Agricultural zoology

I. Subdivision of the Animal Kingdom.

There are animals so like one another that they are given the same name. Such animals are ranked in the same species. Animals which differ so much that they have to be referred to different species, but which notwithstanding agree in the majority of their characters, especially the most important ones, are placed in the same genus. Hare and rabbit, or horse and donkey, are reckoned as different species of the same genus. Genera resembling one another are united into a family; thus, the pine marten and the beech or stone marten both belong to the Marten genus (Martes), while the weasel and stoat are different species of the Weasel genus (Mustela); but these two genera are so similar that they are both placed in the same family, i.e. the Weasel family (Mustelidæ). Nearly related families together build up an order. Thus, the Weasel family, Dog family, Cat family, etc., collectively constitute the order of Carnivora, characterized, speaking generally, by the same kind of teeth, claws, habits, and food. Several related orders are united into a class. Thus, for example, carnivorous animals (Carnivora), ruminating animals (Ruminantia), gnawing animals (Rodentia), etc., constitute different orders of the class of Sucklers (Mammalia); while birds of prey (Raptores), pigeons (Gyrantes), and poultry (Rasores), are included in a second class, that of Birds (Aves). But both Birds and Mammals have a skeleton, of which the chief support is the backbone; on this account they are placed in a larger subdivision, the sub-kingdom of Backboned animals (Vertebrata); while snails are grouped under the sub-kingdom of Molluscs, millipedes and centipedes under that of Jointed-limbed animals (Arthropods).

In this way the animal kingdom is divided into sub-kingdoms, the sub-kingdoms into classes, the classes into orders, the orders into families, the families into genera, and the genera into species. Animals of the same species which differ from one another in more or less constant characters, belong to different races (domestic or geographical races).

There are many species of animals the external features of which are well known to ordinary folk, and which therefore possess a definite English name, but a much larger number, of the smaller forms especially, have no English name. It is, therefore, necessary to devise new names for these species. The English names, however, are liable to cause great confusion, since in different districts the same name is often applied to widely different animals. Besides this, distinct names have usually been given to successive stages in the life history of the same form: “wireworms,” for example, are the young state of the “click beetle.”

By using the scientific method of naming invented by Linnæus, confusion is made impossible. The Latin names of this naturalist have the great advantage that they not only give a perfectly distinct name to any particular species, but also at the same time show the genus to which it belongs. Each kind of animal possesses, in fact, two names; just in the same way as every person possesses at least two names, a Christian name and a surname. The generic name comes first, and is, of course, common to all animals of the same genus. The second name is the specific one, and belongs exclusively to animals of the same species. The hare and rabbit, for example, are both included in the genus Lepus. The Latin name of the first is Lepus timidus; that of the second, Lepus cuniculus. Horse = Equus caballus; ass = Equus asinus.

II. Review of the Structure and Vital Phenomena of Animals.

I select as a point of departure the human body, and the bodies of domestic animals, because my readers are best acquainted with these.

The limbs consist, beginning on the outside, of skin, flesh, and bone. The same parts can also be distinguished in the head, neck, and trunk; but in these divisions of the body they enclose a cavity, the body-cavity, which, again, contains various parts (“organs”), which are not everywhere attached to the body-wall. Fig. 1 represents a longitudinal section through the body. The skin is represented by a line, flesh and internal lining are shaded, while the bones are black. These parts form together the body-wall. In front the body-wall encloses a cavity, the body-cavity (Kh.), which in Mammals is divided into two sections (thoracic cavity, Bz.h, and abdominal cavity, B.h.) by the midriff (diaphragm). In the thoracic cavity are found the lungs and heart (H), also most of the gullet or upper part of the gut; the abdominal cavity contains the remainder of the often much-coiled gut, which in one place widens into the stomach (M), also the kidneys, spleen, and parts connected with the gut (e.g. the liver). The cavities are bounded behind by the backbone (vertebral column), which is made up of many flattened vertebræ. The uppermost vertebra supports the skull, which encloses a cranial cavity (Sch.h.) continuous with a vertebral canal bounded by the vertebræ. Cranial cavity and vertebral canal form together a second body-space, in which are contained the brain and spinal cord.

