Pond-Life--Apparatus and Instructions for Collecting Objects--Methods
of Examination--Sponge--Infusoria.
Of all departments of microscopic research the most fascinating and
the most popular is that which deals with what is known by the generic
name of “pond-life.” The minute forms of the animal creation included
in this term are of such exquisite beauty, and allow the processes of
their life-history to be followed with such facility, from the cradle
(when they have one) to the grave (which is very often the body of
another, larger, organism), that there is none which has attracted
more observers. Indeed, the first application of the microscope, by
Leeuwenhoek, early in the seventeenth century, was to the observation
of these forms of life.
X.
FIG.
1. Skin, Frog
2. Blood, Human
3. Do. Pigeon
4. Do. Proteus
5. Do. Tortoise
6. Do. Frog
7. Do. Fish
8. Human nail
9. Bone, Human
10. White fibrous tissue
11. Epithelial cells from tongue
12. Feather, Peacock
13. Spine, Hedgehog, transverse section
14. Pax-wax
15. Epithelial cells from nose
16. Bone, Ostrich
17. Feather, Shaft of Canary’s
18. Do. Wild Duck
19. Circulation of blood, Frog’s foot
20. Feather, Sparrow
21. Do. Cock’s tail
22. Fibre, crystalline lens of fish
23. Nerve
24. Muscle, Meat
25. Tooth, transverse section
26. Do. Longitudinal section
27. Sweat duct
28. Eye of Haddock
29. Myliobates, palate
30. Gristle, Pig
31. Pigment, Human eye
32. Do. Wing of Bat
33. Do. Shell of Prawn
[Illustration: X.]
A few words may be said, in the first place, as to the outfit. A very
useful part of it is a walking-stick, to which can be attached either
a net for capturing the larger forms of life, or a hook for collecting
the weeds, to which many forms of great interest and beauty are
attached (Fig. 15). The stick is telescopic, and can also have attached
to it a bottle, which, put into the water at any desired spot,--say,
amongst a clump of weeds, or near the bottom, upside down, and then
suddenly reversed,--will bring away samples of the inhabitants of the
neighbourhood. When these are sparsely distributed through the water,
the latter may be concentrated by the use of a bottle round the neck of
which is firmly tied a coarse calico bag, funnel-shaped, and supported
by a wire ring, somewhat as shown in the illustration. Muslin is,
however, too coarse for many organisms. This net is immersed in the
water so that the ring is just above the surface, and one bottleful
after another poured through. The water strains off, the organisms are
left behind. The immersion is necessary to reduce the pressure to which
delicate organisms would otherwise be subjected. When the bottle is
full, or sufficiently concentrated as to its contents, the latter are
poured into one of the ordinary collecting-bottles, of which half a
dozen at least should always be taken.
[Illustration: Fig. 15.]
On reaching home, and as often as possible on the way, the corks should
be removed, as these organisms soon use up the air in the water.
For examination a glass trough of considerable size, say three inches
in length, half an inch in depth, and two inches in height, should be
half filled with the water, and examined with the pocket magnifier.
With a little practice it will be found easy to take up not only
the larger organisms, but even very minute ones, with one of the
dipping-tubes with a long tapering point already referred to. The
organism, when “spotted,” is followed by eye and tube, the finger
being held over the mouth of the latter, and at the critical moment
the finger is removed, and the organism swept into the tube by the
in-rushing water. Now wipe off the excess with a clean handkerchief,
“spot” the organism in the tube again, and carefully absorb the
superfluous water with a piece of blotting paper; and finally, gently
but sharply blow the remainder on to the plate of the live-box, put on
the cover, and examine with a one-inch power. If, as often happens,
the organism sticks to the side of the tube, a little more water must
be drawn in, and the process repeated. The use of the cotton-wool trap
spoken of previously will often be very helpful in the examination of
actively moving organisms.
In the case of weeds, a small portion should be placed in the trough
and carefully examined from end to end, first with the pocket lens and
then with the one-inch power. Let us now consider the objects most
likely to be met with.
