University of Virginia Library

CHAPTER II.

THE ANATOMY OF THE OYSTER.

The most prominent fact in the organization of the
oyster is its shell. Its body is shut in between two
long concave stony doors, which are made of limestone,
and are fastened together at one end, somewhat
in the same way that the covers of a long, narrow
check-book are bound together at the back. One of
these shells, the flat one, is on the right side of the
body, and the other, which is much deeper, on the
left. When oysters are fastened to each other or to
rocks, the left shell is attached, and the oyster lies on
its left side. When it is at home and undisturbed its
shell is open; so that the water circulates within it, but
when disturbed it shuts its shell with a snap, and is
able to keep it firmly closed for a long time. The
snapping drives out the water, together with any irritating
substances which may find their way in, and on
the natural beds the oysters snap their shells shut,
from time to time, for this purpose. The snapping is
popularly called feeding, but it is nothing of the kind.
It serves to drive food out instead of taking it in, and
so long as the shell is open a gentle current of water is
drawn in by a delicate piece of microscopic machinery
which will be explained later on. The food of the

oyster consists of invisible organisms which float in
the water and are drawn in with it.

The apparatus for opening and closing the shell is
very interesting. If you were to open a check-book,
and were to wedge a piece of rubber between the
leaves, close to the back, it would form a spring, which
would be squeezed by closing the book, and would
open it again when released. A book with such a
spring would be open at all times, except when forcibly
closed. Wedged in between the two shells of the
oyster, at their narrow ends, is an elastic pad, the
hinge-ligament, which acts in exactly the same way.
When the shell is forcibly closed the ligament is
squeezed, and it expands when it is released and thus
throws the free edges of the shells apart. The ligament
is not alive. It is formed, like the shell itself, as
an excretion from the living tissues of the oyster, and
its action is not under the control of the animal. It
keeps the shell open at all times, unless it is counteracted,
and for this reason an oyster at rest and undisturbed,
or a dead oyster, always has its shell open.

The active work of squeezing the passive ligament
and closing the shell is done by a large, powerful
muscle, made up of a bundle of contractile fibres
which run across the body between the shells, and are
fastened to their inner surfaces over the dark-colored
spots which are always to be seen on empty oyster
shells. The muscle is known to oyster-openers as the
heart, and they assure you that when this is cut, the vital
point, the seat of the oyster's life, is reached and that
a wound here causes instant death. This is of course
an error, and cutting the muscle causes the shell to

open simply because it destroys the animal's power to
close it; but a fresh oyster on the half-shell is no more
dead than an ox which has been hamstrung. Any
one who has struggled with an oyster-knife to force
open an obstinate thick-shelled specimen, knows the
great strength of this little muscle. It is said that
when fishermen are caught by the feet or hands
between the shells of the giant clam of the Pacific, they
never escape alive, but are held, as if by a vise, until
the tide rises and drowns them; but firmly as the
muscle of the oyster holds the shell together, a little
skill is all that is needed to overcome it. Some years
ago, while on the State Oyster Commission, I stood
with my watch in my hand, in a Crisfield packing-house,
and timed a young man, who, with nothing but
a small thin knife, opened thirty oysters in a minute.
He worked with the precision of a machine, and made
six motions for each oyster. One hand took the oyster
from the pile at his side, the other cut the muscle
from the upper shell; a third movement threw the
shell away; a fourth forced the oyster from the other
shell; a fifth threw it into a tin bucket, and the second
shell was thrown aside by the last movement. He was
very proud of his skill and of the prizes he had taken,
and although he seemed to have abundant assurance,
he explained that his movements were retarded by
his diffidence in the presence of state commissioners,
and he said that, when free from embarrassment, he
could "shuck" thirty-six oysters a minute.

