[40
THE SEAS
_
‘
amount of nourishment from the wood, the fragments
.
oiwhich are actedupon by digestive juices, and sugars'
produced which can be absorbed by the tissues. There
moreover, a large extension of the stomach which is always
lled with shavings of wood and seems to be well adapted
for the storage of such a slowly-digested substance.
Once encased in its burrow no Shipworm can ever leave
it.
As we have seen the opening of the burrow is not
much larger than a pinhead whereas, within, the burrow
widens out quickly and may, in the common species, be
'
some “half an inch in Width. Moreover,—the animal is actually
attached to the edge of its burrow inthe region ofthe
_
,
pallets.
If a Shipworm is taken out of its burrow, ”no matter “
how care£ully this is done, the animal cannot make a new
burrow for itself.
Since this is the case how does it happen
,
that new wood becomes infected, as it does .very quickly
if conditions are favourable for the growth of the Shipworm?
During the spring and summer especially, the
eggs
and
sperms
are
discharged in immense numbers
into the sea-by“ way of the second of the siphons.
There
the young Shipworms develop and at this early stage of
their existence they are exactly like young mussels or
similar bivalves.
After a short time a pair of tiny shell
valves—appear which entirely enclose the
and from
..
"
between. which,a;croWn of small..hairs= or “ cilia ”
can be
__,_
protruded by means of which the little animals are able
.
swim about, in exactly the same manner as the oyster
(Fig. 59). This freely swimming
—“ larval ”
Shipworm is
'shown in Figure 30.
It is not known for exactly how long
the Shipworms remain in this state, but they can probably
do so for several weeks, and‘ during'this-‘time‘they‘may be
'.
carried _for great distances by ocean currents orwind drifts.
Thisearly stage in their existence-is-the-onlytime:.when the
Shipworms are able to _m0ve.about freelÿ in' the sea and