LITTLE BEINGS FROM A MINIATURE OCEAN
[Illustration]
In our last lecture we soared far away into boundless space, and lost
ourselves for a time among seen and unseen suns. In this lecture we will
come back not merely to our little world, nor even to one of the
widespread oceans which cover so much of it, but to one single pool
lying just above the limits of low tide, so that it is only uncovered
for a very short time every day. This pool is to be found in a secluded
bay within an hour's journey by train from this college, and only a few
miles from Torquay. It has no name, so far as I know, nor do many people
visit it, otherwise I should not have kept my little pool so long
undisturbed. As it is, however, for many years past I have had only to
make sure as to the time of low tide, and put myself in the train; and
then, unless the sea was very rough and stormy, I could examine the
little inhabitants of my miniature ocean in peace.
The pool lies in a deep hollow among a group of rocks and boulders,
close to the entrance of the cove, which can only be entered at low
water; it does not measure more than two feet across, so that you can
step over it, if you take care not to slip on the masses of green and
brown seaweed growing over the rocks on its sides, as I have done many a
time when collecting specimens for our salt-water aquarium. I find now
the only way is to lie flat down on the rock, so that my hands and eyes
are free to observe and handle, and then, bringing my eye down to the
edge of the pool, to lift the seaweeds and let the sunlight enter into
the chinks and crannies. In this way I can catch sight of many a small
being either on the seaweed or the rocky ledges, and even creatures
transparent as glass become visible by the thin outline gleaming in the
sunlight. Then I pluck a piece of seaweed, or chip off a fragment of
rock with a sharp-edged collecting knife, bringing away the specimen
uninjured upon it, and place it carefully in its own separate bottle to
be carried home alive and well.
Now though this little pool and I are old friends, I find new treasures
in it almost every time I go, for it is almost as full of living things
as the heavens are of stars, and the tide as it comes and goes brings
many a mother there to find a safe home for her little ones, and many a
waif and stray to seek shelter from the troublous life of the open
ocean.
You will perhaps find it difficult to believe that in this rock-bound
basin there can be millions of living creatures hidden away among the
fine feathery weeds; yet so it is. Not that they are always the same. At
one time it may be the home of myriads of infant crabs, not an eighth of
an inch long, at another of baby sea-urchins only visible to the naked
eye as minute spots in the water, at another of young jelly-fish growing
on their tiny stalks, and splitting off one by one as transparent bells
to float away with the rising tide. Or it may be that the whelk has
chosen this quiet nook to deposit her leathery eggs; or young barnacles,
periwinkles, and limpets are growing up among the green and brown
tangles, while the far-sailing velella and the stay-at-home sea-squirts,
together with a variety of other sea-animals, find a nursery and shelter
in their youth in this quiet harbour of rest.
And besides these casual visitors there are numberless creatures which
have lived and multiplied there, ever since I first visited the pool.
Tender red, olive-coloured, and green seaweeds, stony corallines, and
acorn-barnacles lining the floor, sea-anemones clinging to the sides,
sponges tiny and many-coloured hiding under the ledges, and limpets and
mussels wedged in the cracks. These can be easily seen with the naked
eye, but they are not the most numerous inhabitants; for these we must
search with a magnifying-glass, which will reveal to us wonderful
fairy-forms, delicate crystal vases with tiny creatures in them whose
transparent lashes make whirlpools in the water, living crystal bells so
tiny that whole branches of them look only like a fringe of hair, jelly
globes rising and falling in the water, patches of living jelly
clinging to the rocky sides of the pool, and a hundred other forms, some
so minute that you must examine the fine sand in which they lie under a
powerful microscope before you can even guess that they are there.
[Illustration: Fig. 63.
Group of seaweeds (natural size).
