BRANCH PROTOZOA: THE ONE-CELLED
ANIMALS
Of this group the structure and life-history of the Amœba (_Amœba_
sp.) and the Slipper Animalcule (_Paramœcium_ sp.) have already been
treated in Chapter VI. Another example is the
BELL ANIMALCULE (_Vorticella_ sp.)
TECHNICAL NOTE.--Specimens of _Vorticella_ may usually be found in
the same water with _Amœba_ and _Paramœcium_. The individuals live
together in colonies, a single colony appearing to the naked eye
as a tiny whitish mould-like tuft or spot on the surface of some
leaf or stem or root in the water. Touch such a spot with a
needle, and if it is a Vorticellid colony it will contract
instantly. Bring bits of leaves, stems, etc., bearing Vorticellid
colonies into the laboratory and keep in a small stagnant-water
aquarium (a battery-jar of pond-water will do).
Examine a colony of _Vorticella_ in a watch-glass of water or in a
drop of water on a glass slide under the microscope. Note the stemmed
bell-shaped bodies which compose the colony. Each bell and stem
together form an individual _Vorticella_ (fig. 8.) How are the members
of the colony fastened together? Tap the slide and note the sudden
contraction of the animals; also the details of contraction in the
case of an individual. Watch the colony expand; note the details of
this movement in the case of an individual.
Make drawings showing the colony expanded and contracted.
With higher power examine a single individual. Note the thickened,
bent-out, upper margin of the bell. This margin is called the
_peristome_. With what is it fringed? The free end of the bell is
nearly filled by a central disk, the _epistome_, with arched upper
surface and a circlet of _cilia_. Between the epistome and peristome
is a groove, the _mouth_ or _vestibule_, which leads into the body.
Study the internal structure of the transparent, bell-shaped body.
Note the differentiation of the protoplasm comprising the body into an
inner transparent colorless _endosarc_ containing various dark-colored
granules, vacuoles, oil-drops, etc., and an outer uniformly granular
_ectosarc_ not containing vacuoles. Is the stalk formed of ectosarc or
endosarc or of both? Note the curved _nucleus_ lying in the endosarc.
(This may be difficult to distinguish in some specimens.) Note the
numerous large circular granules, the _food vacuoles_. Note the
_contractile vesicle_, larger and clearer than the food vacuoles. Note
the thin _cuticle_ lining the whole body externally. A high
magnification will show fine transverse ridges or rows of dots on the
cuticle.
[Illustration: FIG. 8.--_Vorticella_ sp.; one individual with stalk
coiled, and one with stalk extended. (From life.)]
Make a drawing showing the internal structure.
Observe a living specimen carefully for some time to determine all of
its movements. Note the contraction and extension of the stalk, the
movements of the cilia of peristome and epistome, the flowing or
streaming of the fluid endosarc (indicated by the movements of the
food vacuoles), the behavior of the contractile vesicle.
Make notes and drawings explaining these motions.
Specimens of _Vorticella_ may perhaps be found dividing, or two
bell-shaped bodies may be found on a single stem, one of the bodies
being sometimes smaller than the other. These two bodies have been
produced by the longitudinal division or fission of a single body. In
this process a cleft first appears at the distal end of the
bell-shaped body, and gradually deepens until the original body is
divided quite in two. The stalk divides for a very short distance. One
of the new bell-shaped bodies develops a circlet of cilia near the
stalked end. After a while it breaks away and swims about by means of
this basal circlet of cilia. Later it settles down, becomes attached
by its basal end, loses its basal cilia and develops a stalk.
"Conjugation occurs sometimes, but it is unlike the conjugation of
_Paramœcium_ in two important points: Firstly, the conjugation is
between two dissimilar forms; an ordinary large-stalked form, and a much
smaller free-swimming form which has originated by repeated division of
a large form. Secondly, the union of the two is a complete and permanent
fusion, the smaller being absorbed into the larger. This permanent
fusion of a small active cell with a relatively large fixed cell,
followed by division of the fused mass, presents a striking analogy to
the process of sexual reproduction occurring in higher animals."
OTHER PROTOZOA
Besides the _Amœba_, _Paramœcium_, and _Vorticella_ there are
thousands of other Protozoa. Most of them live in water, but a few
live in damp sand or moss, and some live inside the bodies of other
animals as parasites. Of those which live in water some are marine,
while others are found only in fresh-water streams and lakes.
[Illustration: FIG. 9.--Sun animalcule, a fresh-water protozoan with a
siliceous skeleton, and long thread-like protoplasmic prolongations.
(From life.)]
=Form of body.=--The Protozoa all agree in having the body composed
for its whole lifetime of a single cell,[7] but they differ much in
shape and appearance. Some of them are of the general shape and
character of _Amœba_, sending out and retracting blunt, finger-like
pseudopodia, the body-mass itself having no fixed form or outline but
constantly changing. Others have the body of definite form, spherical,
elliptical, or flattened, enclosed by a thin cuticle, and having a
definite number of fine thread-like or hair-like protoplasmic
prolongations called flagella or cilia. Many of the familiar Protozoa
of the fresh-water ponds always have two whiplash-like flagella
projecting from one end of the body. By means of the lashing of these
flagella in the water the tiny creature swims about. Others have many
hundreds of fine short cilia scattered, sometimes in regular rows,
over the body-surface. The Protozoan swims by the vibration of these
cilia in the water.
