In popular language an animal is something that moves on four legs and has its body covered with hair. Thus dogs, rabbits, bears and lions are animals; but the rest of the Animal Kingdom is divided into birds, fishes, insects and a number of ill-defined and ill-conceived groups to which the layman often finds difficulty in giving a name. It may not be without profit, therefore, to emphasise at the onset that the Animal Kingdom includes a host of diverse creatures ranging from microscopic organisms, the bodies of which consist only of a single cell, to leviathans such as elephants and whales; and that, strictly speaking, these microscopic one-celled creatures, together with a multitude of other lowly beings, rank equally with fishes, birds, insects and the quadrupeds as animals. Even man himself is compelled to swallow his conceit and admit that he is no more than a close relative of the beasts of the field.
Roughly speaking, an animal can be defined as any living organism capable of moving about, feeding on solid food and having an obvious nervous or muscular reaction to various stimuli; or perhaps we could better express this last quality by saying that an animal possesses in a greater or lesser degree one or more of the senses—taste, sight, smell and hearing. And although there are exceptions to this, especially among the lower animals, the definition does serve as a broad means of distinguishing between the two great groups of living things—the Plants and the Animals.
The next point which should be emphasised at this juncture is what may be best described as the evolutionary sequence in the classification of animals, the manner in which they can be arranged in a series of increasing complexity. This sequence corresponds closely to that revealed in the fossil remains of animals as we progress from the oldest rocks down to the present day, and is probably closely comparable to the order and manner in which the higher animals have been evolved from simple and lowly ancestors. Thus, starting from the unicellular organisms, we have the Sponges, the Coelenterates, including sea-anemones and jelly-fishes, animals with a very low grade of nervous system, without brain or any real centralised sense organs, and differing markedly in appearance and behaviour from the more familiar animals such as fishes, birds and mammals.
The next step takes us to that ill-assorted group commonly referred to as the Worms. These have a centralised nervous
system, a rudimentary brain and definite sense-organs, while other details of their anatomy, the intestine, for example, bear a strong resemblance to those of higher animals. Following them, the Mollusca, Insects, Fishes, Reptiles, Birds, Mammals and Man form so obviously progressive a series, even to those uninitiated in the study of Zoology, that there is little to be gained by pursuing the subject further.
In the following paragraphs, therefore, the various members of the Animal Kingdom will be dealt with in this sequence and some attempt will be made to bring out the salient features of their anatomy, behaviour and relationship to each other.
AMOEBA: THE ANIMAL THAT IS NOT WHAT IT SEEMS WHAT is Amoeba? It is one of the one-celled animals already referred to and may be readily taken as our starting-point in the sequence we are to follow : it is one of the simplest animals known, but is not so simple as it is often held to be. Amoeba is usually described as ‘a colourless, shapeless speck of protoplasm, found in any fresh water, even in the supply cisterns of houses.’ Nearly every work devoted to natural history, and every work devoted to biology or zoology, says much the same thing about it. Yet Amoeba is a fraud, or rather, some who write about it are unintentionally fraudulent and the popular conception of Amoeba is not altogether correct. In the first place, Amoeba is not colourless. It is nearly transparent, but not colourless. Secondly, it is not shapeless, although its shape is constantly changing. And thirdly, it cannot be found in just any fresh water—least of all in cisterns; and those who set out to look for Amoeba are likely to meet with many disappointments before, perhaps in some small pool or pond, they find it after much painstaking search.
But an even greater complaint has yet to be made. In the early days, when the doctrine of evolution was taking concrete form, Amoeba was a perfect godsend to the scientist. It was a form of life that approximated closely to his idea of what the first living matter to appear on the earth looked like. It symbolised to him the primeval blob of jelly from which all living matter was assumed to have arisen, and whose evolution through the ages culminated in the advent of man himself. In this way it served a useful purpose, but this use of Amoeba is now obsolete. Though a very lowly, very primitive organism, it is a well-developed animal with specialised organs and a
complex structure, and resembles to only a very slight degree the original speck of protoplasm from which all living matter is assumed to have sprung. For really low forms of life we have to turn to the bacteria, the lowest forms of plant-life, and by comparison with these Amoeba is a well-advanced organism. Once we have disabused our minds of the old conception of Amoeba we are in a position to realize that the lowest forms of animal life known to us have already reached quite an advanced stage in evolution.