We will now consider the individual parts of the body, beginning with the skeleton. The axis of the skeleton is formed by the vertebral column (spine), which is composed of flat bones, the vertebræ. A vertebra usually consists of (1) the body, which occupies the front; (2) the arch, which possesses several projections or processes (neural spine, transverse processes, articular processes) and encloses the vertebral canal (W.h.). All mammals have seven neck or cervical vertebræ (Fig. 2, 1); while the number of the remaining vertebræ varies according to the species. The cervical vertebræ, which support the head, are followed by the dorsal or thoracic vertebræ (12 in man, Fig. 2, 2), and these by the strong loin or lumbar vertebræ (5 in man, Fig. 2, 3). Cervical, thoracic, and lumbar vertebræ are movable, but, in man, the last-named are followed by five vertebræ immovably united together to make up the sacrum, and these again by tail- or caudal vertebræ. Man has four such vertebræ, all poorly developed, and fused with one another (Fig. 2, 5); but in many animals there are a large number, movably united to make up a tail.

The ribs, which in mammals bound the chest, are jointed to the thoracic vertebræ. Man has 12 pairs of ribs; each rib consists of a bony part behind and a gristly (cartilaginous) part in front. The so-called true ribs (Fig. 2, 14) [the upper pairs] are movably united with the breast-bone, but this is not the case with the false ribs (Fig. 2, 15).

In the head we distinguish the brain-case or cranium, and the skeleton of the face. The first contains the cranial cavity in which the brain is enclosed. We distinguish—2 frontal bones (fused together in man, Fig. 2, 6); 2 parietal bones (7); 2 temporal bones (8); an occipital bone (9) composed of several pieces fused together, perforated by the foramen magnum [where brain and spinal cord unite], and bearing two elevations or condyles [for effecting union with the backbone]; and the sphenoid and ethmoid bones which make up the base of the cranium. The facial skeleton consists of the framework of the jaws and palate, and, together with some of the cranial bones, bounds the cavities in which the eyes are contained (orbits), and the nasal cavities. It consists of the maxillary bones (Fig. 2, 12), the premaxillary bones (Fig. 3, 7,—in man these 4 bones are fused together into one piece), the nasal bones, the lachrymal bones, the ploughshare bone (vomer), the turbinated bones, the cheek-bones (or malars, Fig. 2, 11), the palate-bones, and the lower jaw (Fig. 2, 13). (The last originally consists of two symmetrical halves.)

The upper and lower limbs are built on the same type, and therefore consist of corresponding parts (cp. Fig. 2). The more similar the functions of the two pairs, the closer their resemblance. In the ox they are much more alike than in man; in the bird, on the contrary, the similarity is much less. A distinction can be drawn in both limbs between the bony girdles (shoulder-girdle and hip-girdle), which serve for union with the trunk-skeleton, and the different subdivisions of the limbs themselves. I place side by side the parts of the arm and leg of man.

The differences between arm and leg are explained by their different uses. The bones of the leg, used to support the human body, are firmer and thicker, but less movable than those of the arm, which is employed in grasping. Consequently the union between the hip-girdle and the trunk-skeleton is firmer than that of the shoulder-girdle. The radius can rotate upon the ulna, so as to completely turn the hand over; a similar twisting of the foot would not be of use, and cannot be effected. The leg has a knee-pan (patella) (Fig. 31), with which there is no bone in the arm to correspond. In the foot the toes are short, and the remaining parts long; for instance, one of the tarsal bones, the calcaneum (heel-bone), is strongly developed and projects behind (28*). In the hand, the digits are relatively long, and since the tip of the thumb can be made to touch the tips of all the fingers, are admirably adapted for grasping.

The number of fingers or toes is at most five, but may be less. The horse has a single digit to each limb; the ox, two well developed and two remaining as rudiments; the pig, two large and two small; while the dog has four toes in the hind foot, five in the fore foot.

Man walks on the sole of the foot. Some other animals (dog, cat) on the toes; others again (horse, ox, pig), on the tips of the toes. In the last case there is not simply a horny structure (nail or claw) on the upper side of the toe, but a hoof sheathing the whole of its tip. In many animals the thigh and upper arm are drawn close up to the body, so that the limbs appear quite different from those of man. (Compare Fig. 2 with Fig. 3.)