[Illustration: FIG. 10.--_Stentor_ sp.; a protozoan which may be
fixed, like _Vorticella_, or free-swimming, at will, and which has the
nucleus in the shape of a string or chain of bead-like bodies. The
figure shows a single individual as it appeared when fixed, with
elongate, stalked body, and as it appeared when swimming about with
contracted body. (From life.)]
There is no stagnant pool, no water standing exposed in watering-trough
or barrel which does not contain thousands of individuals of the
one-celled animals. And in any such stagnant water there may always be
found several or many different kinds or species. A drop of this water
examined with the compound microscope will prove to be a tiny world (all
an ocean) with most of its animals and plants one-celled in structure. A
few many-celled animals will be found in it preying on the one-celled
ones. There are sudden and violent deaths here, and births (by fission
of the parent) and active locomotion and food-getting and growth and all
of the businesses and functions of life which we are accustomed to see
in the more familiar world of larger animals.
=Marine Protozoa.=--One usually thinks of the ocean as the home of the
whales and the seals and the sea-lions, and of the countless fishes,
the cod, and the herring, and the mackerel. Those who have been on the
seashore will recall the sea-urchins and starfishes and the
sea-anemones which live in the tide-pools. On the beach there are the
innumerable shells, too, each representing an animal which has lived
in the ocean. But more abundant than all of these, and in one way more
important than all, are the myriads of the marine Protozoa.
Although the water at the surface of the ocean appears clear and on
superficial examination seems to contain no animals, yet in certain
parts of the ocean (especially in the southern seas) a microscopical
examination of this water shows it to be swarming with Protozoa. And
not only is the water just at the surface inhabited by one-celled
animals, but they can be found in all the water from the surface to a
great depth below it. In a pint of this ocean-water there may be
millions of these minute animals. In the oceans of the world the
number of them is inconceivable. And it is necessary that these
Protozoa exist in such great numbers, for they and the marine
one-celled plants (Protophyta) supply directly or indirectly the food
for all the other animals of the ocean.
Among all these ocean Protozoa none are more interesting than those
belonging to the two orders Foraminifera (fig. 11) and Radiolaria. The
many kinds belonging to these orders secrete a tiny shell (of lime in
the Foraminifera, of silica in the Radiolaria) which encloses most of
the one-celled body. These minute shells present a great variety of
shape and pattern, many being of the most exquisite symmetry and
beauty. The shells are perforated by many small holes through which
project long, delicate, protoplasmic pseudopodia. These fine
pseudopodia often interlace and fuse when they touch each other, thus
forming a sort of protoplasmic network outside of the shell. In some
cases there is a complete layer of protoplasm--part of the body
protoplasm of the Protozoan--surrounding the cell externally.
[Illustration: FIG. 11.--_Rosalina varians_, a marine protozoan
(Foraminifera) with calcareous shell. (After Schultze.)]
When these tiny animals die their hard shells sink to the bottom of
the ocean, and accumulate slowly, in inconceivable numbers, until they
form a thick bed on the ocean floor. Large areas of the bottom of the
Atlantic Ocean are covered with this slimy ooze, called Foraminifera
ooze or Radiolaria ooze, depending on the kinds of animals which have
formed it. Nor is it only in present times that there has been a
forming of such beds by the marine Protozoa. All over the world there
are thick rock strata composed almost exclusively of the fossil shells
of these simplest animals. The chalk-beds and cliffs of England, and
of France, Greece, Spain, and America, were made by Foraminifera.
Where now is land were once oceans the bottoms of which have been
gradually lifted above the water's surface. Similarly the rock called
Tripoli found in Sicily and the Barbadoes earth from the island of
Barbadoes are composed of the shells of ancient Radiolaria.
It is thus evident that the Protozoa is an ancient group of animals.
As a matter of fact zoologists are certain that it is the most ancient
of all animal groups. All of the animals of the ocean depend upon the
marine Protozoa and the marine Protophyta, one-celled plants, for
food. Either they feed on them directly, or prey on animals which in
turn prey on these simplest organisms. A well-known zoologist has
said: "The food-supply of marine animals consists of a few species of
microscopic organisms which are inexhaustible and the only source of
food for all the inhabitants of the ocean. The supply is primeval as
well as inexhaustible, and all the life of the ocean has gradually
taken shape in direct dependence on it." The marine Protozoa are the
only animals which live independently; they alone can live or could
have lived in earlier ages without depending on other animals. They
must therefore be the oldest of marine animals. By oldest is meant
that their kind appeared earliest in the history of the world, and as
it is certain that ocean life is older than terrestrial life--that is,
that the first animals lived in the ocean--it is obvious that the
marine Protozoa are the most ancient of all animal groups.
As already learned in the examination of examples of one-celled
animals, it is evident that life may be successfully maintained
without a complex body composed of many organs performing their
functions in a specialized way. The marine Protozoa illustrate this
fact admirably. Despite their lack of special organs and their
primitive way of performing the life-processes, that they live
successfully is shown by their existence in such extraordinary
numbers. They outnumber all other animals. The conditions of life in
the surface-waters of the ocean are easy and constant, and a simple
structure and simple method of performing the necessary life-processes
are wholly adequate for successful life under these conditions.
FOOTNOTE:
[7] In some Protozoa a number of similar cells temporarily unite to form
a colony, but each cell may still be regarded as an individual animal.