The work of closing the shell is done by the muscle,
but we must go very much farther in the study of the
oyster in order to find why it closes. It is opened by

the mechanical properties of the ligament, but the
cause of its closure cannot be the mechanical properties
of the muscle, for these are just the same whether
it is open or at rest. Careful investigation shows the
existence of a wonderful apparatus, consisting of the
muscle which does the work, of nerves which connect
the muscle with the brain, of other nerves which run
to the more exposed parts of the oyster's body, and of
sense organs which are connected with the ends of these
sensory nerves, and these serve to put the animal into
communication with the external world. Though
very much simpler, the mechanism is essentially like
that of our own bodies. The oyster's shell is lined by
a fleshy mantle, which is fringed by a border of dark-colored
sensory tentacles, which are partially exposed
when the shell is opened. The approach of danger is
perceived by these organs, which transmit a sensation
of danger along the sensory nerves to the brain, and
this in turn sends a nervous discharge along another
set of nerves to the muscle, and this shortens under
the stimulus and pulls the shells together and holds
them fast.

The contrast between the opening and the shutting of
the oyster's shell is an excellent illustration of the difference
between vital activity and non-vital action. The
explanation of the movement which opens the shell is
found in the physical properties of the ligament, and
a piece of rubber in the same place would produce the
same effect; but while the closure of the shell is undoubtedly
due to the physical properties of the muscle,
in order to find the reason for its action we must carry
our investigation very much farther, and must learn

what was the change, external to the oyster, which
excited the sense organs, and must ask how the oyster
has learned to associate such a sensation with the
presence of danger, and how it has learned that the
danger may be escaped by closing the shell.

It is much more easy to ask this question than to
answer it. The oyster is by no means a simple animal,
and our efforts to study and understand its structure
bring us, at the first step, face to face with problems of
the most profound character; problems which will tax
all the resources of investigators and philosophers for
many generations. We will not, however, enter into
these deep questions, but will confine ourselves to
simpler matters.

The muscle is attached to the shell at some distance
from the hinge, in order that it may have leverage and
work to advantage; and it must therefore be able to
move as the shell grows, for in an oyster three inches
long its area of attachment is outside what was the
extreme border of the shell when this was only an
inch long. The muscle travels by the addition of new
fibres on its outer surface, together with the absorption
and removal of those on its inner border. As it moves,
the old impression on the shell is gradually covered up
by new deposits of lime, and in an empty shell it may be
traced for some distance up towards the hinge, when it
gradually becomes more faintly marked, as the layers of
new shell grow thicker. A very good idea of the way
the shell grows and keeps pace with the growth of the
body, may be gained by the careful examination of the
muscular impression on its inner surface. Every fool
knows why a snail has a house, but the king could

not tell how an oyster makes his shell. We can now
give a satisfactory answer to what will not, I hope, be
thought a fool's question: "Canst tell how an oyster
makes his shell?"

The shell, on each side of the body, is lined by a
thin, delicate, fleshy fold, the mantle; which may be
compared to the outer leaf on each side of the checkbook,
next the cover. It lies close against the inside of
the shell, and forms a delicate living lining to protect
the body and the gills, and it is also the gland which
makes the shell.

At all times, while the animal is alive, it is laying
down new layers of pearl over its whole inner surface,
and as each successive layer is a little larger in area
than the one before, the shell increases in size as well
as in thickness, and the hinge, where there are many
layers, is very thick, while the edge, which is new, is
quite thin and sharp. Each layer is very thin, hardly
thicker than a sheet of tissue paper, but the deposition
of layer on layer gradually results in a solid box of
stone.

Shells which grow on rough, irregular surfaces conform
to their shape as perfectly as if they had been
moulded into the ridges and furrows, like soft clay.
An oyster growing in the neck of a bottle takes the
smooth, regular curve of the glass, and on the claw of
a crab an oyster shell sometimes follows all the angles
and ridges and spines, as if it were made of wax instead
of inflexible stone. Its apparent plasticity and the
mouldings of its surface are due to the flexibility of
the soft edge of the mantle. When the oyster is at
rest this protrudes a little beyond the edge of the shell,

so that each new layer is a little larger in area than
the last one. The soft mantle readily conforms to the
shape of the body to which the oyster is fastened,
and however irregular this may be, the new shell takes
its shape and closely adheres to it, because the new
deposits are laid down directly upon it.

You will see from this account the error of the
current belief that an old oyster cannot fasten itself.
Since the adhesion takes place around the growing
edge, an oyster may fasten itself at any time, and
clusters of oysters are often found with their shells
soldered together near their tips. This can of course
only occur after they are well grown.