1, _Ulva Linza._ 2, _Sphacelaria filicina._ 3, _Polysiphonia urceolata._
4, _Corallina officinalis._]
So it has proved a rich hunting-ground, where summer and winter, spring
and autumn, I find some form to put under my magic glass. There I can
watch it for weeks growing and multiplying under my care; moved only
from the aquarium, where I keep it supplied with healthy sea-water, to
the tiny transparent trough in which I place it for a few hours to see
the changes it has undergone. I could tell you endless tales of
transformations in these tiny lives, but I want to-day to show you a
few of my friends, most of which I brought yesterday fresh from the
pool, and have prepared for you to examine.
[Illustration: Fig. 64.
_Ulva lactuca_, a green seaweed, greatly magnified to show structure.
(After Oersted.)
_s_, Spores in the cells. _ss_, Spores swimming out. _h_, Holes through
which spores have escaped.]
Let us begin with seaweeds. I have said that there are three leading
colours in my pool--green, olive, and red--and these tints mark roughly
three kinds of weed, though they occur in an endless variety of shapes.
Here is a piece of the beautiful pale green seaweed, called the Laver or
Sea-lettuce, _Ulva Linza_ (1, Fig. 63), which grows in long ribbons in a
sunny nook in the water. I have placed under the first microscope a
piece of this weed which is just sending out young seaweeds in the shape
of tiny cells, with lashes very like those we saw coming from the
moss-flower, and I have pressed them in the position in which they would
naturally leave the plant (_ss_, Fig. 64.).[1] You will also see on this
slide several cells in which these tiny spores _s_ are forming, ready to
burst out and swim; for this green weed is merely a collection of cells,
like the single-celled plants on land. Each cell can work as a separate
plant; it feeds, grows, and can send out its own young spores.
[1] The slice given in Fig. 64 is from a broader-leaved form,
_U. lactuca_, because this species, being composed of only one
layer of cells, is better seen. _Ulva linza_ is composed of two
layers of cells.
This deep olive-green feathery weed (2, Fig. 63), of which a piece is
magnified under the next microscope (2, Fig. 65), is very different. It
is a higher plant, and works harder for its living, using the darker
rays of sunlight which penetrate into shady parts of the pool. So it
comes to pass that its cells divide the work. Those of the feathery
threads make the food, while others, growing on short stalks on the
shafts of the feather make and send out the young spores.
[Illustration: Fig. 65.
Three seaweeds of Fig. 63 much magnified to show fruits. (Harvey.)
2, _Sphacelaria filicina._ 3, _Polysiphonia urceolata._ 4, _Corallina
officinalis._]
Lastly, the lovely red threadlike weeds, such as this _Polysiphonia
urceolata_ (3, Fig. 63), carry actual urns on their stems like those of
mosses. In fact, the history of these urns (see No. 3, Fig. 65) is much
the same in the two classes of plants, only that instead of the urn
being pushed up on a thin stalk as in the moss, it remains on the
seaweed close down to the stem, when it grows out of the plant-egg, and
the tiny plant is shut in till the spores are ready to swim out.
The stony corallines (4, Figs. 63 and 65), which build so much carbonate
of lime into their stems, are near relations of the red seaweeds. There
are plenty of them in my pool. Some of them, of a deep purple colour,
grow upright in stiff groups about three or four inches high; and
others, which form crusts over the stones and weeds, are a pale rose
colour; but both kinds, when the plant dies, leaving the stony skeleton
(1, Fig. 66), are a pure white, and used to be mistaken for corals. They
belong to the same order of plants as the red weeds, which all live in
shady nooks in the pools, and are the highest of their race.
My pool is full of different forms of these four weeds. The green
ribbons float on the surface rooted to the sides of the pool and, as the
sun shines upon it, the glittering bubbles rising from them show that
they are working up food out of the air in the water, and giving off
oxygen. The brown weeds lie chiefly under the shelves of rocks, for they
can manage with less sunlight, and use the darker rays which pass by the
green weeds; and last of all, the red weeds and corallines, small and
delicate in form, line the bottom of the pool in its darkest nooks.
And now if I hand round two specimens--one a coralline, and the other
something you do not yet know--I am sure you will say at first sight
that they belong to the same family, and, in fact, if you buy at the
seaside a group of seaweeds gummed on paper, you will most likely get
both these among them. Yet the truth is, that while the coralline (1,
Fig. 66) is a plant, the other specimen (2) which is called _Sertularia
filicula_, is an animal.
