CHAPTER III

CLASSIFICATION, OR THE SORTING OF THE ANIMAL KINGDOM

Give a child a few handfuls of shells. Probably the first thing he will do with them is to sort out the various kinds and separate them from one another. Each will go into a little heap by itself; and next, our young friend will find names for them. These are Cap-shells and those Sword-shells; these Saucers and those Plates; these Yellow-shells and those Pink-shells--according as some special character or form or colour strikes his fancy.

Now this is what zoologists have been doing with the animal kingdom from the earliest days of science; trying to recognise each distinct kind of animal form, and to give it a name of its own. Unfortunately for the reader, zoologists have been obliged to choose names of Latin and Greek origin, and therefore in writing about animals we are often obliged to burden our pages with long words. This is a disadvantage, but it is a very slight one compared with the great advantage gained by using the learned tongues, which consists in this, that learned men from all countries of the globe can equally understand the names thus brought into use. One particular kind of creature may have one name in English, another in French, another in German, and so on; but the learned world does not trouble itself with this multiplicity of names--it gives the creature a couple of names in Latin, and these names stand good for learned readers in every part of the globe. The importance of this will be fully realised when, in a later page, we shall have to speak of the work done by zoologists, and the way in which they do it. Meantime we must ask our readers to have patience if now and then some long names must be used. These learned names sometimes convey a description of some important characteristic possessed by the animal, and sometimes they are merely fanciful names, such as the child we have spoken of gives to his zoological playthings. It does not greatly matter whether the name is descriptive or not; zoologists describe each animal kind in its most minute details, and the most commonplace or inappropriate name serves its purpose quite efficiently as a means of referring to published descriptions.

We have spoken of sorting the animal kingdom into its various kinds. But how do we know when a number of animals are all of one kind? No two individual animals are ever exactly alike, any more than two persons are ever exactly alike. "It is a matter of common observation that no two individuals of a species are ever exactly alike; two tabby cats, for instance, however they may resemble one another in the general characters of their colour and markings, invariably present differences in detail by which they can be readily distinguished. _Individual variations_ of this kind are of universal occurrence" (T. J. Parker).

Among a host of animals that present so many differences, how do we determine what shall be considered as belonging to one and the same kind? This is a point that nature usually settles thus. If two varieties when mated produce offspring which are perfectly fertile when mated again with another set of offspring similarly produced, then the two varieties, however differing in appearance, belong to one species. If on the other hand, the two belong to a different species, the offspring will be what is called a mule or hybrid, and will not produce offspring if mated with another mule. One of the most familiar examples of a mule is the animal, commonly so-called, which results from mating a horse and an ass, and partakes of the characteristics of both.

Every animal receives two Latin or Latinised names, the first that of the genus, the second that of the species; this system of naming, often referred to as the "binary nomenclature," we owe to the industry of Linnaeus the great Swedish botanist and zoologist. Genera are groups consisting of a number of different species which closely resemble one another. Similarly genera, which are somewhat alike, are again formed into larger groups, and so on. The names of families, orders, and classes used to be given to these groups in ascending order; but it is now generally recognised that such names are arbitrary, and that the divisions into which animals may naturally be grouped are altogether irregular, and not comparable with one another. Those who know a little of botany will readily understand, from their knowledge of wild flowers, that natural groups cannot be arranged in a formal series.

The main branches of the animal kingdom, the largest groups of all, used formerly to be called sub-kingdoms. Now the main divisions are often spoken of as phyla or races. Classifications, although they differ much in detail, according to the preferences of individual zoologists, yet agree as to the main branches of the animal kingdom, the chief of these are:--

1. The Protozoa, or One-celled Animals. 2. The Coelenterata or Two-layered Animals. 3. The Sponges or Porifera. 4. The Vermes or Worms. 5. The Arthropods or Jointed Animals, viz., Insects and Crustacea. 6. The Mollusca or Shell-fish. 7. The Brachiopoda or Lamp-Shells. 8. The Bryozoa or Moss-Corals. 9. The Echinodermata or Sea-Urchins. 10. The Chordata, including--(_a_) the Hemichordata; (_b_) the Ascidians; (_c_) the Vertebrata.