A piece of stick may be coated with a white layer, feeling rough to the
touch, and full of small holes. The chances are that this will be a
piece of fresh-water sponge, _Spongilla fluviatílis_, and by dark-field
illumination particles may be seen to enter at some orifices and be
ejected at others. With a very high power and a very thin section,
properly prepared, these holes will be seen to be the mouths of
channels which are lined by the most delicate organisms possible, each
having a minute body crowned with a tiny crystal cup, in the middle of
which is a long cilium, or flagellum, as it is here called (Plate XIII.
Fig. 1). The currents are produced by the combined action of these
flagella. In point of fact, the sponge is a colony of minute animals
working harmoniously for the common good. If the specimen be found in
winter the sponge will be full of tiny balls, the “gemmules” of the
next season’s growth. The roughness is due to the flinty spicules,
which are at once the scaffolding and the protection of the sponge, and
by boiling the sponge in a mixture of nitric acid and water (half and
half) these spicules will be set free, and may be washed, allowed to
settle, washed again, dried, and mounted in balsam. The gemmules are
coated by very beautiful spicules, consisting of two wheels connected
by a rod. These may be treated in the same way. The life-history of the
common sponge is as yet but imperfectly known.
Perhaps the lowest form of life is the _Amœba_, shown in Plate IX.
Fig. 1, a mere lump of jelly, which flows along, and when it comes
into contact with any likely subject for digestion flows round it,
encloses it, absorbs what it can from it, and leaves it behind. A near
relative of the Amœba is the _Arcella_ (Fig. 2), which is simply an
Amœba with a shell. Being unable to swim, these organisms are naturally
to be most often found at the bottom of the collecting bottle, and
it is always advisable to take up a portion of the débris with a
dipping tube, which is then held upright on a slide with the finger
upon it until the dirt settles on to the slide, when it is removed,
a cover-glass put upon the dirt, and a quarter-inch power used for
examination. Many forms will be discovered in this way which would
otherwise escape observation.
Another curious organism, of great size (comparatively) and extreme
beauty, is the sun animalcule (_Actínophrys_), which has a round body
and long tentacles (Fig. 3), to which free-swimming organisms adhere,
and by the combined action of the neighbouring ones are drawn to the
body and received into it; one cannot say swallowed.
Fig. 6, Plate IX., shows the curious arithmetical process whereby
the Infusoria multiply by division, a groove appearing at one point,
rapidly deepening, and finally separating the animal completely into
two. The species is the _Chílodon_, a flattened creature, ciliated all
over, having a set of teeth arranged in the form of a tube, and at its
fore-part a kind of membranous lip. A similar phenomenon, in an earlier
stage, is shown in Fig. 26, Plate XIII., the organism in this case
being _Euplótes_.
It has been said that sponges are colonies of extremely minute
organisms, each furnished with a membranous collar or funnel, the whole
looking like an exquisite wine-glass without a foot. These organisms
are not always grouped in colonies, however. Many are free-growing,
and may be found attached to the stems of water-plants, but they are
extremely minute, and will hardly be noticed until the microscopist
has acquired considerable experience, nor even then--with such an
instrument as we have postulated--will he see more than a tiny pear,
with a straight line, the margin of the cup, on each side of its
summit. The flagellum will be quite invisible.
Some similar organisms may, nevertheless, be found which, though
still minute, are within the range of a properly managed quarter-inch
objective. Such an one, of extreme beauty, is the _Dinobrýon_ shown in
Plate XIII. Fig. 3. Each “zoöid,” as the separate animals are called,
among the Infusoria, or each generation of zoöids, stands upon its
parent and has two flagella. When alarmed, the zoöid sinks to the
bottom of its cell, and withdraws its flagella. In Fig. 2 (_Eugléna_)
we have a similar zoöid, but of far greater size, and free-swimming.
It is a very common object, and possesses a red eye-speck close to the
“contractile vesicle.” All Infusoria have the latter, some a great
number, as in Fig. 9. The vesicle contracts at regular intervals, and
is then simply blotted out, but reforms in the same place, so that it
is probably the heart or the urinary bladder of these minute animals.
