Marine Life--Sponges--Infusoria--Foraminifera--Radiolaria--Hydroid
Zoophytes--Polyzoa--Worms--Lingual Ribbons and Gills of
Mollusca--Star-Fishes and Sea-Urchins--Cuttle-Fish--
Corallines--Miscellaneous Objects.
Great as is the range of objects presented to the student of
fresh-water life, the latter field is limited indeed as compared with
that afforded by the sea. The Infusoria and Rotifers furnished by the
latter are, indeed, much fewer in number and variety, but the vast host
of sponges, polyzoa, hydroids, crustacea, molluscs, ascidians, and
worms, to say nothing of the wealth of vegetable life, renders the sea
the happy hunting-ground of the microscopist.
Whether it be along the edge of the water, as the tide retreats,
especially after a gale; or in the rock-pools; or, perhaps best of
all, upon those portions of the shore left uncovered only by the
lowest spring-tides, the harvest is simply inexhaustible. Stones
turned up will exhibit a world in miniature. Encrusted with green or
pink sponges, or with gelatinous masses of ascidians, fringed at its
edges with hydroids, coated above with polyzoa, a single one will
often supply more work than could be got through in a week of steady
application.
A description of the fresh-water sponge already given may serve very
well to indicate the general outlines of the organisation of the marine
ones too. The spicules of the latter are, however, not always flinty;
very often, as in the case of _Grantia_ (Plate IX. Figs. 8 and 14),
they are calcareous, a point which can be settled by the application of
a little nitric acid and water. If lime be present there will be strong
effervescence, and the separation of the spicules can only be effected
by gently warming a portion of the sponge in caustic potash solution,
pouring the resulting mass into water, and allowing the spicules to
settle. The washing and settling must be repeated several times, and a
portion of the deposit may then be taken up with a dipping-tube, spread
upon a slide and dried, and then covered in balsam solution. The forms
are endless, and the same sponge will often supply three or four, or
even more. Among them may be seen accurate likenesses of pins, needles,
marlin-spikes, cucumbers, grappling-hooks, fish-hooks, porters’-hooks,
calthrops, knife-rests, fish-spears, barbed arrows, spiked globes,
war-clubs, boomerangs, life-preservers, and many other indescribable
forms. The flinty forms must be prepared by boiling, as described in
speaking of the mounting of diatoms in Chapter XI., except that, of
course, only one settlement is required after thorough washing.
Every one who has been by or on the sea on a fine summer night must
have noticed the bright flashes of light that appear whenever its
surface is disturbed; the wake of a boat, for example, leaving a
luminous track as far as the eye can reach. This phosphorescence is
caused by many animals resident in the sea, but chiefly by the little
creature represented at Fig. 9, the _Noctilúca_, myriads of which may
be found in a pail of water dipped at random from the glowing waves. A
tooth of this creature more magnified is shown immediately above.
A large group of microscopic organisms is known to zoologists under
the name of Foraminifera, on account of the numerous holes in their
beautiful shells, most of which are composed of carbonate of lime,
though some are horny and others are composed of aggregations of minute
grains of sand, the forms in one class often closely imitating those in
another. It is of the shells of these minute animals that the “white
cliffs of old England” are very largely composed, and those who desire
to understand the part which these tiny creatures have played, and are
playing, in geology, will do well to study Huxley’s fascinating essay
on “A Piece of Chalk.”
XIII.
FIG.
1. Grantia compressa
2. Euglena viridis
3. Dinobryon sertularia
4. Synura uvella
5. Peridinium tabulatum
6. Coleps hirtus
7. Trachelocerca viridis
8. Trachelius ovum
9. Amphileptus gigas
10. Bursaria Mülleri
11. Folliculina elegans
12. Stentor polymorphus
13. Trichodina pediculus
14. Vorticella nebulifera
15. Zoothamnium arbuscula
16. Do. do. contracted
17. Vaginicola crystallina
18. Ophrydium versatile (colony)
19. Do. do. (single zoöid)
20. Cothurnia imberbis
21. Pyxicola affinis
22–23. Stylonychia mytilus
24. Acineta grandis
25. Loxophyllum meleagris
26. Euplotes charon (dividing)
27. Chætonotus larus
[Illustration: XIII.]
