oyster in a celebrated oyster-shop. There are Courseilles, Cancales,
Marennes, Ostend, Zeeland, Arcachon, English natives, Côtes Rouges (red
banks), and Black Rocks. And you can eat sea-urchins there, too, if you
wish. They have not, however, got the celebrated oysters from the Lake
Fusaro, near Naples. This was the ancient _Acherusia palus_, and in the
neighbouring Lake Avernus and the Lucrine lake oysters were cultivated
by the ancient Romans, the young oysters being made to affix themselves
at “the fall of the spat” to wooden “stands” or frames, which were then
placed in the lake (a salt-water lake), where they had abundant minute
vegetable food and grew large and fat. The same cultivation, with the
same shape of “stands,” is carried on at the present day in the Lake
Fusaro. My friend, Mr. Günther, of Magdalen College, Oxford, has
published pictures of Roman tiles from this neighbourhood showing the
oysters adhering in rows to the wooden frames. These tiles were
apparently sold to holiday visitors in the time of the Roman emperors as
a memento of a happy day spent at the Lucrine lake, just as a sugar
basin or a mug is now sold at our seaside resorts with the inscription,
“A present from Margate,” or Southport, or Blackpool, and the picture of
a shrimp above it.
The care of the breeding oyster and the plans adopted by the owners of
oyster-beds for catching the “spat,” or young oysters, when they fall to
the bottom, by placing movable tiles or frames for them to fix
themselves to, form an important part of the craft of the oyster-man. It
is a difficult business, and is variously carried out in England,
France, Holland, and America. The young oysters, when they have fixed
themselves, are carried on the movable tiles or frames from one region
to another for the purpose of encouraging their growth and avoiding a
variety of dangers to their life and health (sometimes from the Bay of
Biscay to the mouth of the Thames!). They are often—but not
always—finally fed up in sea-ponds or inlets, which are peculiar in
containing an enormous number of those very minute microscopic plants,
with beautifully shaped siliceous shells, which are known as diatoms.
These are so abundant in such ponds as to form a sort of powder or cloud
near the bottom, and the oysters draw them, day and night, by their
gill-currents into their mouths, digest them, and grow fine and fat. The
district of Marennes, on the west coast of France, is celebrated for
having sea-ponds or tanks in which a wonderful diatom of a bright blue
colour abounds; so abundant are they that the cloud produced by them
in the pools is of a deep cobalt-blue. When oysters are placed in these
tanks to fatten, their gills or beards become rich blue-green in colour.
They lose the colour after ten days, when removed to ordinary tanks.
These are the celebrated green oysters or “Marennes vertes” of French
restaurants. The colouring matter of the little diatoms—swallowed by
the million and digested—is taken up by the blood of the oyster from
its stomach, and is excreted by certain corpuscles on the surface of the
gills—as I showed some twenty-five years ago—just as red madder is
deposited in the bones of a pig fed upon madder, and as the feathers of
the canary take up the colour of cayenne pepper when it is mixed with
the canary’s food. It used to be thought that the green colour of the
green oyster is due to copper—and that opinion was supported by the
curious fact that the blood of all oysters and other molluscs, and also
of lobsters, scorpions, and king-crabs, does really contain a minute
quantity of copper, just as our blood contains iron! It was also
supported by the fact that occasionally a fraudulent fishmonger, when
asked to supply green oysters, has been convicted of colouring the
beards of ordinary oysters with green copper salt, so as to imitate the
real article! The real history of the green-bearded oysters is now quite
certain, and any one interested in the matter should look at the
coloured pictures of the beautiful little blue-coloured _Navicula
ostrearia_—the diatom on which this oyster feeds, published by me in
the _Quarterly Journal of Microscopical Science_ in 1885.
XV
MATERNAL CARE AND MOLLUSCS
The American and Portuguese species of oysters, which are called
respectively _Ostrea virginiana_ and _Ostrea angulata_, as opposed to
the common oyster, which is known as _Ostrea edulis_, are not
hermaphrodite like the latter, but have distinct males and females.
