‘
BOEING LÏFE
139
thehinder muscle, the frontal portions of the shell are drawn!
apart so that the sharp ridges with which they are covered
,
scrape off the surface of the wdod while at the saine time
the ridges on the middle lobe widen the opening behind;
The small muscle between the-frontal lobes now comes into
play and by
in its turn brings the frontal lobes
together again, which is a much easier Operation because
the surface of the wood offers nogresistance to movement
in this direction.
Îhe foot then loosens its hold and moves
a short distance to one side, when the same process is re—
peated.
Thisgoes on time after timeuntil the shell and
‘
front half of the body have twist-ed completely round—
the hinder end cannot do this because it is attached to the
burrow in the region of the pallets—_when the movement
'
is
As a result of this continuous working of the
shell and twisting of the front end of the body rst in” this
direction and then in that, the inside of the-burrow becomes
perfectly smooth and circular in cross section.
.
Immediately
behind the shell the. surface of the wood becomes coveredwith a layer of shelly substancewhich the animalproduces
in’the naked region of its b0dy. The whole process of
boring is a. model of mechanical efciency whichis not to '
be surpassed by any member of the animal kingdom.
The mouth of the Shipwormlies just above the sûCker—
_
like foot,
Into it” pass, not onlythe tiny particles collected
from the sea water which enters through the siphons, but
'
also; the minute fragments of wood which are cut away by
the boring organ,
All of these have to pass through the
gut of the animal before-they can be discharged into the
»
sea
water by way of the second of thesiphons._ It has
-
.
long been a matter of dispute whether
the Shipworm can
actuallydigest the wood or Whether this paSses ‘thr0ugh the
.
body unehan’ged, but recent investigations seem _to show
.
quite denitely that the animal does obtain a cônsiderable
'