Within recent years an attempt has been made to express the relationship these groups bear to one another, by placing them in separate divisions or grades. The first grade includes only the Protozoa, or unicellular animals. The position of second grade has been assigned to the Coelenterata or diploblastic animals, whose bodies consist typically of two layers of cells. A third grade includes only a few groups of the lower worms, among which three body-layers may be distinguished, but no body-cavity is present. While the fourth grade, including practically the rest of the animal kingdom, have three body-layers (see p. 38), and a body-cavity surrounding the internal organs (see p. 38).

This arrangement of groups is an extremely convenient one; all the more convenient because it easily admits of modification. Already, indeed, we might find room for a grade intermediate between I. and II., consisting of what might be termed monoblastic animals, namely, animals consisting of a single layer of cells. For the frequent occurrence of Larvæ of this kind, consisting of a hollow ball of cells, renders zoologists on the alert to find a grown-up organism built in the same way. It is doubtful whether any of the forms that have been supposed to answer to this description really do so. Certain forms of these often claimed as plants by the botanists are, however, in the meanwhile, invited in to fill the blank.

There are also animals in which the internal layer of the body is very much reduced, consisting sometimes in fact of one cell only. Those are the Dicyemidæ and Orthonectidæ, both of them parasitic forms. They differ so completely from all other forms that it has been proposed to make for them a special group, the Mesozoa, or Midway animals, between the Protozoa and all the rest of the animal kingdom. It is, however, possible to group them under the head of Diploblastic animals; but nothing more different from the Coelenterata could well be imagined, and some regard them as a degraded form of worm.

The animals which are higher in structure than the Protozoa, viz. our divisions 2 to 10, are often grouped under the name Metazoa. The Metazoa thus include Grades II., III., and IV.

The meaning of the division of the animal kingdom into grades will be more apparent if we give an example of each.

GRADE I. _The One-Celled Animals._--_Amoeba_, the Mobile animal, is the typical example of these. It consists of a single microscopic cell. In this cell is seen a dark irregular speck, the nucleus, which is an essential character of cells, whether they are independent or form part of the body of a larger animal. There is often visible also a clear rounded space, called the "contractile vacuole," which squeezes out fluid, disappears, and reappears again, serving the purpose of excretion. The cell-substance, called protoplasm, is practically identical in this and in cells of all other kinds. It is jelly-like, and capable of a slow movement, which may be watched under the microscope. It suggests the flowing of treacle or thick gum. The movement may be traced by the change in outline of the cell and by the change in position of any granules that it may have taken in; for particles which touch the creature sink in and are surrounded; thus it obtains its food. These slow flowing movements of the protoplasm result in continual changes of shape; hence the name, Amoeba, the mobile animal. Sometimes the island of protoplasm, as it changes its shape, throws out, as it were, capes and headlands. These projections, which are presently drawn in again, are called pseudopodia or false feet. They are characteristic of the whole group of Amoeba-like animals, which are consequently called Rhizopoda, the root-footed. The production of new individuals is accomplished by the division of the old cell into two. Thus it may be said that there is always a bit of the old cell remaining, though divided into fragments; and for this reason the Amoeba-like Protozoans have been fancifully called "immortal."

GRADE II. _The Two-layered, or Diploblastic Animals._--The type of these usually chosen is _Hydra_, a two-layered animal, which is further described on p. 54. A section through Hydra (Fig. 4) shows (1) the outer or skin layer of cells, called the ectoderm, and (2) the inner or stomach layer of cells, called the endoderm (literally outer skin and inner skin). The clear recognition of the primary body-layers of the simpler invertebrates as identical with the primary body-layers of the embryo of higher forms, is largely owing to the teaching of Professor Huxley, the importance of whose work on this and in many other respects, is little guessed at by many readers who know his name merely as a popular exponent of scientific ideas. The two-layered body of Hydra encloses a hollow digestive space; from this the Coelenterata receive their name, which means "possessing a hollow space only, by way of intestines." The name of Acoelomata, animals without a body-cavity, has therefore been given to the Coelenterata and sponges. The meaning of the term body-cavity will be explained in the next paragraph but one. The Hydra, like all animals of its grade, and all those of the succeeding grades, reproduces itself by means of ova or egg-cells, and spermatozoa which fertilize them.

GRADE III. _The Triploblastic Animals without Body-Cavity._--This is a small section including only some of the lowest worms, such as the forms called Planarians. Between the Ectoderm and Endoderm lies an intermediate layer the Mesoderm. There are the beginnings of this in the Coelenterata and Sponges, but here it is further established. It includes a very thick layer of muscles.