Moreover, the young are not fertilised within the parent’s body, nor do
they pass their earliest stages of growth within the parent’s shell
adhering to the “beard,” or gills, as in the common oyster. The eggs
(Fig. 31) are, on the contrary, discharged by the females into the sea,
and at the same time the males discharge a cloud of microscopic sperm
filaments, or spermatozoa (Fig. 32), which dart about in the water and
fertilise the eggs. That is a more prodigal and less certain process
than that pursued by the common oyster. The American and Portuguese
oyster have to pay for it. The female produces in one season not a
million eggs, as does the common oyster, but nine millions. And out of
every fifty million so produced (in some five or six years) only a
single male and a single female individual, taking the whole oyster
population of these species into consideration, survive to maturity.
This enormous excess of egg-production in order to ensure the survival
of a single pair to replace their parents is a very frequent thing in
aquatic animals. But there are many devices by which the necessity for
such lavish scattering of a new brood is avoided. The common oyster is
already a step in advance of the American in this matter, since it
protects its young in the very earliest stages within the shelter of its
shell. A further advance in this direction is found in the fresh-water
mussels (not to be confused with the very different sea-mussels, since
they are bitter and tough, and quite inedible, though used as bait in
sea-fishing). The pond-mussel (_Anodon_) and the river-mussel (_Unio_)
are of distinct sexes, and the gills of the female become swollen up at
the breeding season so as to form two large bags, into which the eggs
are laid by her, as many as 500,000 in number. They are fertilised by
the sperm filaments discharged by the males, which are carried into the
female’s shell by currents produced by the vibrating hairs on the gills,
as in the common oyster. But the young remain much longer in the
mussel’s gills than do the young oysters in those of their parent’s.
Late in the season you find the bag-like gills of the female pond- and
river-mussels full of extraordinary little creatures one-thirtieth of an
inch long, each provided with a pair of triangular shells. They are
discharged into the water, and swim very actively by rapidly opening and
shutting the little shells (Fig. 35). The common scallop (_Pecten_, or
Pilgrim’s shell) swims every now and then in the same way as do these
young mussels, and so do some other bivalves. The young fresh-water
mussels produce a long, sticky thread, which trails from the shell (Fig.
35 _by_). Very few have the good chance to get further on in life than
this stage, for all depends on their stumbling across fish—a
stickleback, or a perch, or a pike—as they blindly snap their shells
and wobble through the water. The lucky triangular mite whose sticky
thread happens to touch a fish’s body becomes immediately fastened by
it to the fish and then grips the skin with its snapping shells, the
edges of which are provided with a few long, sharp teeth. The fish
probably is quite unaware of the lodgment of the young mussel on its
skin, but there it remains, and gets buried for a time in the soft
tissues of the fish, becoming thus actually a parasite for some two or
three months during the winter season. It nourishes itself on the juices
of the fish, and grows to the size of a pin’s head, whilst it is carried
away from its birthplace by the peregrinations of its host, the fish.
Its shell now ceases to be triangular, and becomes like that of its
parents. Eventually the young mussel drops off the fish and rests on the
muddy bottom of pond or river, where it remains for many years, growing
vastly in size, and barely moving during its long life from the spot
where it fell.
[Illustration: FIG. 35.—Young of the pond-mussel after escaping from
the maternal gill-pouch: A, as it escapes, swimming by opening and
shutting the shells; _sh_, shell of one side; _al_, shell-muscle; _t_,
teeth of the shell’s edge; _by_, adhesive filament. B, after it has
fixed to a fish; _mt_, mantle; _f_, muscular foot; _br_, gill processes;
_pad_, _aad_, _al_, muscles; _auv_, heart. (From drawings by the late
Frank Balfour.)]
A beautiful little bivalve common in weedy streams in England is known
as _Cyclas_ (it has no English name); it has a pair of shells shaped
like those of a cockle, but smooth, and only half as big as one’s
little finger-nail. The nursing of the young in the gill-sacs is carried
to a much further point by _Cyclas_ than by the pond- or river-mussel.
Before they are ejected by the parent they are quite large—like their
parent in appearance, and half as big as a hemp-seed. Necessarily there
are not many produced in a season—there is not room for more than
twenty or thirty young in the gill-sacs.
