# Encyclopædia Britannica, Ninth Edition/Animal Kingdom

See also Animal on Wikipedia; Protozoa and Animal on Wikipedia, Animal and Zoology in the 11th Edition; and the disclaimer.
ANIMAL KINGDOM
​ANIMALS, Classification of. The object of classification is to bring together those things which are like, and to separate those which are unlike. Each science has its own classification of the objects with which it deals, the kinds- of likeness and unlikeness according to which these objects are grouped varying in relation to the special qualities or properties of matter with which the science is concerned. Thus, the physicist classifies bodies according to their mechanical, electrical, thermic, or other physical properties ; the chemist, regards their composition ; while the zoologist and the botanist group them according to their likenesses and unlikenesses of structure, function, and distribution. {{ti|1em|As soon as the labours of anatomists had extended over a sufficiently great variety of animals, it was found that they could be grouped into separate assemblages, the members of each of which, while varying more or less in minor respects, had certain structural features in common, and these common morphological characters became the definition of the group thus formed. The smallest group thus constituted is a Morphological Species. A certain number of species having characters in common, by which they resemble one another and differ from all other species, constitutes a Genus; a group of genera, similarly associated, constitutes a Family; a group of families, an Order; a group of orders, a Class; a group of classes, a Sub-Kingdom; while the latter, agreeing with one another only in the characters in which all animals agree, and in which they differ from all plants, make up the Animal Kingdom.
The formation of a morphological classification is therefore a logical process, the purpose of which is to throw the facts of structure into the smallest possible number of general propositions, which propositions constitute the definitions of the respective groups. A perfect classification will fulfil this end, and, in order to form it, two conditions are necessary : Firstly, we must have a full knowledge of the adult structure of every animal, recent and extinct ; secondly, we must know all the modifications of structure through which it has passed, in order to attain the adult condition, or in other words, the mode of development of the animal. For it is the sum of all the structural con ditions of an animal which constitutes the totality of its structure ; and if two animals, similar in their adult state, were unlike in their development, it is clear that the latter circumstance would have to be taken into account in deter mining their position in a classification.
Linnaeus, living at a time when neither comparative anatomy nor embryology can be said to have existed, based his classification of animals upon such broad resemblances of adult structure and habit as his remarkable sagacity and wide knowledge enabled him to detect. Cuvier and his school devoted themselves to the working out of adult structure, and the Lemons d Anatomie Comparee and the Regne Animal are wonderful embodiments of the results of such investigations. But the Cuvierian system ignores development ; and it was reserved for Von Baer to show the importance of developmental studies, and to inaugurate the marvellous series of researches which, in the course of the last fifty years, have made us acquainted with the manner of development of every important group of animals. The splendid researches of Cuvier gave birth to scientific palaeontology, and demonstrated that, in some cases, at any rate, extinct forms of life present characters intermediate between those of groups which are at present widely different. The investigations of Agassiz upon fossil fishes tended in the same direction, and further showed that, in some cases, the older forms preserve, as permanent features, structural characters which are embryonic and transitory in their living congeners. Moreover, Darwin, Owen, and Wallace proved that, in any great area of geographical distribution, the later tertiary extinct forms are clearly related to those which now exist in the area. As Taxonomic investigations increased in accuracy and in extent, the careful examination of large suites of specimens revealed an unexpected amount of variability in species ; and Darwin s investigation of the phenomena presented by animals under domestication proved that forms, morpholo gically as distinct as admitted natural genera, could be produced by selective breeding from a common stock.
Upon the foundation thus furnished, the doctrine of Evolution, first scientifically formulated by Lamarck, has been solidly built up by Darwin, and is now, with various modifications and qualifications, widely accepted. But the acceptance of this doctrine introduced a new element into Taxonomy. If all existing animals are the last terms of a long series of developmental stages, represented by the animals of earlier ages of the earth s history, the starting point of which has been a primordial form of the extremest simplicity consistent with animal life, then every animal has an "ancestral" as well as what may be termed a "personal" embryology ; and the same considerations which oblige the Taxonomist to take account of the latter phenomena, compel his attention to the former stages of development. Two animals belong to the same group, when they are similar in structure, personal development, and ancestral development, and not otherwise. Hence it follows that a perfect and final zoological classification cannot be made until we know all that is important con cerning 1, the adult structure ; 2, the personal develop ment ; and 3, the ancestral development of animals. It is hardly necessary to observe that our present knowledge, r.s regards even the first and second heads, is very imperfect ; while, as respects the third, it is utterly fragmentary.
