The most important discoveries of the laws, methods, and progress of Nature have nearly always sprung from the examination of the smallest objects which she contains, and from apparently the most insignificant enquiries. Lamarck.
There is probably no better introduction to the study of the biology of an animal than that afforded by Paramecium, a common organism of ponds, ditches, and decaying vegetable infusions. Paramecium is a representative of some 10,000 kinds of single-celled animals, or Protozoa.
Members of this group are found in almost every niche in nature and, like the Protophyta, as the unicellular plants are sometimes called, are important because in numbers there is strength.
STRUCTURE AND LIFE HISTORY OF PARAMECIUM
Paramecium is a giant among the Protozoa, though just visible to the naked eye as a whitish speck if the water in which it is swimming is properly illuminated. But to make out the details of structure it is necessary to magnify it several hundred times. This done, it appears as a more or less cigar-shaped organism which one would not consider, at first glance, a single cell because it shows highly differen tiated parts. However, careful study reveals the fact that the organism really consists of a single protoplasmic unit differ entiated into cytoplasm and nucleus, though each of these regions shows specializations. The nuclear material, instead of forming a single body as it does in most cells, in Paramecium is distributed in two parts: a larger body, or MACRO NUCLEUS, and a smaller body, or MiCRONUCLEUS.
Strictly speaking, the macronucleus and micronucleus together constitute the nucleus of the cell, and rep resent a sort of physiological division of labor in the chromatin complex. But it is in the cyto plasm that specialization is most conspicuous. Not only are there general differentiations into ectoplasm and endoplasm, but these regions also have local specializations such as Cilia for locomotion, TRICHOCYSTS for defense, PERISTOME, MOUTH, and GULLET for the intake of solid food, GASTRIC VACUOLES for digestion, and CONTRACTILE VACUOLES for excretion. And withal, recent in vestigations indicate that various parts of the cell are coordinated by a 'neuromotor' apparatus.
Paramecium, under favorable conditions, grows rapidly and, when it has attained the size limit characteristic of the species, cell division takes place, with the result that from the single large cell there are formed two smaller individuals which soon become complete in all respects. These, in turn, Grow and repeat the process in about ten hours so that, as in the case of Sphaerella, within a few days the original Paramecium has divided its individuality, so to speak, among a multitude of descendants.
This process of multiplying by dividing can go on indefinitely under optimum environmental conditions. But periodically Paramecium undergoes an internal nuclear reorganization process (ENDOMIXIS). Also now and then individuals temporarily fuse in pairs and interchange nuclear material (CONJUGATION) an expression of the same fundamental sex phenomenon which is exhibited in Sphaerella.
Paramecium thus affords some idea of the complexities of structure and function which a cell may exhibit when it forms the whole animal organism. The Protozoa are the simplest, though by no means simple, animals. But the great structural differences between Sphaerella and Paramecium, though to a certain extent representative of plants on the one hand and animals on the other, are not essentially diagnostic, because, as we have suggested before, in the last analysis. Position as^ is a matter of metabolism. And it assumed by conjugating largely for this reason that Sphaerella and Paramecia.
Food Taking By Animals - Paramecium
The food of Paramecium is chiefly microscopic, colorless plants known as BACTERIA which are present in countless numbers in decaying vegetable infusions. As Paramecium swims about by means of its cilia, a current of water laden with Bacteria is whirled down the peristome on one side of the animal and some passes through the mouth and gullet into the endoplasm. Here the Bacteria, surrounded by a droplet of water, form a gastric vacuole, into which the endoplasm secretes chemical substances (enzymes, etc.) which gradually break down that is, digest the complex proteins, carbohydrates, etc., of the plant cells. Finally, this material which shortly before was Bacteria protoplasm is incorporated into Paramecium protoplasm matter and energy is supplied and the animal lives and grows.
This is, in most regards, a strikingly different condition from that which we have seen in Sphaerella. In Paramecium solid particles of food Bacteria are taken into the cell, and since the chief organic constituents of protoplasm are proteins, associated with carbohydrates and fats, it is clear that the income of the animal organism is, unlike that of the green plant, chiefly ready-made complex foodstuffs. In other words, Paramecium, like all animals, requires relatively complex chemical compounds rich in potential energy: proteins, carbohydrates, and fats. Of these, proteins or their constituent amino acids are absolutely indispensable because it is only from this source that nitrogen is available for the animal. But the green plant, through its chlorophyll apparatus, is able to take materials largely devoid of energy and to rearrange them and endow them with potential energy which it has received in the kinetic form from sunlight.
Respiration and Excretion
Of course, during life, the animal, like the green plant, is continually breaking down its food and its own protoplasm by a process of combustion which involves an intake of free oxygen and the liberation of carbon dioxide and water.
Nitrogenous wastes, chiefly urea, as well as inorganic salts, are also excreted. So the animal, like the plant, returns to its environment the elements in simple combinations which are devoid or nearly devoid of energy. We have stated that green plants are essentially constructive and animals essentially destructive agents in nature. It is now apparent that green plants are both constructive and destructive, while animals are essentially destructive.
A little consideration of the income and outgo of green plants and animals will show that, although the animals are dependent on the plants for their complex foodstuffs, they do not return, for example, the nitrogen to the outer world in a form simple enough to be available for green plants. In other words the urea, (NH^CO, which still has a little energy left which the animal is unable to extract, must be transformed into nitrates.
Furthermore, since plants die, which are not consumed by animals, and animals die, which are not devoured by other animals, large stores of matter and energy are locked up in the complex compounds of their dead tissues. Clearly, there must be some way of completing the cycle of the elements, for if there were not, life, as we know it, could not have continued long on the Earth. This gap is filled by the so-called COLOR LESS PLANTS, that is plants which, because chlorophyll is not present, lack the power of photosynthesis and so in most cases are dependent for food on more complex substances than green plants demand, though not so complex as animals require.