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THE SEA 'SHORE
43
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In all these bases we must remember that the co'nceal—‘
is not Only from foes but also from the prey, -i-.e.,
for attack as well as defence. For protection alone are the
limy, sandy or parchment-like tubes of the worms, while
:
the borers probably nd the interior of stone or wood safer
than a more exposed habitat.
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The replacement of lost parts of the body is a very
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familiar occurrence on the shore.
Crustaceans‘ have an
almost unlimited power of growing new limbs.
It is quite
common
for them to be faced With the alternative of
one or more limbs or else losing their lives.
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FIG. 5.—Diagram to illustrate breaking plane of CmStàcean limb; a, inuscle
responsible for breaking; b, detached ring of third segment of limb; f, breaking
furrow (after Paul).
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They do not hesitate to do the former and the limb is cast
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off at a special line
called the “ breaking plane,”
the fracture. being caused by the deliberate Contraction of
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the muscles at this point
Owing to the depth of the
furrow, the wound -is small and blood quickly coagulates
and closes the opening. ,So_1the animal remains, with a
stump in place of a limb, until: the next time it moul_ts,
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when the rudiments of the new limb forcé their way out
quickly before- the new shell has had time to harden.
Ar
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each successive moult the process
is carried a little further
until—the new limbis as large”as those which were uninjured.