The bones are usually surrounded by flesh. This consists of a number of different pieces united together by a delicate, elastic, fibrous mass (connective tissue). The different pieces are termed muscles, each of which is again made up of a large number of muscle-fibres, all taking a longitudinal direction. Each fibre can contract, and a muscle becomes shorter and thicker by simultaneous contraction of all its fibres. The contraction and subsequent relaxation of muscles move other parts. There are some muscles, the hollow muscles, which surround a cavity, and by their contraction propel the liquid or solid substances found in their cavity. The heart, for example, is a large muscle of this sort, serving to propel the blood, while the hollow muscular coat of the gut moves on the contained food. Other muscles are fixed by their ends to other parts of the body, which they move by their contraction. We distinguish between dermal muscles and skeletal muscles, attached respectively to the skin or by one end to an integumentary structure (hair, feather, scale), and to parts of the skeleton. The animals which are devoid of any internal skeleton, the invertebrates (i.e. all animals except vertebrates), naturally possess no skeletal muscles. Examples of dermal muscles are those by means of which a bird erects its feathers (tail-coverts of peacock!), and those which enable a hedgehog to roll itself into a ball and stick out its spines. Each end of a skeletal muscle is connected with a bone. If such a muscle contracts the more easily movable bone is drawn towards the less easily movable one (Fig. 4). In order that the bones may be movable upon one another they are united together by joints.

According as muscular movements are, or are not, under the influence of the will, they are distinguished as voluntary and involuntary. To the latter kind belong the movement of the heart, and the movements of the muscles in the wall of the gut by which the food is made to progress.

To destroy the contractile power of a muscle it is not necessary to injure the muscle itself. Every muscle is related to a nerve, which sends its fine branches to the fibres making up the muscle. If we cut the nerve, the corresponding muscle loses its power of contraction. But the nerve arises from the central nervous system, which in vertebrates principally consists of the brain and spinal cord. The muscle will therefore lose its contractile power if the connection with these central parts is broken. The true cause of movement resides in these parts. A sort of change, the essential nature of which is unknown to us, takes place in them, and is propagated along the nerve to the muscle, causing it to contract. The central nervous system is, therefore, the origin, the centre from which the order to contract proceeds; hence its name. The nerves which run from these central parts to the muscles are known as the nerves of movement (motor nerves).

There is still, however, a second group of nerves, the nerves of sensation (sensory nerves), which arise in the sense-organs (skin, mucous membrane of tongue, nose, ear, eye), and convey to the central nervous system the impressions they receive from the outer world by the aid of these sense-organs. In the appended diagram (Fig. 5), C represents the central nervous system; B.N., a motor nerve, branching in the muscle M; G.N., a sensory nerve, which runs from the blood-bathed inner skin or dermis (L.h.), underlying the outer skin or epidermis (O.h.), to the central system. (The arrows indicate the direction in which impulses are conveyed along the corresponding nerve.)

Men or animals lose in weight if they take no food. The reason for this is that certain substances leave the body either as gases (through the lungs), or as liquids (by the kidneys and sweat-glands), without a corresponding compensation. An animal or human being could not live without taking in fresh substances, which, according as they are solid or liquid, are known as food or drink. The different kinds of food and drink, which, with few exceptions (salts, water), are taken from the animal and plant kingdoms, cannot, however, as such, replace the gradually diminishing body substance, for, to begin with, they contain useless matters, which pass out of the body in the fæces (dung). And even the nutritious parts of the animal and vegetable substances taken into the stomach, are not always in a form in which they can be used at once. Digestion, which in all the higher animals takes place in a food-tube (gut), serves to reduce them to a suitable condition, at the same time separating the useless matters. The action of several fluids (saliva, gastric juice, bile, etc.) secreted by glands, extracts the useful (nutritious) substances from the food and drink, converting them also into a suitable form. The smaller the pieces into which the food is separated, the better can this purpose be effected. In mammals the teeth serve to break down the food; in birds and many Invertebrates the same part is played by special secretions of the stomach or intestine provided with hard ridges.

So long as the nutritious food-stuffs remain in the food-canal, even though in a completely suitable form, they cannot nourish the body. And since waste of the substance of the body everywhere takes place, it is absolutely necessary that the food-stuffs should pass after digestion into a system of organs going to all parts of the body. This system is the circulatory, or vascular system. Food-stuffs enter it from the gut directly or indirectly, reaching it in the latter case through the lymphatic (lacteal) system.