[Illustration: Fig. 66.
Coralline and Sertularia, to show likeness between the animal Sertularia
and the plant Coralline.
1, _Corallina officinalis._ 2, _Sertularia filicula._]
This special sertularian grows upright in my pool on stones or often on
seaweeds, but I have here (Fig. 67) another and much smaller one which
lives literally in millions hanging its cups downwards. I find it not
only under the narrow ledges of the pool sheltered by the seaweed, but
forming a fringe along all the rocks on each side of the cove near to
low-water mark, and for a long time I passed it by thinking it was of no
interest. But I have long since given up thinking this of anything,
especially in my pool, for my magic glass has taught me that there is
not even a living speck which does not open out into something
marvellous and beautiful. So I chipped off a small piece of rock and
brought the fringe home, and found, when I hung it up in clear sea water
as I have done over this glass trough (Fig. 67) and looked at it through
the lens, that each thread of the dense fringe, in itself not a quarter
of an inch deep, turns out to be a tiny sertularian with at least twenty
mouths. You can see this with your pocket lens even as it hangs here,
and when you have examined it you can by and by take off one thread and
put it carefully in the trough. I promise you a sight of the most
beautiful little beings which exist in nature.
[Illustration: Fig. 67.
_Sertularia tenella_, hanging from a splint of rock over a water trough.
Also piece enlarged to show the animal protruding.]
Come and look at it after the lecture. It is a horny branched stem with
a double row of tiny cups all along each side (see Fig. 67). Out of
these cups there appear from time to time sixteen minute transparent
tentacles as fine as spun glass, which wave about in the water. If you
shake the glass a little, in an instant each crystal star vanishes into
its cup, to come out again a few minutes later; so that now here, now
there, the delicate animal-flowers spread out on each side of the stem,
and the tree is covered with moving beings. These tentacles are feelers,
which lash food into a mouth and stomach in each cup, where it is
digested and passed, through a hole in the bottom, along a jelly thread
which runs down the stem and joins all the mouths together. In this way
the food is distributed all over the tree, which is, in fact, one animal
with many feeding-cups. Some day I will show you one of these cups with
the tentacles stretched out and mounted on a slide, so that you can
examine a tentacle with a very strong magnifying power. You will then
see that it is dotted over with cells, in which are coiled fine
threads. The animal uses these threads to paralyse the creatures on
which it feeds, for at the base of each thread there is a poison gland.
In the larger Sertularia (2, Fig. 66) the whole branched tree is
connected by jelly threads running through the stem, and all the
thousands of mouths are spread out in the water. One large form called
the sea-fir _Sertularia cupressina_ grows sometimes three feet high, and
bears as many as a hundred thousand cups, with living mouths, on its
branches.
The next of my minute friends I can only show to the class in a diagram,
but you will see it under the fourth microscope by and by. I had great
trouble in finding it yesterday, though I knew its haunts upon the green
weed, for it is so minute and transparent that even when the weed is in
a trough a magnifying-glass will scarcely detect it. And I must warn you
that if you want to know any of the minute creatures we are studying,
you must visit one place constantly. You may in a casual way find many
of them on seaweed, or in the damp ooze and mud, but it will be by
chance only; to look for them with any certainty you must take trouble
in making their acquaintance.
Turning then to the diagram (Fig. 68) I will describe it as I hope you
will see it under the microscope--a curious tiny, perfectly transparent
open-mouthed vase standing upright on the weed, and having an equally
transparent being rising up in it and waving its tiny lashes in the
water. This is really all one animal, the vase _hc_ being the horny
covering or carapace of the body, which last stands up like a tube in
the centre. If you watch carefully, you may even see the minute atoms of
food twisting round inside the tube until they are digested, after they
have been swept in at the wide open mouth by the whirling lashes. You
will see this more clearly if you put a little rice-flour, very minutely
powdered and coloured by carmine, into the water; for you can trace
these red atoms into some round spaces called _vacuoles_ which are
dotted over the body of the animal, and are really globules of watery
fluid in which the food is probably partly digested.