The lovely rosette shown in Fig. 4 is the _Synura_, a spherical colony
of zoöids, each of which has two flagella, and is in addition clothed
with rows of cilia. A beautiful sight it is to watch these colonies
rolling through the field of view. Not uncommon, especially in brackish
water, is the _Peridinium_ (Fig. 5), with its plate armour, long
flagellum, and girdle of cilia. A gigantic species of the same family
is common in sea-water, and will be easily recognised by its body, not
much larger than that of Peridinium, being furnished with three long
arms, curiously bent. It is called _Ceratium_, and is sometimes present
in such abundance as to thicken the water, near the surface of which it
swims.
We now come to a class of Infusoria which is characterised by the
possession of a complete covering of cilia, arranged in rows all over
the body. The number of these is enormous; we can only glance at a
few types, by mastering which the observer will, at all events, know
whereabouts he is. The first we will take is the _Coleps_ (Fig. 6),
a very common kind, whose body is marked by a series of geometrical
lines, so that the organism looks very much like an elongated
geographical globe. These markings are on the tunic, which is of a
brownish colour. Very different is the _Trachelocerca_ (Fig. 7), with
its long flexible neck, which is in constant movement from side to side
as the creature swims along. As seen in the figure, the neck is clear
and the head has a fringe of longer cilia.
The _Trachelius_ (Fig. 8) is perhaps the largest of all the Infusoria,
being readily visible to even an inexperienced eye. Its body is richly
furnished with contractile vesicles, and the protoplasm is curiously
reticulated. We may here remark that the Trachelius is especially
prompt in doing what most of these organisms do when put under pressure
in a live-box, namely, in performing a kind of _harakiri_. The outline
first becomes irregular, then the body rapidly swells and finally comes
to pieces, the fragments dancing mockingly away under the influence of
their still-moving cilia. The remedy is to use the cotton-wool trap and
the lightest possible pressure.
A very elegant organism is shown in the bottom right-hand corner of the
Plate (Fig. 25). It is the _Loxophyllum_, and has a strongly marked
contractile vesicle.
Another large form is _Amphileptus_ (Fig. 9), already referred to as
having a large number of contractile vesicles arranged in a regular
row; and more massive still is _Bursaria_ (Fig. 10), a very curious
organism, very much like a purse indeed, and possessing a wonderful
arrangement of cilia inside the funnel. These are arranged like a
ladder, a series of rows of short stiff cilia, which move at short
intervals in unison, and tend to sweep down into the cavity any
small particles of food. This arrangement is here described for the
first time, and appears to be quite unlike anything else among the
Infusoria. Not unlike Bursaria, but having no ladder, and being
furnished with a delicate membranous pouch in front of the slit of the
purse, is _Condylostoma_, which we shrewdly suspect to be the young
form of Bursaria. This is a point which requires elucidation.
One of the most beautiful of all these forms is shown in Fig. 11,
_Folliculina_, a type of a large group characterised by the possession
of a transparent case, of extremely elegant form, within which the
animal retreats on the slightest alarm.
Fearless and independent, as becomes its size, is the trumpet-shaped
_Stentor_ (Fig. 12), which may easily be seen when present, as it is in
almost every good gathering of water-weed. The particular form drawn
(_S. Mülleri_) does not make a case, but many members of the genus
do, and it is very common to see a stem almost covered with them.
Such a sight, once seen under dark-field illumination, will never be
forgotten. The method of multiplication of the Stentors (by division)
is extremely easy to watch, and very instructive.
A curious organism is _Trichodina_ (Fig. 13), which, though a
free-swimmer, is always parasitic upon the body of some higher animal.
We have found it sometimes upon Hydra, and always in hundreds upon the
stickleback. The next group of Infusoria is distinguished by the body’s
being only ciliated at particular points, usually round the mouth, or
what acts as such. The first form is Vorticella (Fig. 14), a beautiful
vase-like creature upon a stem. Down the stem runs a muscular fibre,
and on the least shock the fibre contracts and draws the stem into a
beautiful spiral, whilst the cilia are drawn in, and the zoöid assumes
the appearance of a ball at the end of a watch-spring. An exquisite
sight is a colony of Vorticellæ, for these actions are always going on,
as, for example, when one member of the family touches another, which
is quite sufficient to provoke the contraction.