GRADE IV. _The Coelomata or Triploblastic Animals with a Body-Cavity._--This grade includes all the remainder of the animal kingdom. As an example of it, we may take the Common Frog. If we open from the lower surface the dead body of a frog, we first cut through the skin, next the muscles; then we come to the viscera, lying neatly packed in a cavity from which we can dislodge them. This cavity is the Body-Cavity. The skin corresponds with the ectoderm of Hydra, although it is a vastly more complicated affair. The glandular lining of the alimentary canal corresponds with the endoderm of Hydra; although this, too, is a more complicated affair. The mass of the body, lying between these two layers, is considered to correspond somewhat with the mesoderm of Grade III., and has received the collective term of Mesoblast. This description applies equally to the earthworm, for the higher worms differ immensely from the lower worms, and stand on a level with more important members of the animal kingdom (see Fig. 41, p. 139). The body-cavity may be formed in different ways in different animal groups; but there is reason to believe that in certain cases it originates by a folding off of part of an original cavity corresponding with that of Hydra; so that part went to form the intestine, and part the cavity surrounding it.

The above arrangement of the main great groups of animals into four grades is that given by Professor Arnold Lang.

It should be added, that there are a few exceptional forms that present a departure from these broad rules of structure. They are, however, so few that they need only be named as curiosities. For instance, there are parasites in which the inner body-layer is practically done away with, because they are fitted to absorb food through the outer layer. And in one division of the Moss-Corals there is no body-cavity to be seen, although it is to be found in the other division.

What is the outcome of all this sorting of the animal kingdom? This most important result: that a classification of the animal kingdom into the four grades we have named, presents, in serial order, the stages through which young animals of the higher forms pass in the course of their growth. Every creature begins as a unicellular organism--the fertilised egg-cell. A vast number of creatures belonging to the higher groups present, later on, a two-layered condition, comparable with that of Grade II. Later on they acquire a third layer, and therefore correspond with Grade III. By degrees the body-cavity is formed, and they then present the adult body-structure of Grade IV. The development of the chicken in the egg, for instance, presents these four stages.

It will be sufficiently apparent that this coincidence is too striking to be without a meaning. Zoologists are all agreed in their interpretation of this meaning: it is, that the history of the individual presents a summary of the history of the race, and goes through the stages of structure which its ancestors presented in their adult forms. The story of the gradual upward struggle of the animal kingdom, from its humble beginnings to its present wonderful complexity, is written in the growing tissues of every young creature.

The principle that ancestral traits betray themselves is accepted as a truism in common life. Do we see young people rude and stupid? We say, perhaps, "No wonder; their grandfather was a drunken, worthless lout." Do we see a family of the poorest class clever, and industrious, and refined? We say, "They come of a good stock." When we speak in this way, we reason from the common experience of mankind, that children resemble their ancestors. Similarly, when zoologists find an embryo starting its existence from one cell, they say, "No wonder; its ancestors were unicellular." And when they find it assuming a two-layered form, they say, "Its ancestors were two-layered creatures." So certain are zoologists of the existence of an ancestral two-layered form, the parent at once of the existing Coelenterata and of the higher forms, that Professor Hæckel has given it a special name--Gastræa. The two-layered young stage of higher creatures, when it has a free-swimming existence, is called a Gastrula (Fig. 6). Both names, meaning stomach-animal, refer to the structure, which is, in a still simpler form, that of _Hydra_--a two-layered bag of cells, of which the inner layer, lining the cavity, performs the work of digestion. The lowest of the Vertebrata, the Lancelet (see p. 140), has a larva of this kind. The same reasoning which suggests the existence of an ancestral Gastræa-animal, suggests that of an ancestral Planula-animal; for the two-layered animals, on their part, present us with a monoblastic larva of the form already described (p. 34), called a Planula. Hence it is that zoologists look with such eagerness for forms, of which it can be said that they consist of one layer of cells only. The name Planula signifies "wandering animal," because the Planula larva swims about by means of cilia.