[Illustration: Fig. 68.
_Thuricolla folliculata_ and _Chilomonas amygdalum_. (Saville Kent.)
1, Thuricolla erect; 2, retracted; 3, dividing. 4, _Chilomonas
amygdalum_. _hc_, Horny carapace. _cv_, Contractile vesicle. _v_,
Closing valves.]
You will notice, however, one round clear space (_cv_) into which they
do not go, and after a time you will be able to observe that this round
spot closes up or contracts very quickly, and then expands again very
slowly. As it expands it fills with a clear fluid, and naturalists have
not yet decided exactly what work it does. It may serve the animal
either for breathing, or as a very simple heart, making the fluids
circulate in the tube. The next interesting point about this little
being is the way it retreats into its sheltering vase. Even while you
are watching, it is quite likely it may all at once draw itself down to
the bottom as in No. 2, and folding down the valves _v_, _v_ of horny
teeth which grow on each side, shut itself in from some fancied danger.
Another very curious point is that, besides sending forth young ones,
these creatures multiply by dividing in two (see No. 3, Fig. 68), each
one closing its own part of the vase into a new home.
There are hundreds of these _Infusoria_, as they are called, in my pond,
some with vases, some without, some fixed to weeds and stones, others
swimming about freely. Even in the water-trough in which this Thuricolla
stands, I saw several smaller forms, and the next microscope has a
trough filled with the minutest form of all, called a Monad (No. 4, Fig.
68). These are so small that 2000 of them would lie side by side in an
inch; that is, if you could make them lie at all, for they are the most
restless little beings, darting hither and thither, scarcely even
halting except to turn back. And yet though there are so many of them,
and as far as we know they have no organs of sight, they never run up
against each other, but glide past more cleverly than any clear-sighted
fish. These creatures are mostly to be found among decaying seaweed, and
though they are so tiny, you can still see distinctly the clear space
(_cv_) contracting and expanding within them.
But if there are so many thousands of mouths to feed, on the tree-like
_Sertulariae_ as well as in all these _Infusoria_, where does the food
come from?
Partly from the numerous atoms of decaying life all around, and the
minute eggs of animals and spores of plants; but besides these, the pool
is full of minute living plants--small jelly masses with solid coats of
flint which are moulded into most lovely shapes. Plants formed of jelly
and flint! You will think I am joking, but I am not. These plants,
called _Diatoms_, which live both in salt and fresh water, are single
cells feeding and growing just like those we took from the water-butt
(Fig. 29, p. 78), only that instead of a soft covering they build up a
flinty skeleton. They are so small, that many of them must be magnified
to fifty times their real size before you can even see them distinctly.
Yet the skeletons of these almost invisible plants are carved and
chiselled in the most delicate patterns. I showed you a group of these
in our lecture on magic glasses (p. 39), and now I have brought a few
living ones that we may learn to know them. The diagram (Fig. 69) shows
the chief forms you will see on the different slides.
[Illustration: Fig. 69.
Living diatoms.
_a_, _Cocconema lanceolatum_. _b_, _Bacillaria paradoxa_.
_c_, _Gomphonema marinum_. _d_, _Diatoma hyalina_.]
The first one, _Bacillaria paradoxa_ (_b_, Fig. 69), looks like a number
of rods clinging one to another in a string, but each one of these is a
single-celled plant with a jelly cell surrounding the flinty skeleton.
You will see that they move to and fro over each other in the water.
[Illustration: Fig. 70.
A diatom (_Diatoma vulgare_) growing.
_a_, _b_, Flint skeleton inside the jelly-cell. _a_, _c_ and _d_, _b_,
Two flint skeletons formed by new valves, _c_ and _d_, forming within
the first skeleton.]
The next two forms, _a_ and _c_, look much more like plants, for the
cells arrange themselves on a jelly stem, which by and by disappears,
leaving only the separate flint skeletons such as you see in Fig. 16.