The blood is the fluid into which the food-stuffs are taken up. It consists of an almost colourless liquid, together with an innumerable number of exceedingly minute blood-corpuscles.

The blood flows through the body in a system of tubes, or blood-vessels, which branch repeatedly, and at last become merged in the microscopic capillary blood-vessels. These capillaries are present in nearly all parts of the body except the epidermis and epidermal structures (hairs, feathers, scales, etc.). They have exceedingly thin walls, which present no resistance to the passage of the nutritious substances contained in the blood, so that these can be absorbed by those parts of the body which lie between the individual capillary vessels. The central organ of the circulation is the heart, an enlarged part of the vascular system, possessing thick muscular walls. By contraction of these, the blood is driven out of the heart (Fig. 6, H); and its exit is possible on one side only (a), as at the other side (b) there is a valve, which closes when the heart contracts. The vessel into which the blood leaving the heart enters is termed an artery (S.A.) It divides into several branches, also known as arteries, and the smallest arteries pass into capillaries, which again are connected with veins, which join larger and larger veins, until finally one or a few open into the heart (A).

Since the blood in the course of its circulation gives up some of its nutriment to the various parts of the body, it would in the end become useless for the purposes of nutrition if it did not receive a fresh supply of food-stuffs from the gut, either directly or indirectly (through the lacteal system). But apart from this, the blood would ultimately become useless, and that very quickly, if it did not traverse the lungs, kidneys, and sweat-glands. It is well known to every one that a man or animal cannot live without air, or at any rate without a certain gas, oxygen, that is contained in air. This oxygen must be able to penetrate into the minutest particles of the body, and the blood, in the corpuscles of which it is contained, carries it everywhere. In the smallest particles (molecules) of the body an oxidation (combustion) of body substance takes place, which not only causes an evolution of heat, but also renders the body capable of doing work. But if now the blood passes from the capillaries into the veins, it contains too little oxygen. And besides, it has taken up from the molecules of the body several substances, developed in those molecules, which would be fatal to the animal if they were not removed from the body. Now, when the blood streams through the lungs, it gets rid of the poisonous gaseous matter, and when it traverses the kidneys and sweat-glands it parts with the injurious liquid and solid substances. But in the lungs the blood takes up at the same time fresh oxygen; and since in this way the air in the lungs becomes poor in oxygen, the movements of breathing (respiration) provide for the passage of a fresh supply of oxygen into the lungs. Only the higher Vertebrates breathe by means of lungs; fishes and numerous aquatic Invertebrates breathe by gills, and insects by air-tubes (tracheæ).

While Nutrition is the life-process which shields the individual from death, Reproduction serves to maintain the species. It is familiarly known that the offspring generally resemble their parents. But it is also a fact recognized by the stock-breeder, that a particular animal will not only transmit several of its own characteristics to its offspring, but perhaps also various characteristics of the grandparents or of animals belonging to still more remote generations, although these characteristics are not visible in the animal which is actually breeding (Reversion, Atavism). Among insects and the lower animals there are species which, as adult animals, appear not in one form, but two or several. In this case, as a regular thing, the offspring does not resemble the parents, but the grandparents, great-grandparents, or a still earlier generation. The older observers have placed the offspring and the parents, and sometimes the grandparents too, of the same animal species in different species, or even genera or families, until newer researches on the reproduction and development of these animals have proved them to belong to one and the same species (see Fig. 7 and explanation). The method of reproduction by which a species appears in two or several forms is distinguished as heterogeny and metagenesis, or alternation of generations. In the first (Fig. 7) sexually reproducing animals alternate with other sexual animals. It may be that these are of separate sexes, or else they may possess both male and female organs (hermaphrodite). In metagenesis a sexual generation regularly alternates with one or several generations reproducing asexually.

The animal kingdom falls (cf. p. 2) into sub-kingdoms or main divisions. Seven of these are commonly distinguished: I. Backboned animals; II. Jointed-limbed animals; III. Worms; IV. Molluscs; V. Echinoderms; VI. Cœlenterates; VII. Protozoa.