Young Folks' Library, Volume XI (of 20) by Various

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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, 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
harbor 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-colored, and green seaweeds, stony corallines, and
acorn-barnacles lining the floor, sea-anemones clinging to the sides,
sponges tiny and many-colored 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. 1. 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. 2. _Ulva lactuca_, A GREEN-SEAWEED, GREATLY
MAGNIFIED TO SHOW STRUCTURE. (_After Orested)._

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
colors 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. 1),[1] 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. You will also
see on this side several cells in which these tiny spores 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.

[Footnote 1: The slice given in Fig. 2 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. 1), of which a piece is
magnified under the next microscope (2, Fig. 3), 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.

Lastly, the lovely red threadlike weeds, such as this _Polysiphonia
urceolata_ (3, Fig. 1), carry actual urns on their stems like those of
mosses. In fact, the history of these urns (see 3, Fig. 3), 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.

[Illustration: FIG. 3. THREE SEAWEEDS OF FIG. 1 MUCH MAGNIFIED TO SHOW
FRUITS. (_Harvey._)

2, _Sphacelaria filicina._ 3, _Polysiphonia urceolata._ 4, _Corallina
officinalis._]

The stony corallines (4, Figs. 1 and 3), 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
color, 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 color; but both kinds, when the plant dies, leaving the stony
skeleton (1, Fig. 4), 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.

[Illustration: FIG. 4. CORALLINE AND SERTULARIA, TO SHOW LIKENESS
BETWEEN THE ANIMAL SERTULARIA AND THE PLANT CORALLINE.

1, _Corallina officinalis._ 2, _Sertularia filicula._]

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. 4) is a plant, the other specimen (2), which is called
_Sertularia filicula_, is an animal.

This special sertularian grows up right in my pool on stones or often
on seaweeds, but I have here (Fig. 5) 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. 5) 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. 5. _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. It is a horny-branched stem with a double row of
tiny cups all along each side. Out of these cups there appear a row of
tiny cups all along each side (see Fig. 5), 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 paralyze the creatures on
which it feeds, for at the base of each thread there is a poison
gland.

In the larger Sertularia 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 _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 know 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.

[Illustration: FIG. 6. _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.]

Turning then to the diagram (Fig. 6) 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 colored 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.

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 _w_ 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. 6), 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. These are so small that two thousand of them could 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 contracting and expanding within them.

[Illustration: FIG. 7. LIVING DIATOMS.

_a, Cocconema lanceolatum. b, Bacillaria paradoxa. c, Gomphonema
marinum. d, Diatoma hyalina_.]

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, 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, and now I have brought
a few living ones that we may learn to know them. The diagram (Fig. 7)
shows the chief forms you will see on the different slides.

The first one, _Sacillaria paradoxa_ (_b_, Fig. 7), 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.

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. 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.

[Illustration: FIG. 8. 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.]

Another species of Diatoma (Fig. 8) 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 _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 the 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-colored seaweed,
but is really composed of many thousands of grottos, the homes of tiny
sea-animals.

[Illustration: FIG. 9. _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. 9). 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. 9), and from it, inside the ball hangs a long bag or stomach,
which opens below into a cavity, 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
colors, till, as it moves more and more rapidly these colors,
reflected in the jelly, seem to tinge the whole ball with colors 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. 9),
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
colors; 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 the 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 gray 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.

[Illustration: FIG. 10. THE SEA-MAT OR FLUSTRA (_Flustra foliacea_).

1, Natural size. 2, Much magnified, _s_, Slit caused by drawing in of
the animal _a_.]

First of all I will pass round pieces of the dry leaf (1, Fig. 10),
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 gray 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 you 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, 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.

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