The last form, _d_, is something midway between the other forms, the
separate cells hang on to each other and also on to a straight jelly
stem.
Another species of Diatoma (Fig. 70) called _Diatoma vulgare_, is a very
simple and common form, and will help to explain how these plants grow.
The two flinty valves _a_, _b_ inside the cell are not quite the same
size; the older one _a_ is larger than the younger one _b_ and fits over
it like the cover of a pill-box. As the plant grows, the cell enlarges
and forms two more valves, one _c_ fitting into the cover _a_, so as to
make a complete box _ac_, and a second, _d_, back to back with _c_,
fitting into the valve _b_, and making another complete box _bd_. This
goes on very rapidly, and in this plant each new cell separates as it is
formed, and the free diatoms move about quite actively in the water.
If you consider for a moment, you will see that, as the new valves
always fit into the old ones, each must be smaller than the last, and so
there comes a time when the valves have become too small to go on
increasing. Then the plant must begin afresh. So the two halves of the
last cell open, and throwing out their flinty skeletons, cover
themselves with a thin jelly layer, and form a new cell which grows
larger than any of the old ones. These, which are spore-cells, then form
flinty valves inside, and the whole thing begins again.
Now though the plants themselves die, or become the food of minute
animals, the flinty skeletons are not destroyed, but go on accumulating
in the waters of ponds, lakes, rivers, and seas, all over the world.
Untold millions have no doubt crumbled to dust and gone back into the
waters, but untold millions also have survived. The towns of Berlin in
Europe and of Richmond in the United States are actually built upon
ground called "infusorial earth," composed almost entirely of valves of
these minute diatoms which have accumulated to a thickness of more than
eighty feet! Those under Berlin are fresh-water forms, and must have
lived in a lake, while those of Richmond belong to salt-water forms.
Every inch of the ground under those cities represents thousands and
thousands of living plants which flourished in ages long gone by, and
were no larger than those you will see presently under the microscope.
These are a very few of the microscopic inhabitants of my pond, but, as
you will confuse them if I show you too many, we will conclude with two
rather larger specimens, and examine them carefully. The first, called
the Cydippe, is a lovely, transparent living ball, which I want to
explain to you because it is so wondrously beautiful. The second, the
Sea-mat or Flustra, looks like a crumpled drab-coloured seaweed, but is
really composed of many thousands of grottos, the homes of tiny
sea-animals.
[Illustration: Fig. 71.
_Cydippe Pileus._
1, Animal with tentacles _t_, bearing small tendrils _t'_. 2, Body of
animal enlarged. _m_, Mouth. _c_, Digestive cavity. _s_, Sac into which
the tentacles are withdrawn. _p_, Bands with comb-like plates. 3,
Portion of a band enlarged to show the moving plates _p_.]
Let us take the Cydippe first (1, Fig. 71). I have six here, each in a
separate tumbler, and could have brought many more, for when I dipped my
net in the pool yesterday such numbers were caught in it that I believe
the retreating tide must just have left a shoal behind. Put a tumbler on
the desk in front of you, and if the light falls well upon it you will
see a transparent ball about the size of a large pea marked with eight
bright bands, which begin at the lower end of the ball and reach nearly
to the top, dividing the outside into sections like the ribs of a melon.
The creature is so perfectly transparent that you can count all the
eight bands.
At the top of the ball is a slight bulge which is the mouth (_m_ 2, Fig.
71), and from it, inside the ball, hangs a long bag or stomach, which
opens below into a cavity c, from which two canals branch out, one on
each side, and these divide again into four canals which go one into
each of the tubes running down the bands. From this cavity the food,
which is digested in the stomach, is carried by the canals all over the
body. The smaller tubes which branch out of these canals cannot be seen
clearly without a very strong lens, and the only other parts you can
discern in this transparent ball are two long sacs on each side of the
lower end. These are the tentacle sacs, in which are coiled up the
tentacles, which we shall describe presently. Lastly, you can notice
that the bands outside the globe are broader in the middle than at the
ends, and are striped across by a number of ridges.