The inhabitants of these shells are Amœbæ, mere masses of jelly,
and some forms may be found sliding over the weeds in almost every
rock-pool. The anchor-mud, already spoken of, always contains some,
and many forms may be found in the sand from sponges, which should be
passed through a series of wire sieves, of increasing fineness, and
the residuum in each case be examined dry under a one-inch power. The
shells may be picked up with a needle which has been slightly greased
by being passed over the hair, and they may be mounted by sticking them
to the slide with thin starch paste, putting on a cover-glass properly
supported, and then running turpentine under the cover-glass, heating
to expel the air, and finally filling up with balsam. Or, as opaque
objects, they may be mounted in a cell dry. The forms are endless, but
all are beautiful, and a few examples are given in Plate IX. Fig. 4
(_Miliolína_), and Plate XII. Fig. 7, which is a portion of the shell
to show the holes, Fig. 13 (_Polystomella_), Fig. 14 (_Truncatulína_),
Fig. 15 (_Polymorphína_), Fig. 16 (_Miliolína_, partly fossilised),
Fig. 18 (_Lagéna_). and Fig. 20 (_Biloculína_).
Allied to these are the lovely Radiolaria, whose shells, constructed
on a similar plan, are composed of flint. They are found in remarkable
profusion in the deposit from Cambridge, Barbados, but also in a
living state at even enormous depths in the ocean. The present writer
has obtained many forms from _Challenger_ soundings, and the great
authority on this subject is Haeckel’s report in the official accounts
of the expedition of the above-named vessel.
The Hydroid Zoophytes are represented by several examples. These
creatures are soft and almost gelatinous, and are furnished with
tentacles or lobes by which they can catch and retain their prey.
In order to support their tender structure they are endowed with a
horny skeleton, sometimes outside and sometimes inside them, which is
called the polypidom. They are very common on our coasts, where they
may be found thrown on the shore, or may be dredged up from the deeper
portions of the sea.
Fig. 13 is a portion of one of the commonest genera, _Sertularia_,
showing one of the inhabitants projecting its tentacles from its
domicile. Fig. 15 is the same species, given to show the egg-cells.
This, as well as other zoophytes, is generally classed among the
sea-weeds in the shops that throng all watering-places.
The form just referred to is a near relative of the Hydra, already
described, and belongs to the same great family as the sea-anemones.
One form, shown in Fig. 26, is the _Hydra Tuba_, long thought to be a
distinct animal, but now known to be the young form of a jelly-fish,
or Medusa. The Hydra Tuba throws off joints at intervals, each of
which becomes a perfect jelly-fish. One of them is shown in Fig. 27.
Fig. 28 represents a very small and pretty Medusa, the Thaumantias.
When this animal is touched or startled, each of the purple globules
round the edge flashes into light, producing a most beautiful and
singular appearance. Fig. 29 exhibits the so-called compound eye of
another species of Medusa, though it would appear that these are really
connected with the nervous system of the animal, and have to do with
the pulsating contractions of the bell by which it is propelled through
the water.
In my _Common Objects of the Sea-Shore_ the Actíniæ, or Sea-Anemones,
are treated of at some length. At Fig. 16 is shown part of a tentacle
flinging out the poison-darts by which it secures its prey; and Fig. 17
is a more magnified view of one of these darts and its case.
Much more might be said under this head, but we must pass on to another
group, which, whilst possessing a certain general resemblance to the
hydroid zoophytes, differs utterly from them in internal organisation.
We have already referred to the fresh-water polyzoa. The marine forms
are vastly more numerous, and more easily found, since not only
pieces of weed upon which they grow are to be found upon every beach,
but whole masses of leaf-like colonies, forming what is known as
horn-wrack, may be plentifully found. Instead of the tentacles armed
with sting-cells, like the anemone’s, possessed by the Hydrozoa, the
Polyzoa have arms clothed with active cilia, by which the food is swept
into the mouth, passing on into the stomach, and then through the
intestine to another opening.
Fig. 19 is a very curious zoophyte called _Anguinaria_, or snake-head,
on account of its shape, the end of the polypidom resembling the head
of the snake, and the tentacles looking like its tongue as they are
thrust forward and rapidly withdrawn. Fig. 21 is the same creature
on an enlarged scale, and just below is one of its tentacles still
more magnified. Fig. 23 is the ladies’-slipper zoophyte (_Eretea_);
and Fig. 24 is called the tobacco-pipe or shepherd’s-purse zoophyte
(_Notamia_).
Fig. 22 is a portion of the _Bugula_, with one of the curious
“birds’-head” processes. These appendages have the most absurd likeness
to a bird’s head, the beak opening and shutting with a smart snap (so
smart, indeed, that the ear instinctively tries to catch the sound),
and the head nodding backward and forward just as if the bird were
pecking up its food. On Plate XII. Fig. 2, is a pretty zoophyte called
_Gemellaria_, on account of the double or twin-like form of the cells;
and Fig. 5 represents the _Antennularia_, so called on account of its
resemblance to the antennæ of an insect. Fig. 22 is an example of a
pretty zoophyte found parasitic on many sea-weeds, and known by the
name of _Membranipora_. Two more specimens of zoophytes may be seen
on Plate XI. as they appear under polarised light. Fig. 17 is the
_Cellularia reptans_; and Fig. 20 is the _Bowerbankia_, one form of
which occurs in fresh water.