Oysters are able to close up broken places in their
shells, and most molluscs sometimes absorb and rebuild
parts of the shell. If any foreign body gets in between
the shell and the mantle, shelly matter is deposited
upon it. The pearls of the pearl oyster are formed in
this way. Some small particle, such as a grain of
sand, works its way in, and forms a nucleus which is
gradually covered by layer after layer of pearl. The
brilliant lustre, as well as that of mother-of-pearl,
which is nothing but polished shell, is due to the
interference of light caused by the laminated structure.

It is said that the Chinese manufacture pearls, or
rather make the pearl oyster do the work for them, by
inserting between the shell and the mantle strings of
small shot. Did you ever see one of the sacred clam
shells which the Chinese Buddhists believe to have a
miraculous origin? They are often found in collections.
The inside of the shell has a beautiful pearl lustre,
and along it is a row of little fat images of Buddha,

squatting with his legs crossed under him, and his
elbows on his knees: they are formed of pearl precisely
like that which lines the rest of the shell, a little
raised above its surface and outlined in faint relief,
but they are part of the shell, with no break nor joint.
In the process of manufacturing them, the shell of
the living animal is wedged open, and thin images,
punched out of a sheet of bell-metal, are inserted.
The animal is then returned to the water, and is left
there until enough new shell has been formed to
cover them with a varnish of pearl thick enough to
fasten them, and to hide the metal, while permitting
the raised outline to be seen.

Several years ago it occurred to me that a series of
microscopic specimens of stages in the growth of the
shell might be obtained in the same way, and that, by
studying them, the whole history of the process might
be traced. One of my students, at my suggestion, put
into the shells of a number of oysters thin glass circles,
such as are used to cover microscopic specimens. The
oysters were then returned to the water, and were left
undisturbed until new shell began to be formed on the
glasses. These were then taken out and studied under
the microscope.

At the end of twenty-four hours the glass was found
to be covered by a transparent, faintly brown film of
thin gummy deposit, which exhibited no evidences of
structure, and contained no visible particles of lime,
although it effervesced when treated with acids, thus
showing that it contained particles too small to be
visible with a microscope. The gummy film is poured
out from the wall of the mantle, and in forty-eight

hours it forms a tough leathery membrane fastening
the glass cover to the inside of the shell. At about
this time the invisible particles of lime begin to aggregate
and to form little flat crystals, hexagonal in outline,
and about 1/2500 of an inch long. The crystals
grow and unite into little bundles or groups, and new
ones appear between the old ones, until, at the end of
six days, the film has completely lost its leathery character
and has become stony, from the great amount of
lime present in it. In three or four weeks the glass
cover is completely built into the shell and can no
longer be seen, and its place is only to be traced by
its form, which is perfectly preserved upon the inner
surface of the shell. When broken out it is found to
be coated with a thick plate of white shell, which is
beautifully smooth and pearly upon the side nearest
the glass.

Microscopic examination of this plate shows that it
is made up of an immense number of minute crystals,
packed and crowded together into a solid mass, without
any regular arrangement. These observations
show that the new layers are thrown off in the form
of a gummy excretion from the mantle, with the lime
in solution, and that the particles unite with each other
and form crystals while the gum is hardening.

The oyster obtains the lime for its shell from the
water, and while the amount dissolved in each gallon
is very small, it extracts enough to provide for the
slow growth of the shell. It is very important that
the shell be built up as rapidly as possible, for the
oyster has many enemies continually on the watch for
thin-shelled specimens. In the lower part of the bay

I have leaned over a wharf and watched the sheepshead
moving up and down with their noses close to
the piles, crushing the shells of the young oysters
between their strong jaws and sucking out the soft
bodies. As I watched them I have seen the juices
from the bodies of the little oysters streaming down
from the corners of their mouths, to be swept away by
the tide.