XVI
THE HEART’S BEAT
The beat of the heart is one of those great and elemental features of
man’s life which, in spite of our familiarity with it and its momentary
recurrence, never loses its quality of mystery and isolation. The
ceaseless accompaniment to our lives which the heart is always beating,
like the inexorable stroke of an unseen pendulum, fills even the
stoutest and bravest at times with a sense of awe. It seems now and then
as though an independent living thing were in our breasts, and when it
quickens and struggles, as it were, with its work, or languishes and
hesitates in its efforts we have a sense of helpless domination by an
existence—a living thing—over whose vagaries we have no control.
The heart of man is no special endowment of the human race, nor even of
the higher animals. As I mentioned a few pages back, the oyster and
other shell-fish have a heart which keeps time and beats the seconds for
their uneventful lives, as does that of man for his more varied career.
Not only the molluscs, but the insects, the spiders, the crabs,
lobsters, and shrimps, and even the worms, have each a rhythmically
beating heart. In all of them the significance of this heart and its
beat are the same—it is driving the nourishing, oxygen-carrying blood
through the great vessels (arteries), which branch from it like a tree
into the living tissues of the body, whence it returns by other vessels
(the veins) back to the heart.
In man and the warm-blooded quadrupeds, in birds, reptiles, and fishes,
the blood is of a splendid red colour, and the transparent vessels can
be easily traced in their graceful ramifications and intricate networks,
in consequence of the red blood showing through their walls. The red
colour is due to a peculiar body, which can be easily separated from the
blood as crystals. It has the special duty of carrying oxygen gas
dissolved and attached to it; and of giving up that essential element to
cause slow burning or oxydation in all parts of the body whilst taking
up fresh supplies of oxygen on its passage through the lungs or the
gills. In many of the lower animals (for instance, the oyster) the blood
is devoid of this red crystalline substance (which, by the bye, is
called hæmoglobin), and accordingly we cannot easily catch sight either
of the heart or the blood-vessels (see, however, Fig. 30). But in
shell-fish the blood has a very pale blue tint, and this colour is due
to a substance like hæmoglobin, which also can be crystallised, and is
the oxygen-carrier. Some sea-worms have a green substance of a similar
nature dissolved in their blood, and one can trace their blood-vessels
as a beautiful green network. A good many worms, for instance the common
earth-worm and the leeches (a discovery made by Cuvier, and referred to
by him on his deathbed), and many sea-worms have deep-red-coloured
blood, due to the presence of the same crystalline substance which we
find in man’s blood. And even a snail, common in the ponds at Hampstead
and such places—the flat coiled snail known as _Planorbis_—has blood
of a fine crimson colour, due to the presence of the same red
oxygen-carrier, as an exception to the colourless or pale-blue blood
found in most shell-fish. Perhaps if oysters, too, had red blood,
there would be a prejudice against eating them in the uncooked
condition.