Among the worms we may refer to the beautiful little _Spirorbis_,
whose tiny coiled spiral tube may be found attached to almost every
sea-weed, and which, when placed in a trough of sea-water, protrudes
its beautiful crown of plumes. In chalk or other soft rocks, again,
the tubes of _Spio_, with its two long waving tentacles, may be found
by hundreds. Then there are the centipede-like worms, which may be
found under nearly every stone, and which belong to the great family of
Nereids, provided with formidable jaws and stiff bristles of various
forms. The Serpulæ are allied to the Spirorbis already mentioned. Parts
of the so-called feet of one of these worms are shown in Fig. 36,
where the spears or “pushing-poles” are seen gathered into bundles, as
during life. One of them, on a larger scale, is shown in Fig. 32. The
gorgeous hairs of Aphrodite have already been alluded to.
In the sea the few species of Crustacea which fresh water offers to
the observer in the shape of Cyclops and its allies become thousands,
and their changes during development are numerous and puzzling. Who,
for example, would suppose that the young stage of the Cyclops was
indistinguishable in habits, and almost in form, from that of the
barnacle which adheres to the rocks? Yet such is the case, and there
are other metamorphoses even more startling. Fig. 25 is the larva of
the common crab, once thought to be a separate species, and described
as such under the name of _Zoæa_.
The Mollusca proper will not afford us many objects, except in the form
of their lingual ribbon, which may be extracted from the mouth, gently
heated in _liquor potassæ_, and mounted in balsam after well washing,
when the rows of teeth form splendid objects by polarised light. The
palate of a whelk is shown in Plate XI. Fig. 19.
Again, the gills of the mussel will afford a beautiful illustration of
ciliary action. If a portion of the thin plates which lie along the
edge of the shell be examined in a little of the liquor, the action may
be splendidly seen, and watched for a long time (Fig. 39).
The structure of shell, _e.g._ oyster-shell, is well shown in three
examples: Fig. 34 is a group of artificial crystals of carbonate of
lime; and on Figs. 38 and 39 may be seen part of an oyster-shell,
showing how it is composed of similar crystals aggregated together.
Their appearance under polarised light may be seen on Plate XI. Figs. 1
and 6.
We now pass on to the Echinoderms, including the star-fishes and
sea-urchins.
The old story of the goose-bearing tree is an example of how truth may
be stranger than fiction. For if the fable had said that the mother
goose laid eggs which grew into trees, budded and flowered, and then
produced new geese, it would not have been one whit a stranger tale
than the truth. Plate IX. Fig. 33, shows the young state of one of
the common star-fishes (_Comátula_), which in its early days is like
a plant with a stalk, but afterwards breaks loose and becomes the
wandering sea-star which we all know so well. In this process there
is just the reverse of that which characterises the barnacles and
sponges, where the young are at first free and then become fixed for
the remainder of their lives. Fig. 30 is the young of another kind of
star-fish, the long-armed Ophiúris, or snake-star.
Fig. 37 is a portion of the skin of the common sun-star (_Solaster_),
showing the single large spine surrounded by a circle of smaller
spines, supposed to be organs of touch, together with two or three
of the curious appendages called pedicellariæ. These are found on
star-fishes and Echini, and bear a close resemblance in many respects
to the bird-head appendages of the zoophytes. They are fixed on
foot-stalks, some very long and others very short, and have jaws which
open and shut regularly. Their use is doubtful, unless it be to act as
police, and by their continual movements to prevent the spores of algæ,
or the young of various marine animals, from effecting a lodgment on
the skin. A group, of pedicellariæ from a star-fish is shown on a large
scale on Plate XII. Fig. 6, and Fig. 9 of the same Plate shows the
pedicellariæ of the Echinus.
Upon the exterior of the Echini, or sea-urchins, are a vast number of
spines having a very beautiful structure, as may be seen by Fig. 35,
Plate IX., which is part of a transverse section of one of these
spines. An entire spine is shown on Plate XII. Fig. 12, and shows the
ball-and-socket joint on which it moves, and the membranous muscle that
moves it. Fig. 8 is the disc of the snake-star as seen from below.
Fig. 1 is a portion of skin of the sun-star, to show one of the curious
madrepore-like tubercles which are found upon this common star-fish.
Fig. 3 is a portion of cuttle “bone,” very slightly magnified, in order
to show the beautiful pillar-like form of its structure; and Fig. 4
is the same object seen from above. When ground very thin this is a
magnificent object for the polariscope.