The only genus of animals of which we possess a satisfactory, though still not quite complete, ancestral history, is the genus Equus, the development of which in the course of the Tertiary epoch from an Anchitherioid ancestor, through the form of llipparion, appears to admit of no doubt. And all the facts of geology and palaeontology not only tend to show that the knowledge of ancestral development is likely long to remain fragmentary, but lead us to doubt, whether even such fragments as may be vouchsafed to us by the extension of geological inquiry will ever be sufficiently old, in relation to the whole duration of life on the earth, to give us positive evidence of the nature of the earliest forms of animals.
While holding the doctrine of evolution to its fullest extent, and having no doubt that Taxonomy ought to be the expression of ancestral development, or pkyloyeny, as well as of embryogeny and adult structure, and while con ceiving that the attempts at founding a scientific phylo- geny, which have been made by Haeckel and others, are of much interest and importance as guides to and suggcs- tors of investigation, the present writer looks upon all such attempts as provisional hypotheses ; and he conceives that, at any rate for the present, it is a mistake to introduce considerations of this purely hypothetical kind into classi fication, which should be based on verifiable data.
mine its adult structure and its development, and there fore to assign its place relatively to other animals, the ​structure and development of which are also known ; and, in the case of an extinct animal, it is possible to ascertain certain facts of its structure, and sometimes certain facts of its development, which will justify a more or less positive assignment of its place relatively to existing animals. So far, Taxonomy is objective, capable of proof and disproof, and it should leave speculation aside, until speculation has
In the present rapidly shifting condition of our knowledge of the facts of animal structure and development, however, it is no easy matter to group these facts into general pro positions which shall express neither more nor less than is contained in the facts ; and no one can be more conscious of the manifold imperfections of the following attempt at such a classification than the author of it.
In certain of the lower animals, the substance of the body is not differentiated into histogenetic elements ; that is, into cells which, by their metamorphoses, give rise to tissues. In all other animals, on the other hand, the protoplasmic mass, which constitutes the primitive body, is converted into a multitude of cells, which become meta morphosed into the tissues of the body.
For the first of these divisions the old name of Protozoa may be retained ; for the second, the title of Metazoa, recently proposed by Haeckel, may be conveniently employed.
I. Protozoa.
II. Metazoa.
The subjoined synopsis indicates the general relations of the different groups of the Animal kingdom, in accordance with the views which have been put forward in the pre ceding pages.
Those who are familiar with the existing condition of our knowledge of animal morphology, will be aware that any such scheme must needs, at present, be tentative and subject to extensive revision, in correspondence with the advance of knowledge. Nor will they regard it as any objection to the scheme of classification proposed, that the divisions sketched out may be incapable of sharp definition the constant tendency of modern investigations being to break through all boundaries of groups, and to fill up the gaps between them by the discovery of transitional forms. In the place of assemblages of distinctly definable groups, which it has hitherto been the object of the Taxonomist to define and co-ordinate in precise logical categories, we are gradually learning to substitute series, in which all the modifications by which a fundamental form passes from lower to higher degrees of organic complication, are summed up.
(t. h. h.)
ANIMALIA.
I. PROTOZOA.
i. MONERA.
ProtamcebidiE. Protomonadidae. Myxastridre, &c.
ii. ENDOPLASTICA.
Amcebidse. Flagellata. Gregarinidse. Acinetida. Ciliata. Radiokria.
A. Gastraeae.
i. POLYSTOMATA.
Porifera (or Spongida).
ii. MONOSTOMATA.
1. ARCH.EOSTOMATA. a. Scolecimorpha.
Eotifera.
Turbellaria.
Trematoda.
Nematoidea.
2. DEUTEROSTOMATA. a. Schizocoela.
II. METAZOA.
Hirudinea. Oligochseta .
8. Coslenterata.
Hydrozoa. Actinozoa.
b. Enterocoela.
Annelida (Polychceta). Gephyrea.
Arthropoda. Mollusca.
B. Agastrsese (provisional).
Acanthocephala. Cestoidea.
Brachiopoda (?) Polyzoa (?)
Enteropneusta. Chcetognatha. EchinoderiKata.
c. Epicoela. Tunicata. Vertebrata.