The heart is essentially an enlargement of the great stem or main
blood-vessel which, like the trunk of a tree, has branching roots at one
end of it and ordinary branches at the other. The trunk branches, and
roots of the “heart-tree” are, of course, hollow blood-holding tubes,
not solid fibrous structures, as are the woody branches and trunk of a
vegetable tree. Further, the finest rootlets and the finest terminal
branches in the case of the heart-tree are connected to one another by
the network of very fine branches or by great blood-holding cavities,
which occupy all parts of the body of an animal. The enlarged part of
the trunk of the tree-like system of blood-vessels—the heart—has
powerful muscles forming its walls, the fibres disposed so as to
surround the contained chamber. When these muscular fibres contract,
they squeeze the walls of the chamber together and drive the blood out
of it into the forward branches, called “arteries.” It is prevented from
going backwards into the hinder branches called “veins” (which we
compare to the roots of a tree) by flaps which are so set on the inside
of the great vessel at the entrance to those branches that the flaps are
made to move out across the space by the backward current, and thus
prevent any backward flow, whilst a forward current merely presses them
flat against the wall of the vessel, and thus no obstruction to a
forward flow is presented. These flaps are called the valves of the
heart. The consequence of this arrangement is that whilst blood flows
freely into the heart from the veins or hinder (root-like) set of
vessels, it is driven by the muscular contraction of the heart—only in
one direction—namely, forwards into the arteries. This movement in
one direction is helped in some elongated hearts by the contraction of
the wall of the heart beginning behind and spreading quickly forward
like a wave. The heart of the common earth-worm and of small transparent
worms with red blood like it, which are common in the mud of ponds and
rivers and can be easily watched with the microscope so that one can see
through their glass-like skin what is going on inside them, shows very
beautifully this wave of contraction. The heart in these worms is a long
contractile vessel which runs the whole length of the body along the
back. You can watch the red blood flowing into it through the veins in
each ring or segment of the worm’s body—slowly swelling it out—so that
it looks like a long red cord. Then, suddenly, there is a movement like
a flash in its rapidity, passing from behind forwards! The walls of the
red cord-like heart contract so as to drive the blood forward into the
arteries, which also are present in every ring of the worm’s body. At
the same time you can see the valves, which hang at the entrance of the
veins to the heart, swing with a sudden “chuck” and close those vessels
against the driven blood. The red cord becomes colourless progressively
from behind forwards, owing to the squeezing out of the blood, and by
the time the movement has reached the head of the worm, the hinder part
of the cord-like heart is beginning slowly to dilate again with the
influx of red blood from the veins.
What causes the muscles of the heart to contract at regular intervals?
There is no doubt that the “stimulus” which excites the heart muscles to
contraction is in these simpler animals merely the tension or strain
produced by the presence of a sufficient quantity of blood which has
flowed into the heart from the veins. The heart muscle, after its rapid
contraction, rests; it has no other rest, no sleep, as have all the
other parts of the body. It must rest and take refreshment after each
effort. Whilst it rests the blood quietly flows in and dilates the
heart’s cavity; then the rested muscular wall of the heart, gently
stretched by the recovery after compression of its elastic components,
nourished and oxygenated by the blood, is ready for another “stroke,”
and again it contracts tightly, emptying its cavity of blood, which is
driven into the arteries. So it goes on—effort and rest, effort and
rest alternating without cease. Whilst it is the stroke of the heart
which causes the blood to flow through the arteries into the finest
network of hair-like vessels, what is it that causes the blood to flow
on through the collecting veins, to reach the heart, and actually to
distend that collapsed cavity after its stroke? It must be remembered
that a very low pressure is enough to effect this. In the simplest
arrangements of worms and such-like animals, there is probably some
pressure transmitted to the blood in the veins by the heart-stroke; but
the elasticity of the heart-wall and its necessary tendency to resume
its dilated condition after its squeezing by its rings of muscle, is
what is chiefly effective in drawing on the blood in the veins into the
heart.
In man and the higher animals the whole mechanism of the heart is
greatly complicated by the action of the nervous system upon it and upon
the contraction or expansion of the blood vessels. In this way the rate
of the beat of the heart is affected and brought into relation with the
needs of the blood circulation in remote parts of the body. The beat of
the heart in the human species is more rapid in children than in adults,
and more rapid in women than in men, and it differs in all individuals
under differing conditions. Before birth it is 140 per minute, in the
first month after birth 130, and gradually diminishes to 90 at nine
years of age, and at twenty-one to 70 in man and to 80 in woman. But
these figures only represent a general average; there are healthy men
whose pulse usually is less than 45 per minute, and there are
individuals who, without being invalids, yet have the movement of the
heart so liable to increase in rapidity through mental or other
excitement, acting by nerves directly on the heart muscle, that the
pulse often goes up to 120. In the horse and the ox the pulse or heart
beat is 36 to 40 a minute; in the sheep 60 to 80; in the dog 100 to 120;
in the rabbit 150; and in small creatures, like mice and moles, 200, and
even more! I do not know what is the record for the elephant, but as it
seems that the larger the mammal the slower the pulse, one would not
expect more than 20 to 25 beats a minute in his case.