One or two miscellaneous objects now come before our notice. Fig. 11
is one of those curious marine plants, the Corallines, which are
remarkable for depositing a large amount of chalky matter among their
tissues, so as to leave a complete cast in white chalk when the
coloured living portion of the plant dies. The species of this example
is _Jania rubens_.
Fig. 19 is part of the pouch-like inflation of the skin, and the hairs
found upon the rat’s tail, which is a curious object as bearing so
close a similitude to Fig. 22, the sea-mat zoophyte. Fig. 23 is a
portion of the skin taken from the finger, which has been injected
with a coloured preparation in order to show the manner in which the
minute blood-vessels or “capillaries” are distributed; and Fig. 26 is a
portion of a frog’s lung, also injected.
The process of injection is a rather difficult one, and requires
considerable anatomical knowledge. The principle is simple enough,
being merely to fill the blood-vessels with a coloured substance, so as
to exhibit their form as they appear while distended with blood during
the life of the animal. It sometimes happens that when an animal is
killed suddenly without effusion of blood, as is often seen in the case
of a mouse caught in a spring trap, the minute vessels of the lungs and
other organs become so filled with coagulated blood as to form what is
called a natural injection, ready for the microscope.
Before leaving the subject I must ask the reader to refer again for a
moment to the frog’s foot on Plate X., and to notice the arrangement
of the dark pigment spots. It is well known that when frogs live in a
clear sandy pond, well exposed to the rays of the sun, their skins are
bright yellow, and that when their residence is in a shady locality,
especially if sheltered by heavy overhanging banks, they are of a deep
blackish-brown colour. Moreover, under the influence of fear they will
often change colour instantaneously. The cause of this curious fact is
explained by the microscope.
Under the effects of sunlight the pigment granules are gathered
together into small rounded spots, as seen on the left hand of the
figure, leaving the skin of its own bright yellow hue. In the shade the
pigment granules spread themselves so as to cover almost the entire
skin and to produce the dark brown colour. In the intermediate state
they assume the bold stellate form in which they are shown on the right
hand of the round spots. Very remarkable forms of these cells may be
found in the skin of the cuttle-fish.
Figs. 24 and 25 are two examples of coal, the former being a
longitudinal and the latter a transverse section, given in order to
show its woody character. Fig. 17 is a specimen of gold-dust intermixed
with crystals of quartz sand, brought from Australia; and Fig. 21 is a
small piece of copper-ore.
Every possessor of a microscope should, as soon as he can afford it,
add to his instrument the beautiful apparatus for polarising light.
The optical explanation of this phenomenon is far too abstruse for
these pages, but the practical application of the apparatus is very
simple. It consists of two prisms, one of which, called the polariser,
is fastened by a catch just below the stage; and the other, called an
analyser, is placed above the eye-piece. In order to aid those bodies
whose polarising powers are but weak, a thin plate of selenite is
generally placed on the stage immediately below the object. The colours
exhibited by this instrument are gorgeous in the extreme, as may be
seen by Plate XI., which affords a most feeble representation of the
glowing tints exhibited by the objects there depicted. The value of the
polariser is very great, as it often enables observers to distinguish,
by means of their different polarising properties, one class of objects
from another.
If the expense of a polarising apparatus be too great for the means
of the microscopist, he may manufacture a substitute for it by taking
several thin plates of glass, arranging them in a paper tube so that
the light may meet the surface of the lowest one at an angle of about
52°, and placing the bundle above the eye-piece to act as an analyser;
whilst, by using a plate of glass, and so arranging the lamp that the
light falls upon it at the above angle, and is reflected up the tube
of the microscope, he will find on rotating the extemporised analyser
that the phenomena of polarisation are to a great extent reproduced;
whilst by splitting an extremely thin film from the surface of a sheet
of mica, such as is employed for making smoke-screens above glass
globes, he will have a substitute for the selenite by means of which
alone can the gorgeous effects be produced. The extemporised apparatus
will not, of course, give such perfect effects, but this is sometimes
an advantage, and the present writer has used the same means with
considerable success in photographing starch-granules.
XIV.
FIG.
1. Dero latissima
2. Slavina serpentina
3. Pristina longiseta
4. Lophopus crystallinus
5. Fredericella sultana
6. Stylaria proboscidea (head)
7. Hydatina senta
8. Synchœta mordax
9. Asplanchna Brightwellii
10. Notommata aurita
11. Triarthra longiseta
12. Euchlanis triquetra
13. Brachionus amphiceros
14. Salpina mucronata
15. Pterodina patina
16. Anurœa brevispina
17. Dinocharis tetractis
18. Noteus quadricornis
19. Floscularia ornata
20. Young Melicerta
21. Macrobiotus (sp.?)
22. Cypris fusca
23. Cyclops quadricornis
[Illustration: XIV.]