The bodies of the great majority of animals are built up of thousands or even millions of living cells, each one of which bears a strong resemblance to the body of Amoeba—in structure at all events, though not always in behaviour or appearance. The white corpuscles of the blood, for example, are really nothing more than Amoeba? imprisoned within our bloodvessels. It may, therefore, be of interest to consider closely the structure of the single cell that makes the body of Amoeba.
The cell is, when at rest, a rounded mass of protoplasm, visible only under a strong hand-lens or under a microscope, with the interior granular, and suggesting, when magnified, a jelly impregnated with sand-grains. At the centre, or thereabouts, is a rounded body, the nucleus, which is the seat of control. As the animal moves, the substance of the cell is thrown out into finger-like processes, called pseudopodia, and the animal moves in a creeping way.
A CREATURE THAT FOLDS ITSELF OVER ITS FOOD LIVING as it does at the bottom of muddy pools the animal As constantly coming into contact with all manner of fine organic debris and particles of mud. The indigestible mud is ignored, but the organic matter is ingested simply by the protoplasm flowing round it and engulfing it. The fact that a selection is exercised in the matter of what is and what is not taken in, points definitely to a sense of taste in Amoeba, however feeble this sense may be. Once inside the body, a rounded space forms about the food particle and digestive juices are poured into it from the body proper. When digestion is completed, the indigestible remains are got rid of by a very simple process; the Amoeba simply flows away from it.
An imitation of this simple animal can be made by mixing various fats and oils and allowing drops of the mixture to fall on a damp surface of glass. These artificial Amoebas move about in the same way as the living Amoebas and in appearance at all events are strikingly like them, but they cannot feed or reproduce themselves, and in this is demonstrated the fundamental difference between living and nonliving things : non-living things can never reproduce themselves. It is true that reproduction in Amoeba begins with an extremely simple process, a mere splitting in two of the original body to form two new individuals. It is no complicated union between two parents to produce an offspring, but a simple fission of the parental body. In other words, in this lowly animal we approach, theoretically, very close to true immortality since there is neither birth nor death in the sense used of higher animals.
ANIMALS MADE OF A SINGLE CELL
THE number of animals which, like Amoeba, have a body composed of a single cell only is legion. Many are found in fresh water, and an even greater number in the sea, where
the surface is often rendered phosphorescent for miles by the presence of vast shoals of Noctiluca, representing teeming millions of these animals. Some idea of the enormous quantities of unicellular animals that have inhabited the world in past times may be gained from the fact that many of the large masses of rock composing the earth’s crust are composed almost exclusively of the limy skeletons of Protozoa, as all one-celled animals are called. The Nummulitic limestone, extending in a wide belt across Northern Africa, from the stone of which the pyramids were built, is a case in point.
Even to-day extensive deposits of similar skeletons cover the bottom of the sea for thousands of square miles, and the sandy beaches at many points on our coasts and the coasts of other countries throughout the world are composed mainly of the shells of Foraminifera, a group of unicellular animals. These shells are of surprising beauty and diversity, and few more fascinating diversions can be found than to examine a
handful of sand under a strong hand-lens or under the microscope and to see the beautiful shells of Foraminifcra among the sand-grains.
To a lesser extent, we may gain some idea of the infinite variety and the vast numbers of the Protozoa in the world by simply taking a dipping of water from practically any pond or stream, or, for the matter of that, from any stagnant pool or water butt, and examining a drop of it under a microscope. In it we shall see numerous small animals, some spherical, some oval, others pointed at each end. Some are crawling about with a slug-like motion, others swimming round and round, propelled by a coating of fine protoplasmic hairs covering the body, others moving forward in a straight line drawn along by a vortex created by the lashing of a single long, whip-like thread of protoplasm. In fact, a single drop of such water is as abundantly peopled with these unicellular animals as any garden is with insects.
Not the least important of the Protozoa are those that have forsaken a free existence for one parasitic in the bodies of other animals and of plants. Some of them do little damage to their hosts, but others are responsible for some of the most damaging and widespread diseases known to us. Dysentery, for example, is caused by the presence in the intestine of hosts of unicellular animals very closely related to Amoeba. Malaria is due to the invasion of the blood cells by Protozoa that periodically set up a poisoning of the system, while sleeping sickness is the result of an infection of Try-panosomes, another group of Protozoa.