The sooner a young oyster can make a shell thick
enough to resist such attacks the better, not only for
the oyster but for us also; for once past this dangerous
stage of development, there is a prospect that it may
live to complete its growth; although it is true that
the fully grown oyster has many enemies which either
crush the shell or pull it apart, or else bore holes
through it in order to reach the delicate flesh within.
At all times in its life its chance of survival is greatest
when the supply of lime is so abundant that it is able
to construct rapidly a thick, massive shell. The rate of
growth of any animal must be regulated by the supply
of that necessary ingredient of its food which is least
abundant, as may be illustrated in many ways. To
run a locomotive the engineer must have fuel and
water and oil. He needs very little oil, but that little
he must have. After this is gone, an unlimited supply
of fuel and water will not help him. He must have
oil or stop. So, too, if he have plenty of oil and fuel,
but only a little water, he must stop as soon as the
water fails. In general, the amount of work he can
do is determined by his supply of that of which he has
least. If food in general is abundant while there is a
scarcity of one necessary article, growth can take place

only so fast as the scarce article can be procured. A
superfluity of other things is of no value, for it cannot
be utilized.

There are many reasons for believing that the
growth of oysters is limited by the supply of lime,
and that all the other necessary ingredients of their
food are so abundant that an increase in the supply of
lime would cause more rapid growth, greater safety
from enemies, and an increase in the number of oysters.
All kinds of shelled molluscs grow more rapidly, and
reach a greater size, and have stronger and thicker
shells in coral seas, where the supply of lime is
unlimited, than in other waters. In some parts of the
Bahamas the large pink-lipped conch, the one which
we often see for sale in the fruit stores of Baltimore,
is so abundant that whole islands, large enough to be
inhabited, are entirely made up of the broken fragments
of these beautiful shells, which have been
pounded to pieces and heaped up by the waves.

The fresh-water mussels of our western rivers are
very large in limestone regions, and so abundant that
the bottom is almost paved with them, while in another
river, perhaps only a few miles away, but flowing
through a country where there is little lime, they are
few and very small, with thin, fragile shells.

If you turn over the old bones which are sometimes
found in the woods and fields, you will nearly always
find a number of snails which have been drawn to
them for the sake of the lime.

In order that the oyster may grow rapidly, and may
be securely protected from its enemies, it must have
lime. The lime in the water of the bay is derived in

great part from the springs of the interior, which, flowing
through limestone regions, carry some of it away
in solution, and this is finally carried down the rivers
and into the bay. Some of it is no doubt derived from
deposits of rock in the bed of the ocean, and some from
the soil along the shores. Now, the geologist will
tell you that the limestone rock has all of it at one time
been part of the bodies of living animals. Limestone
is either old reefs of fossil coral, or beds of extinct
shells, or the skeletons of other animals and plants
which lived in remote ages and stored up the lime
from the ocean at a time when it was more abundant
than it is now. The oyster gets the greater part of its
lime from these sources in this roundabout way, but a
very considerable portion is obtained in a much more
direct way, by the decomposition of old oyster shells.

We save up egg shells to feed laying hens, but we
waste our oyster shells in every possible way, and
treat them as if they were of no value. Some are
burned for lime, some are used for making roads and
wharves, some are used for filling in low land, some
are dumped in great piles at convenient spots in the
bay, where they sink far down into the mud and are
lost.

I shall show you soon that there is another far more
important reason why they should be returned to the
beds, but their value as food for the oyster is very
great, and should lead us to return them to the beds.
On the oyster-beds an old shell is soon honeycombed
by boring sponges and other animals, and as soon as
the sea-water is thus admitted to its interior, it is
rapidly dissolved and diffused. In a few years nothing

is left. It has all gone back into a form which makes
it available as oyster food, and it soon begins its transformation
into new oyster shells. In this way the
oysters obtain some of their lime directly without
being compelled to draw on the inland beds of ancient
fossils, and if all the shells could be returned to the
beds, this source of supply would be greatly increased.

The difference between the right and the left shells
of the oyster has a very profound significance, for in
science nothing is trivial or unimportant. Most of
the near relations of the oyster, like the clam and the
fresh-water mussel, have the two sides of the body, and
the two shells, alike. These animals are not fastened
nor stationary like the oyster. They move from place
to place in search of food, and their line of locomotion
lies in the plane which divides the body into halves.
They are erect and bilaterally symmetrical like other
locomotor animals, such as the horse, the fish, the
butterfly and the crab. The full-grown oyster has no
locomotor power and it lies on its left side, but in the
early part of its life it is very active, and is then bilaterally
symmetrical like the clam. When it ceases
its wanderings and settles down for life, it topples
over, falls on its left side, and fastens itself by its left
shell, which soon grows deep and spoon-shaped, while
the right becomes a flat, movable lid. The body,
which was originally symmetrical, becomes distorted
or twisted to fit the difference in the shells,
and naturalists see in the fact that the locomotor
relations of the oyster are symmetrical through life,
while the oyster loses its symmetry as soon as it
settles down, one of the proofs that it is descended