In moving the tumblers the water has naturally been shaken, and the
creature being alarmed will probably at first remain motionless. But
very soon it will begin to play in the water, rising and falling, and
swimming gracefully from side to side. Now you will notice a curious
effect, for the bands will glitter and become tinged with prismatic
colours, till, as it moves more and more rapidly these colours,
reflected in the jelly, seem to tinge the whole ball with colours like
those on a soap-bubble, while from the two sacs below come forth two
long transparent threads like spun glass. At first these appear to be
simple threads, but as they gradually open out to about four or five
inches, smaller threads uncoil on each side of the line till there are
about fifty on each line. These short _tendrils_ are never still for
long; as the main threads wave to and fro, some of the shorter ones coil
up and hang like tiny beads, then these uncoil and others roll up, so
that these graceful floating lines are never two seconds alike.
We do not really know their use. Sometimes the creature anchors itself
by them, rising and falling as they stretch out or coil up; but more
often they float idly behind it in the water. At first you would perhaps
think that they served to drive the ball through the water, but this is
done by a special apparatus. The cross ridges which we noticed on the
bands are really flat comb-like plates (_p_, Fig. 71), of which there
are about twenty or thirty on each band; and these vibrate very rapidly,
so that two hundred or more paddles drive the tiny ball through the
water. This is the cause of the prismatic colours; for iridescent tints
are produced by the play of light upon the glittering plates, as they
incessantly change their angle. Sometimes they move all at once,
sometimes only a few at a time, and it is evident the creature controls
them at will.
This lovely fairy-like globe, with its long floating tentacles and
rainbow tints, was for a long time classed with the jelly-fish; but it
really is most nearly related to sea-anemones, as it has a true central
cavity which acts as a stomach, and many other points in common with the
_Actinozoa_. We cannot help wondering, as the little being glides hither
and thither, whether it can see where it is going. It has nerves of a
low kind which start from a little dark spot (_ng_), exactly at the
south pole of the ball, and at that point a sense-organ of some kind
exists, but what impression the creature gains from it of the world
outside we cannot tell.
I am afraid you may think it dull to turn from such a beautiful being as
this, to the grey leaf which looks only like a dead dry seaweed; yet you
will be wrong, for a more wonderful history attaches to this crumpled
dead-looking leaf than to the lovely jelly-globe.
First of all I will pass round pieces of the dry leaf (_r_, Fig. 72),
and while you are getting them I will tell you where I found the living
ones. Great masses of the Flustra, as it is called, line the bottom and
sides of my pool. They grow in tufts, standing upright on the rock, and
looking exactly like hard grey seaweeds, while there is nothing to lead
you to suspect that they are anything else. Yesterday I chipped off very
carefully a piece of rock with a tuft upon it, and have kept it since in
a glass globe by itself with sea-water, for the little creatures living
in this marine city require a very good supply of healthy water and air.
I have called it a "marine city," and now I will tell you why. Take the
piece in your hand and run your finger gently up and down it; you will
glide quite comfortably from the lower to the higher part of the leaf,
but when you come back you will feel your finger catch slightly on a
rough surface. Your pocket lens will show why this is, for if you look
through it at the surface of the leaf you will see it is not smooth, but
composed of hundreds of tiny alcoves with arched tops; and on each side
of these tops stand two short blunt spines (see 2, Fig. 72), making four
in all, pointing upwards, so as partly to cover the alcove above. As
your finger went up it glided over the spines, but on coming back it met
their points. This is all you can see in the dead specimen; I must show
you the rest by diagrams, and by and by under the microscope.
[Illustration: Fig. 72.
The Sea-mat or Flustra (_Flustra foliacea._)
1, Natural size. 2, Much magnified. _s_, Slit caused by drawing in of
the animal _a_.]