XI.
POLARIZED LIGHT.
FIG.
1. Carbonate of Lime
2. Starfish
3. Thistle down
4. Starch, Wheat
5. Do. Potato
6. Prawn-shell
7. Starch, “Tous les mois”
8. Bone, cancellous
9. Gun-cotton
10. Cow’s hair
11. Hoof, donkey, longitudinal
12. Do. transverse
13. Nitre, Crystals
14. Scale, Eel
15. Wing, Water-Boatman
16. Chlorate of Potash, Crystals
17. Cellularia reptans
18. Star-shaped hair, Stalk of Yellow Water-Lily
19. Teeth, Palate of Whelk
20. Zoophyte, Bowerbankia
21. Raphides, _i.e._ crystalline formations in
vegetable cells, Bulb of Hyacinth
22. Do. Rhubarb
23. Sulphate of Magnesia, Crystals
24. Bone, Skate
25. Cherrystone, transverse section
26. Sugar, Crystals in honey
27. Tendon, Ox
28. Calcareous plates. Tooth of Echinus
[Illustration: XI.]
Many compound tree-like forms of Vorticella are known, one of which,
_Carchesium_ (Fig. 15), may serve as a type of all. In the case of this
organism, the colony contracts in sections on a moderate shock; in the
second, _Zoothamnium_, as a whole; whilst in _Epistylis_ the stalks are
rigid, and the individuals contract singly. When the shock is violent,
the appearance presented by the two former is that shown in Fig. 16. In
all three cases the colonies are usually so large that they are visible
as trees to the naked eye, and some members of the group are extremely
common. Moreover, they are often parasitic, as, for example, upon
Cyclops, which is frequently loaded with them.
Another compound form is _Ophrydium_, a colony of which (not unusually
large) is shown of the natural size in Fig. 18, with a single zoöid,
magnified, by the side of it, in Fig. 19.
Lastly, we have an exquisite group of organisms related to Vorticella,
but possessing a transparent envelope, the forms of which are most
varied, but always graceful. _Vaginicola_ (Fig. 17) is a good example
of this, and _Cothurnia_ (Fig. 20) still more so. Many of these
organisms, too, are furnished with a plate, attached either to the
head or to the body, which plate, when they withdraw into their cases,
closes the latter perfectly, as in the case of the exquisite _Pyxicola_
(Fig. 21).
A very interesting but singularly obtrusive organism is the
_Stylonychia_ (Figs. 22, 23). How often has it happened to us to have
an interesting object nicely in the field of view, and then to have it
knocked out of sight by the blundering incursion of this burly fellow,
who runs so rapidly by means of his “styles” that he gives nothing
time to get out of the way. He is of interest to us, however, as the
representative of a class in which the body is not ciliated, or very
partially and slightly so, usually round the mouth. We have frequently
found Stylonychia, in company with Vorticella and _Paramœcium_ (Plate
IX. Fig. 6), in the water in which flowers have been standing for a few
days; sometimes the numbers are so great as to make the water quite
milky.
One more form must conclude this short sketch of the great Infusorial
family. It is the _Acineta_ (Fig. 24), which, attached by its
foot-stalk, and devoid of cilia, patiently waits, with outspread arms,
to receive and embrace smaller members of the family as they dance
merrily about. Alas! its embrace is as fatal as that of the image of
the Virgin which bore beneath its robe spikes and daggers, for the
victim struggles vainly to escape, and the nourishment from its body is
rapidly absorbed.
And here we take our leave of a group which, simple as is the
construction of the animals which it includes (for every one, great
and small alike, is composed of a single cell), is yet full of beauty
and interest. He who wishes to pursue the matter further will find in
Saville Kent’s _Manual of the Infusoria_ a perfect mine of information,
to which we gladly acknowledge our indebtedness, both now and in time
past.