198
_
THE SEAS
'
_
during the process.
Those worms which live in calcareous -
or parchment—like tubes bear round their head end a crown
of foliaceous tentacles, often of great delicacÿ and beauty,
which nOrmally wave about freely in the water, but which are
withdrawn in a ash on the approach of enemies (Plate 77).
These tentacles are covered with cilia and also with a sticky
substance in which ne particles are entangled, being later
carried to the mouth by currents produced by the beating of
'
the cilia.
But it is the bivalve molluscs, such as the oyster
and mussel, which have carried this method of feeding to
the greatest degree of perfection.
As shown in Plate 7 5,
_
on either side of the body there are two pairs of delicate
membranes known to scientists, owing to the mistakes of
eaflie‘r naturalists, as gills, and to the oyster merchant as the
'
_
.“ beard,”_ but which really form an apparatus for collecting
f00d.
Each of these membranes is double and consists of a
great number of ne laments laid side by side and covered
,
with rows of cilia.
The cil—ia on the SideS ‘of the laments
beat inwardsSo that they cause a strong current of water to
_
pass
into the interior of the membrane and so upwards and
'
out of the body.
The cilia beat continuouëlÿ So that as long
as the shell is open there is a continuous stream ofwater
entering at
point, passing through the membrane of the
gills, and then out at“ another. Only the water passes
«
through the gills ; all particles in suspension are retained on
—
,
the surface, where they are entangled' in the sticky mucus
—
"
there produced and carried by the cilia on the“ outer surface
'
of the laments to.the edge or base of the gills, whence they
are conducted by ‘fu'rther cilia to the mouth, whiCh , is
protected by two pairs of
aps known as
_
“
palps.”
These are ridged on their inner surfaces and
.
covered with many Series Of cilia beating in every possible
t
'
direction, the whole formig an extremely cOmplièa‘ted, but
'
very, efcient, ‘meChanism- for sorting out the particles.
_
THE SEAS
'
_
during the process.
Those worms which live in calcareous -
or parchment—like tubes bear round their head end a crown
of foliaceous tentacles, often of great delicacÿ and beauty,
which nOrmally wave about freely in the water, but which are
withdrawn in a ash on the approach of enemies (Plate 77).
These tentacles are covered with cilia and also with a sticky
substance in which ne particles are entangled, being later
carried to the mouth by currents produced by the beating of
'
the cilia.
But it is the bivalve molluscs, such as the oyster
and mussel, which have carried this method of feeding to
the greatest degree of perfection.
As shown in Plate 7 5,
_
on either side of the body there are two pairs of delicate
membranes known to scientists, owing to the mistakes of
eaflie‘r naturalists, as gills, and to the oyster merchant as the
'
_
.“ beard,”_ but which really form an apparatus for collecting
f00d.
Each of these membranes is double and consists of a
great number of ne laments laid side by side and covered
,
with rows of cilia.
The cil—ia on the SideS ‘of the laments
beat inwardsSo that they cause a strong current of water to
_
pass
into the interior of the membrane and so upwards and
'
out of the body.
The cilia beat continuouëlÿ So that as long
as the shell is open there is a continuous stream ofwater
entering at
point, passing through the membrane of the
gills, and then out at“ another. Only the water passes
«
through the gills ; all particles in suspension are retained on
—
,
the surface, where they are entangled' in the sticky mucus
—
"
there produced and carried by the cilia on the“ outer surface
'
of the laments to.the edge or base of the gills, whence they
are conducted by ‘fu'rther cilia to the mouth, whiCh , is
protected by two pairs of
aps known as
_
“
palps.”
These are ridged on their inner surfaces and
.
covered with many Series Of cilia beating in every possible
t
'
direction, the whole formig an extremely cOmplièa‘ted, but
'
very, efcient, ‘meChanism- for sorting out the particles.