from locomotor ancestors. There are many other
proofs that this has been its history, and that it has, in
comparatively modern times, learned to fasten itself to
rocks above the soft mud of our bays and estuaries,
in order to avail itself of the rich vegetation; that it
has lost its symmetry in order to fit it for this mode
of life. The oyster is a very ancient animal, and its
sedentary habits belong to the more modern part of
its history; although this change took place very long
ago, so far as human chronology goes, and fossil
oysters are found in many parts of the world.

In order to understand the anatomy of the oyster, a
clear conception of the structure and significance of
its gill is most important. In all the bivalve molluscs
the gills are very complicated, and they dominate
over the whole structure of the body in such a
way that an anatomical sketch of the animal is of
necessity little more than an account of the gills. A
thorough knowledge of the oyster-gill will not only
throw light on the purpose and use of all its other
organs: it will at the same time help us to understand
the great value of the animal as a means for making
the microscopic inhabitants of our waters useful, and
it will also show how well it is adapted for cultivation,
and why it is impossible for natural oysters to
stock the whole bay without aid from man.

The labor which is necessary before we can have a
clear, accurate picture of them, of their complicated
structure, their relation to other parts of the body,
their use and their origin, is considerable, but it is
well worth while, for the gills give us the key to the
whole significance of the oyster; but this requires

close attention to all the details of a long, complicated
and minute description, which from the nature of the
case cannot be stated briefly, although it may all be
put in simple language.

A gill is, of course, a breathing organ, for aerating
the blood by exposing it to the oxygen in the water;
and the oyster has a heart, for driving to the various
organs of the body the blood which has been purified
in the gills. It is easy to see and study the oyster's
heart, but in order to do so the animal must be
opened with great care, by cutting the muscle with a
thin sharp blade, as near the shell as possible. If this
is done, a small semi-transparent space will be seen
close to the inner edge of the muscle. The thin membrane
which covers the space is the pericardium, or
the chamber which holds the heart, Plate I, d, and
through its transparent wall this may be seen slowly
pulsating, for an oyster is not killed by opening its
shell, and its heart continues to beat for hours, or,
under favorable conditions, for days. If the pericardium
be gently lifted and cut with sharp scissors,
the heart, Plate II, d, with its blood-vessels, will be
exposed. It consists of two chambers, the auricle,
which receives the pure blood from the gills, and
a ventricle, which drives it through arteries to the
various organs of the body.

While the gill of an oyster is a breathing organ,
like the gill of a fish or crab or conch, this is only one
of its many uses. The fish and the crab and the conch
have other organs for supplying the gills with a stream
of fresh water, but the gills of the oyster, besides purifying
the blood, keep up a circulation of water for

themselves. They are also organs for gathering up
food from the water, and after it has been gathered
they become organs for carrying it to the mouth.
They are also reproductive organs, adapted for securing
the fertilization of the eggs, and thus providing for
the propagation of the species. In the European oyster
and in the mussel they are also brood-chambers, in
which the young are held and protected and nourished
during their early stages of growth, until they are large
enough to care for themselves.

An organ which is at once a gill, a pump for supplying
the gills with water, a food-collector, an organ
for carrying the food into the mouth, a reproductive
organ, and a nursing-chamber, must, of course, be
complicated. The oyster's gill does all these things,
and does them all well. It is not a jack-of-all-trades,
but a machine which is beautifully adapted for carrying
them all on at the same time, in such a way that each
use helps the other uses, instead of hindering them.
This is the more remarkable since an ordinary mollusc,
such as the conch, has distinct organs for all
these purposes, although the oyster's gill does everything
just as well and just as readily as the various
organs of the conch.

There are four gills in the oyster, two on each side
of the body. They are long, flat, thin, leaf-like organs,
Plates I and II, g, placed side by side, and nearly
filling the mantle chamber, in which they hang.
Each gill is made up of two leaves, so that there are
in all eight gill-leaves.

