Animal Kingdom

Animal Kingdom, a term of comparatively recent introduction, the exact extent of which it is impossible to define, and for which it would be well to substitute the term "organic kingdom" - embracing all organisms, animal or vegetable, as distinct from the inorganic world. The Linnaean aphorism, "Stones grow; plants grow and live; animals grow, live, and feel," is ambiguous; for, as Professor Huxley points out, "the word grow, as applied to stones (i.e. minerals), denotes a totally different process from what is called growth in plants and animals." The growth of minerals is effected purely by the external addition of new matter, as may be observed in crystals; the growth of the other two is the result of a process of molecular intussusception - the interposition of new molecules between those already existing - to such an extent that the process of reconstruction is more rapid than that of disintegration. Then the chemical constitution of living matter, which, in its primary unmodified state, is known as protoplasm (q.v.), distinguishes it absolutely from all other kinds of things, and the present state of knowledge furnishes us with no link between the living and the not-living. Moreover, an individual living body is constantly changing its substance by waste and reconstruction, and its size and form undergo continual modifications, ending in decay and death; while the perpetuation of the species is secured by the detachment of portions that tend to run through the same cycle as the parent form. Thus it is easy to distinguish animals and plants from inorganic bodies. One of the results of modern biology is the conviction that there is essential unity between all living organisms; and traced down to their lowest terms the series of plant forms gradually lose more and more of their distinctive features, while the series of animal forms part with more and more of their distinctive animal characters, and the two converge to a common term. Professor Jeffrey Bell thus enumerates the points of differences between animals and plants: -

1. The form of an animal is oblong and rounded; that of a plant diffuse and arborescent.

2. An animal requires albuminoid foods; a plant lives ou carbonic and mineral salts.

3. In all but the lowest animals there is a distinct mouth; plants take in food by the porous tissues.

4. Some of the waste products of an animal always contain nitrogen; the secretions of a plant are non-nitrogenous.

5. Animals are locomotive; plants are fixed.

6. The wall of an animal cell is derived directly from the cell protoplasm; the cell-wall of plants is formed by cellulose.

To nearly all these statements, however, exceptions may be found.

1. Polyps are arborescent or diffuse; cacti and fungi are not.

2. Fungi appear to require a more complex compound than carbonic acid and mineral salts.

4. Though plants do not give off nitrogenous excreta, their protoplasm is capable of forming them.

5. Polyps and many of the stalked Echinodermata are fixed: Volvox (q.v.) is locomotive.

6. The Cilio-flagellata have cellulose in the cell-wall, while some of the lowest plants have their protoplasm naked.

This list - imperfect as it is - will serve to show the broad general characteristics of animals and plants; but it must be borne in mind that sensibility appears not to be an exclusive animal characteristic, and that some (the sun-dews and Venus's fly-trap) have the power to absorb and digest animal matter. For forms which stand as it were upon the border of these two groups of organisms, it has been proposed by Hackel to erect a third group, Protista (q.v.). The classification adopted in this book is as follows: -

Sub-Kingdom 1. - Protozoa.

Sub-Kingdom II. - Mctazoa.

  (a) Coelenterata.

  (b) Coelomata.

    1. Echinodermata.

    2. Vermes.

    3. Arthropoda.

    4. Molluscoida.

    5. Mollusca.

    6. Chordata.