First, then, in the living specimen which I have here, those alcoves are
not open as in the dead piece, but covered over with a transparent skin,
in which, near the top of the alcove just where the curve begins, is a
slit (_s_ 2, Fig. 72). Unfortunately the membrane covering this alcove
is too dense for you to distinguish the parts within. Presently,
however, if you are watching a piece of this living leaf in a flat
water-cell under the microscope, you will see the slit slowly open, and
begin to turn as it were inside out, exactly like the finger of a
glove, which has been pushed in at the tip, gradually rises up when you
put your finger inside it. As this goes on, a bundle of threads appears,
at first closed like a bud, but gradually opening out into a crown of
tentacles (_a_, Fig. 72), each one clothed with hairs. Then you will see
that the slit was not exactly a slit after all, but the round edge where
the sac was pushed in. Ah! you will say, you are now showing me a polyp
like those on the sertularian tree. Not so fast, my friend; you have not
yet studied what is still under the covering skin and hidden in the
living animal. I have, however, prepared a slide with this membrane
removed (see Fig. 73), and there you can observe the different parts,
and learn that each one of these alcoves contains a complete animal, and
not merely one among many mouths, like the polyp on the Sertularia.
[Illustration: Fig. 73.
Diagram of the animal in the Flustra or Sea-mat.
1, Animal protruding. 2, Animal retracted in the sheath. _sh_, Covering
sheath. _s_, Slit. _t_, Tentacles. _m_, Mouth. _th_, Throat. _st_,
Stomach. _i_, Intestine. _r_, Retractor muscle. _e_, Egg-forming parts.
_g_, Nerve-ganglion.]
Each of these little beings (_a_, Fig 72) living in its alcove has a
mouth, throat, stomach, intestine, muscles, and nerves starting from the
ganglion of nervous matter, besides all that is necessary for producing
eggs and sending forth young ones. You can trace all these under the
microscope (see 2, Fig. 73) as the creature lies curiously doubled up in
its bed, with its body bent in a loop; the intestine _i_, out of which
the refuse food passes, coming back close up to the slit. When it is at
rest, the top of the sac in which it lies is pulled in by the retractor
muscle _r_, and looks, as I have said, like the finger of a glove with
the top pushed in. When it wishes to feed, this top is drawn out by
muscles running round the sac, and the tentacles open and wave in the
water (1, Fig. 73).
Look now at the alcoves, the homes of these animals; see how tiny they
are and how closely they fit together. Mr. Gosse, the naturalist, has
reckoned that there are 6720 alcoves in a square inch; then if you turn
the leaf over you will see that there is another set, fixed back to back
with these, on the other side, making in all 13,440 alcoves. Now a
moderate-sized leaf of flustra measures about three square inches,
taking all the rounded lobes into account, so you will see we get 40,320
as a rough estimate of the number of beings on this one leaf. But if you
look at this tuft I have brought, you will find it is composed of twelve
such leaves, and this after all is a very small part of the mass growing
round my pool. Was I wrong, then, when I said that my miniature ocean
contains as many millions of beings as there are stars in the heavens?
You will want to know how these leaves grew, and it is in this way.
First a little free swimming animal, a mere living sac provided with
lashes, settles down and grows into one little horny alcove, with its
live creature inside, which in time sends off from it three to five
buds, forming alcoves all round the top and sides of the first one,
growing on to it. These again bud out, and you can thus easily
understand that, in this way, in time a good-sized leaf is formed.
Meanwhile the creatures also send forth new swimming cells, which settle
down near to begin new leaves, and thus a tuft is formed; and long after
the beings in earlier parts of the leaf have died and left their alcoves
empty, those round the margin are still alive and spreading.
With this history we must stop for to-day, and I expect it will be many
weeks before you have thoroughly examined the specimens of each kind
which I have put in the aquarium. If you can trace the spore-cells and
urns in the seaweeds, observe the polyps in the Sertularia, and count
the number of mouths on a branch of my animal fringe (_Sertularia
tenella_); if you make acquaintance with the Thuricolla in its vase, and
are fortunate enough to see one divide in two; if you learn to know some
of the beautiful forms of diatoms, and can picture to yourselves the
life of the tiny inhabitants of the Flustra; then you will have used
your microscope with some effect, and be prepared for an expedition to
my pool, where we will go together some day to seek new treasures.