THE ONE-CELLED ANIMALS OR PROTOZOA
Some idea of the general characteristics of the Protozoa has already
been given by the description of _Amoeba_. We may now say something
about special groups of the Protozoa, which have minor characteristics
of their own.
Amoeba belongs to the class Rhizopoda, as has been already stated; but
there are many of the Rhizopoda that greatly differ from Amoeba in
appearance. The possession of a shell or skeleton gives a special
importance to several groups. For, as the reader has no doubt already
learnt from an earlier volume in this series, such skeletons or shells
have played an important part in the history of the earth's surface,
building up geological strata of vast extent, by the accumulation of the
shells left after the decay of the owners' tiny bodies, during long
periods of time. The chalk rocks that form the "white cliffs of Albion,"
and that are so widely distributed in other parts of the globe, are
formed in this manner; while the ooze of the Atlantic and other oceans,
similarly composed of Protozoan _débris_, is now at the present time
building up what will be the chalk rocks of future ages. Some of these
Protozoans attain a remarkable size, instead of being microscopic, as is
the case typically with the one-celled animals. Some forms of the
Foraminifera found on the coast of North America measure as much as
one-fifth of an inch across, while in warmer seas there are kinds which
attain, as did the extinct Nummulite of Egypt, the size of a bean. Two
inches across is mentioned as the maximum diameter, however, of either
extinct or living forms. The Foraminifera are sometimes named
Reticularia, because their pseudopodia interlace.
[Illustration: FIG. 8.--Fossil Skeletons of Polycystina, from the
so-called "Infusorial Earth" of Barbadoes, highly magnified.]
The Foraminifera have shells composed of carbonate of lime, but there
are other forms that build up geological deposits, in which the shell
is flinty. The diagram (Fig. 8) shows some fossil shells of Protozoa
from the marl of Barbadoes. These constitute a deposit which was named
"Infusorial earth," in the earlier days of microscopic observation, when
all Protozoans were spoken of as Infusoria. The name, Infusoria, it must
be recollected, is now restricted to a special class, to which the forms
in question do not belong. These fossil forms were named Polycystina,
and are still often spoken of under that name, although the animals that
present the peculiar feature of possessing "more than one cyst" now are
called Radiolarians. The "cyst" consists of a basket-work supporting
skeleton of flint; there may be several, one inside the other, and
connected by radial bars. A living species named _Actinomma_ has three
such layers of basket-work, one in the outer layer of protoplasm, one in
the inner layer, and a central one. It will perhaps be remembered by the
reader that the animals of this group, Radiolaria, are forms described
in a previous volume of the series, as so curiously associated in
Symbiosis with the algæ known as Yellow Cells.
The famous polishing slate of Bilin in Bohemia consists of flinty
Protozoan shells; it is 14 feet thick, and a cubic inch has been
estimated to contain 41,000,000,000 of the shells.
While the Radiolarians are marine, the Heliozoa, a group in which
the skeleton is also present, but not usually so greatly developed,
are predominantly fresh-water forms. Both classes take their name
(Ray-animals, Sun-animals) from the stiff radiating rods of the
skeleton.
Strongly to be contrasted with the above groups belonging to the
Rhizopoda are the Infusoria proper, which are characterized by the usual
possession of cilia. Cilia (literally "eyelashes") are fine hair-like
processes of the protoplasm of the cell, which fringe its exterior; by
their constant movement they enable the animal to swim, and at the same
time they create a current in the water, which washes up to the region
of the mouth particles which may serve for food; for these creatures
have this very great advantage over Amoeba, and the other forms above
referred to, that they possess something which may be called a mouth.
That is to say, there is one particular spot of the surface where
particles are taken in. This may seem to be a restriction, when we
compare the Infusorian with Amoeba, which is apparently able to take in
food at any part of the surface. But it is a restriction which is
associated with an advantage; the Infusorian cell, namely, has a firm
exterior with a definite outline, instead of being soft and mobile all
over. The firmer exterior layer of protoplasm, which is in turn covered
by a thin cuticle or limiting membrane, is called the cortex or rind.
