 00uld be discovered; Who, for instance, without
'
knowing beforehand, would dream of suggesting that the
little animal gured in Plate 43 was the young of the common
.
édible crab ?
_
When rst discovered it was not recognized
as such and was giVen the name “
Zoea,” Onthis 'account
_
it is*11ow known as the zoea stage of the’crab.