If you gum together the ends of a folded sheet of
foolscap paper, so as to make a flat pocket, this, when

held vertically, with the opening above, will form a
pretty good model of a single gill.

The closed portions of the four gills hang down
into the mantle-chamber, side by side, but their upper
edges are fastened to each other, and to the inside of
the mantle, in such a way that they form a folded partition,
something like a double W, which divides
the mantle-chamber into two parts: a lower chamber,
in which the gills hang, known as the gill-chamber,
and an upper chamber, into which the pockets open.
This chamber is known as the cloaca, the Latin word
for a sewer, or channel for waste water, and I hope to
show you the fitness of the name soon.

The partition between the two chambers is formed
somewhat in this way. The upper edge of the outer
leaf of the outer gill is united, along its whole length,
to the inner surface of the mantle. The upper edge of
the inner leaf of the outer gill is united to the same
edge of the outer leaf of the inner gill. The upper
edges of the inner leaves of the two inner gills are
united to each other on the middle line of the body.

If you were to make four pockets out of four sheets
of paper, and were then to gum two of them together
along their free edges, you would make a double
pocket, which might be opened out so that a section
through it would be like a W. This would serve as
a model of the two gills on one side of the body, and
two more sheets, treated in the same way, would
make a model of the other two gills. Now gum two
W's together, side by side, and the double W will be
a model of the four gills. Now open a very large
book-cover, just far enough to gum the upper outer

edge of one W to the inside of one cover, and the
opposite edge of the other W to the other, and you
will have a rough model of the coarse anatomy of the
oyster's gills, like the diagram in Fig. 1 of Plate III.
The space between the covers is the mantle-chamber,
divided by the gills into a lower portion or gill-chamber,
in which the gills hang, and an upper cloacal
chamber, into which the pockets open.

So far I have spoken of the gills as if the pockets
reached, without interruption, from end to end, but
this is not the case. Each pocket is divided up, by a
series of vertical partitions, into a number of small
cavities—the water tubes, each of which ends blindly
below, and opens above into the cloaca.

To represent them in our model we must gum the
two leaves of each pocket together from top to bottom
along a series of vertical lines about an inch
apart. Our model is very much larger than the actual
gill, of course.

The spaces between the partitions which are thus
formed will represent the water tubes, w, in Figs. 1
and 3 of Plate III, closed below and opening above
into the cloaca, and our model will now illustrate the
anatomy of the gill, so far as it can be made out
without a microscope.

I must now speak of the minute anatomy. If a
small piece of one of the gills be cut out, and spread
flat upon a glass slide so that its surface may be
examined under a microscope, it will be found to be
thickly covered with parallel ridges running from top
to bottom, like the lines on the sheet of paper, each
ridge being separated from the next one by a deep

furrow. Fig. 3 of Plate III is a greatly magnified
drawing of a cross-section of a small part of a gill,
including one water tube, w, and the partitions a, a,
between it and the adjacent tubes; r, r, r are the
ridges, and p, p water pores. In the bottoms of the
furrows there are many minute openings—the water
pores, which pass through the wall of the gill into
the water tubes, and thus form the channels of communication
between the two divisions of the mantle-chamber.

The ridges themselves are hollow, or, rather, each
one contains a minute blood-vessel, which runs
throughout its entire length, so that each wall of each
gill is practically a grating of parallel, vertical blood-vessels,
in which the blood is purified by contact with
the water which fills the gills and the chamber in
which they hang. The purified blood is then forced
into larger vessels, which carry it to the heart, by
which it is pumped to all parts of the body, to be
again returned to the gills after it has become impure.

The gills are therefore easily intelligible, so far as
they are simply organs of respiration; they hang in
the water which fills the mantle-chamber, and their
walls are filled with blood-vessels in which the blood
comes into close contact with the water.

The way in which the current of fresh water is kept
up to bathe the gills continually with a new supply is
more complicated.

When one of the ridges on the surface of the gill is
examined with a high power of the microscope, it is
found to be fringed on each side by a row of fine
hairs, Plate III, Fig. 2, c, c, each one less than 1/500 inch

long, and so fine that they are invisible under a low
magnifying power. They project from the sides of the
ridges, over the furrows between them, and therefore
overhang the water pores in the bottoms of the furrows.