Mention has been made above of larval forms. It is perhaps advisable to explain clearly what is meant by this term. It is a matter of every-day knowledge that in some animals the young form presents an appearance and structure very different from that of the grown-up form, and adapted for a different mode of life; the commonest instances are the caterpillar of the butterfly and the tadpole of the frog. We are apt to think of these creatures as somewhat exceptional in this respect. But the zoologist, in viewing the whole range of the animal kingdom, finds a vast number of animals with larvæ, differing much from the adult, and adapted for a different mode of life. It is, in fact, a very common arrangement; but often these larvæ are very minute, perhaps absolutely microscopic, therefore only known to the scientific observer. The two familiar instances we have named are fortunately big enough to be known to everyone. Now it is an axiom with modern zoologists (as has been explained above), that the history of the individual is a summary of the history of its ancestors; larval forms are therefore of special interest in this connection. A very wide-spread form of larva, more advanced in its structure than the little Gastrula that has been already named, has received the name of Trochosphere or Wheel-ball (Fig. 7), because it swims round and round, by means of cilia, usually distributed in bands. Its inner or stomach-layer, forms a definite alimentary canal, and is separated by a very simple mesoderm from the outside ciliated layer, which presents certain differences in form, according as the creature belongs to one group of animals or to another. The main characters of the Trochosphere are, however, the same in very widely differing groups. These little larvæ give rise to one of the most eagerly debated problems of zoology. Are we to suppose that animals which possess a Trochosphere larva are all descended from one common ancestor? Or are we to think that the Trochosphere is a form of body very convenient for the necessities of juvenile existence in the sea, and therefore independently evolved by animals which are not directly related to each other? Some authorities take the latter view; the former is perhaps more widely accepted, and has even been expressed by the application of the name Trochophora (Wheel-carriers), as a general term for those groups in which such larvæ are found. These include some of the higher worms, which present the typical Trochosphere, the Brachiopoda, and the Polyzoa; while variations of the Trochosphere type are shown by the earliest larvæ of Mollusca, the larvæ of the Echinoderms, and those of the Hemichordata (see p. 33), the latter bringing us, as it were, within eye-shot of the Vertebrata themselves. It will be seen, therefore, that the range of the Trochosphere larva covers a large portion of the ground occupied by our Grade IV. There is, however, one marked exception: the Arthropoda, which seem to have a prejudice against cilia in any form (since they include but one animal which possess any) have no example of a ciliated larva. Even their simplest larval forms belong to a higher type of structure, in which the shelly, jointed structure characteristic of the group is already indicated.

When we speak, however, of the occurrence of the Trochosphere throughout a wide range of animal life, it must be understood that its presence is not necessarily uniform throughout a group in which it occurs. Larval forms are adaptations which conform with the conditions of life for the particular animal in question: and nearly related kinds of animal may be without a larva. The Trochosphere larva is, of course, only adapted for aquatic existence, and is necessarily absent in the case of terrestrial forms.

TABLE OF THE CLASSIFICATION OF THE ANIMAL KINGDOM[A]

=Grade I.=--UNICELLULAR ANIMALS. PROTOZOA.

(Intermediate forms, see p. 34.)

=Grade II.=--TWO-LAYERED { SPONGES. ANIMALS. { COELENTERATA.

=Grade III.=--THREE-LAYERED { PLATYHELMINTHES, OR FLAT-WORMS. ANIMALS. { VERMES, THE HIGHER FORMS.

{ ARTHROPODA. { MOLLUSCA. =Grade IV.=--COELOMATA, OR { BRACHIOPODA. THREE-LAYERED ANIMALS WITH A { BRYOZOA. BODY-CAVITY. { ECHINODERMATA. { TUNICATA OR ASCIDIANS. } =Chordata.= { VERTEBRATA. }

[A] In the subsequent tables which show the respective sub-divisions of these chief groups, the larger only of the sub-divisions are named.

When an animal has no free larva, but quits the egg in a form practically identical with that of the adult, the development is said to be "direct." But changes equally startling with those displayed when a larva develops into the adult form, may take place while the young animal is enclosed within the egg itself. To these also zoologists apply the axiom referred to above, that the history of the individual summarises the history of the race. Thus, for example, the Amphibian larva, _e.g._ the tadpole of a frog (p. 153) has gills, which disappear in the adult form: the young reptile, bird, or mammal, which has no larval stage, has gills during a comparatively early stage; and loses them at a later period of its development. In each case zoologists conclude that the animal is descended from a fish-like ancestor, which possessed gills all its life, and that the more immediate ancestors in the family tree, have lost their gills by degrees.

The study of the progressive changes of young forms, whether larval, or enclosed within the egg, is called Embryology, and constitutes, in these days, the major branch of zoological science. That it is of paramount importance to the student of classification, engaged upon the sorting of the animal kingdom, will be apparent from what has been stated above.