It is easy to watch the beating of the heart of a flea or other small
insects—under the microscope—since the skin is sufficiently
transparent. It is not usually much more rapid than in man, but in the
very transparent little fresh-water shrimps which are called water-fleas
(_Entomostraca_) I have seen the heart beating so rapidly that I could
not count its rate. The heart in insects and shrimps and their like is
remarkable for the fact that whilst it pumps out blood through arteries
both in front and behind, it has no actual veins opening into it. All
the veins, which in their ancestors entered the heart in a row on each
side of it, have united, and their walls broken down, so that the heart
lies in a sac full of venous blood from which it draws its fill, when it
dilates, through a series of valve-bearing openings on its surface,
openings which, in an earlier stage of development, were connected with
individual veins.
The heart of the _Ascidians_ or sea-squirts, common sac-like marine
creatures of most varied form, size, and colour, is perhaps the most
extraordinary in the whole animal series. I have often watched it in
transparent individuals of this group. It is an oblong sac with
branching vessels at either end. It beats for some thirty or forty
strokes so as to drive the blood forwards; it then pauses, and the
onlooker is astounded to see the wave of movement changed, and the heart
steadily beating the same number of strokes in the reversed direction.
What were arteries become veins, and the veins become arteries. Then
again there is a pause—which seems like a moment of hesitation and
doubt—and the original direction of movement is resumed; then again
there is a pause and a reversal, and so on, with absolute regularity. It
is still a matter for investigation as to why and how this altogether
exceptional alternating reversal of the heart’s action is brought about.
It is a curious fact in illustration of the essential character of the
heart and its beat that “hearts” are produced in some animals by
dilatation of the lymph-vessels—a system of delicate vessels, difficult
to see, which take up the colourless fluid which the blood-vessels exude
into the tissues and return it to the heart. The eel has a pair of these
“lymph-hearts” in its tail, and the common frog has a pair near the
shoulder-blades and another pair at the hips. These sacs have muscular
walls, and pulsate rhythmically like the blood-heart, driving on the
lymph fluid through the lymph vessels to join the blood-stream.
The simplest thing in the animal world which can claim the name of a
heart—or, at any rate, be compared with that organ—is found in those
microscopic animalcules which consist of only a single “cell” or
corpuscle of living protoplasm. These animalcules may be compared to a
single brick or unit of structure, whereas all other animals consist of
thousands, or even millions, of such corpuscles or units aggregated and
fitted together as are the bricks and planks of a house. In most of
these uni-cellular animalcules you may observe with a high-power
microscope a little spherical liquid-holding cavity, which slowly
enlarges, then bursts at the surface and collapses. After a brief
interval it forms again, and again bursts to the exterior. In the
“bell-animalcule”—a beautiful active little creature only
one-thousandth of an inch in diameter—it may be seen to form, swell,
collapse, and re-form as often as twenty times in a minute (see Fig.
41). Soluble colouring matter taken in by the animalcule with food is
excreted by the liquid accumulated in and ejected to the exterior by
this spherical chamber. It is called the “pulsating” or “contractile”
vacuole, and by its rhythmical pulsating movement of dilatation and
collapse presents definite points of similarity to the alternately
dilating and contracting hearts of higher animals. The entering flow of
liquid here, as in the veins and heart of higher animals, is continuous.
The rhythm is due, as is the rhythm of the heart, to the alternation of
a brief period of activity or contraction, and a brief period of
consequent exhaustion, rest, and repair on the part of living
contractile substance.
XVII
SLEEP
An enterprising journalist has recently published the replies of a
number of well-known men to an inquiry as to how many hours’ sleep they
are in the habit of taking, and what they find to be the best remedy for
sleeplessness. Such an inquiry naturally leads on to further thoughts
about “Sleep.” What a mysterious, yet sweet and lovable thing it is! How
strange it is that we all regularly and gladly abandon ourselves to it!
How terrible is the state of those who cannot do so! And then one is led
to ask, what is it? and why is it? Do all living things sleep for some
part of the twenty-four hours? How does it differ from mere resting, and
in what does its virtue consist?
Shakespeare has said the most beautiful words that have ever been
uttered about sleep, and that because he knew what it was to seek for it
in vain—