In a fragment cut from a fresh gill, each one of these
hairs is constantly swaying back and forth, with a
motion like that of an oar in rowing, quick and strong
one way, and slower the other way. They all move in
time, but they do not keep stroke, for each one comes
to rest an instant before the one on one side of it, and an
instant after the one on the other side. So that waves
of motion are continually running from one end of
each ridge to the other, like the waves which you have
seen running over a field of ripe grain, as each stalk
bends before the wind and then recovers.

What would happen if a boat's crew were to row
with all their strength, with the boat tied to a wharf?
As they could not pull the boat through the water,
they would push the water past the boat. This is
exactly what the cilia do. They set up a current in
the water. Each one is so small that its individual
effect is inconceivably minute, but the innumerable
multitude causes a vigorous circulation, and each one
is set in such a position that it drives the water before
it from the gill-chamber into one of the water pores,
and so into one of the water tubes inside the gill;
and as these are filled they overflow into the cloaca
and fill that. If the mantle were closed, all the water
would soon be pumped out of the gill-chamber into
the cloaca, but you remember that an oyster at rest
always has the mantle open. As fast as the gill-chamber
is emptied by the cilia, fresh water streams in from

outside, to be, in its turn, driven through the water
pores into the water tubes, and through them into the
cloaca, where it is driven out between the open shells
and away from the oyster.

So much for the gills as organs for maintaining a
current of water. We come now to the way in which
they procure food.

The food of the oyster consists of microscopic organisms,
minute animals and plants, which swim in the
water. They are pretty abundant in all water, but
those who do not work with the microscope have very
erroneous ideas on the subject. When a professional
exhibitor shows you, under the microscope, what he
calls a drop of pure water, it is nothing of the sort.
It is either a collection made by filtering several barrels
of water, or else it is a drop squeezed from a piece of
decayed moss, or from some other substance in which
they have lived and multiplied.

Sea water is like fresh water in this respect, and an
oyster must strain many gallons of water to get its
daily bread; but the gills, with their hundreds of thousands
of microscopic water pores, are most efficient
strainers.

The surface of the gills is covered by an adhesive
excretion, for entangling the microscopic organisms
contained in the water, and as this circulates over and
through the gills, they stick fast like flies on fly-paper.
The cilia which drive the water through the gills, push
the slime, with the food which has become entangled
in it, towards the mouth, which is well up towards the
hinge; for it is hardly necessary to say that what the
oystermen call the mouth is only the opening between
the halves of the mantle.

On each side of the mouth, Plate II, m, there is a
pair of fleshy organs, Plates I and II, w, called the
lips, although they are more like mustaches than lips,
for they hang down on each side of the mouth. One
on the right is joined to one on the left, above the
mouth, while the other two are joined below it, so that
the mouth itself lies in a deep groove or slit between
the lips.

The ends of the gills fit into this groove, and as the
cilia slide the food forward, it slips at last between the
lips and slides into the mouth, which is always open.
As this process is going on whenever the oyster is
breathing, the supply of food is continuous, and while
it consists, for the most part, of invisible organisms,
the oyster's stomach is usually well filled. It is not
necessary to describe the oyster's stomach and intestine,
and dark-colored liver, as these will be understood
from the figure. The chief purpose of this
anatomical sketch is to show the wonderful way in
which the gills of the oyster fit it for gathering up the
microscopic life of our bay, and for turning it into
valuable human food. Looked at from this point of
view, the minute anatomy of the animal becomes eminently
practical, as it enables us to understand its true
relation to man.

In view of the very exceptional fertility of the bay,
and its boundless capacity for producing microscopic
vegetation, the immense importance of an animated
strainer perfectly adapted for filtering very great
quantities of water, for gathering up the microscopic
life which it contains, for digesting and assimilating
it, and for converting it into food of the most attractive

and nutritious character, cannot be overestimated; but
after we have studied the embryology of the oyster,
we shall understand why the natural oysters alone
can never utilize all the resources of our waters.
We shall see why it is that the oyster is so well fitted
for domestication and cultivation, and why the cultivation
of oysters will render the Chesapeake Bay incomparably
more valuable than it has ever been, even
before our natural beds began to deteriorate.