For this reason the name Corticata is sometimes given to the group,
_i.e._, Protozoa with a rind.
_Vorticella_, the Bell Animalcule, is a stalked form living in ditches,
which is usually selected as a typical form of the Infusoria. It
receives its name, the Whirlpool Animal, from the current which its
cilia create in the water. The purpose of this current is to wash food
particles into the mouth. Associated with the Infusoria under the name
of Corticata are the Gregarina and some other parasitic forms.
It is interesting to note that the main types of the unicellular animals
are repeated again in the cells of different parts of the bodies of
multicellular animals. Amoeboid cells, so called because of their
mobility and general resemblance to Amoeba, are found in various parts
of the higher animals. The lymph corpuscles of vertebrata, and the white
corpuscles of vertebrate blood, as well as the blood corpuscles of
invertebrates, are among the instances of this. There are cells, on the
contrary, such as those that line the mucous tracts, which are of a
Vorticella type, so to speak; fixed to their bases, and presenting cilia
on the free aspect.
Two things must be noticed before we leave the subject of the Protozoa.
One is, that some forms present the beginning of a multicellular
condition. Several units sometimes join together, and in this way a
complex object may be formed, in which there are several nuclei; or the
original unit may keep on growing till it consists of many successive
portions, and in some of them a fresh nucleus may arise. This occurs in
some of the Foraminifera.
The next thing to be noticed is, that there are a number of organisms
which constitute a debateable ground, and are claimed now by the
botanist, and now by the zoologist. While the latter insists on calling
them Protozoa (Primitive Animals) the former would have them Protophyta
(Primitive Plants). The fact is that in these organisms of the first
grade, the distinction between "plant" and "animal" has not become a
hard and fast line; and the disputed forms may be best described as
links between the two. The chemistry of nutrition is probably more to be
relied upon as a distinction, than the difference of structure. It is
here that the two groups, plants and animals, start upon different
roads, and many of the differences in structure must be regarded as the
direct result of the fundamental difference in the mode of nutrition.
The following very instructive remarks on the subject are taken from
Professor Hertwig's valuable book "The Biological Problem of To-Day,"[B]
pp. 111, 112.
[B] "The Biological Problem of To-Day, Preformation or Epigenesis," by
Professor O. Hertwig. Translated by P. C. Mitchell. Heinemann, 1896.
"The different mode of nutrition of animals results in a totally
different structural plan. Animal cells absorb material that is already
organised, and that they may do so their cells are either quite naked,
so affording an easy passage for solid particles, or they are clothed
only by a thin membrane, through which solutions of slightly diffusible
organic colloids may pass. Therefore, unlike plants, multicellular
animals display a compact structure with internal organs adapted to the
different conditions which result from the method of nutrition peculiar
to animals. A unicellular animal takes organic particles bodily into its
protoplasm, and forming around them temporary cavities known as food
vacuoles, treats them chemically. The multicellular animal has become
shaped so as to enclose a space within its body, into which solid
organic food-particles are carried and digested thereafter in a state of
solution, to be shared by the single cells lining the cavity. In this
way the animal body does not require so close a relation with the medium
surrounding it; its food, the first requirement of an organism, is
distributed to it from inside outwards. In its further complication the
animal organisation proceeds along the same lines. The system of
internal hollows becomes more complicated by the specialisation of
secreting surfaces, and by the formation of an alimentary canal, and of
a body-cavity separate from the alimentary canal. In plants it is the
external surface that is increased as much as possible. In animals, in
obedience to their different requirements, increase takes place in the
internal surface. The specialisation of plants displays itself in organs
externally visible--in leaves, twigs, flowers, and tendrils. The
specialisation of animals is concealed within the body, for the internal
surface is the starting-point for the formation of the organs and
tissues."
TABLE SHOWING THE CLASSIFICATION OF THE PROTOZOA
=Grade I.= { RHIZOPODA, OR
{ GYMNOMYXA.
UNICELLULAR ANIMALS. =PROTOZOA= {
{ INFUSORIA, OR
{ CORTICATA.