THE ' SEA SHORE
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49
and sea urchins all have special devices which enable them
to worktheir way throughthe sand Or-mud.
Many shes migrate
distances, usually because their
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feeding and spawning grdunds, are far apart. There are
also migrations on the sea shore, though on_a much smaller
scale.
The -movements of edible crabs have-been studied,
and it has been found that they move from near-the shore
into depths of twenty or thirty fathoms about the beginning
of autumn and remain there until February, when they
begin to return to the shore again… The eggs are laid in the
winter in deep water and. remain attached to_ the body of
the female, nally
out during the summer in the
warmer water near the shore.
Both temperature and food,
'
therefore, play a part in determining the yearly travels of the
crab and also of lobsters and-prawns which, though we know
less of their habits,appeaf to migrate in a;si_milar manner.
The shore shes also move into deepér water in the winter,
while many of thebigger…sea slügs, Such as the Sea Hare
-
(Aplysia) and the large Plume-bearer (Oscam‘us) co'me on to
,
the
shore
during “the summer specially for -spawning.
_
Attention has already been drawn to the more localised
movements, apparently concerned with feeding only, of_the
.
.1impèts, and this habit is also found in allied beasts.
,
SPAWNING
.
In such a difcult region as the sea shore, Spawning
has to be carefully attended to if the race is to be continued; '
Spring and early summer are the "times usually chosen for
spawning, for the water is then teeming with microscopic'al
_
plant life, which forms ideal food for the newly—hatched
young.
We may divide shore animals into three classes
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'
according to their methods of reproduction;
those which
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discharge their reproductive products freely into the sea
_
Where they oat helplessly until swimming organs are.-