MORPHOLOGICAL classification, then, acquires its highest importance as a statement of the empirical laws of the correlation of structures; and its value is in proportion to the precision and the comprehensiveness with which those laws, the definitions of the groups adopted in the classification, are stated. So that, in attempting to arrive at clear notions concerning classification, the first point is to ascertain whether any, and if so, what groups of animals can be established, the members of which shall be at once united together and separated from those of all other groups, by well-defined structural characters. And it will be most convenient to commence the inquiry with groups of that value which are commonly called CLASSES, and which are enumerated in an order and arrangement, the purpose of which will appear more fully by and by, in the following table.
It is not necessary for my purpose that the groups which are named on the preceding table should be absolutely and precisely equivalent one to another; it is sufficient that the sum of them is the whole of the Animal Kingdom, and that each of them embraces one of the principal types, or plans of modification, of animal form; so that, if we have a precise knowledge of that which constitutes the typical structure of each of these groups, we shall have, so far, an exhaustive knowledge of the Animal Kingdom.
I shall endeavour, then, to define --or, where definition is not yet possible, to describe a typical example of -- these various groups. Subsequently, I shall take up some of those further classificatory questions which are open to discussion; inquiring how far we can group these classes into larger assemblages, with definite and constant characters; and, on the other hand, how far the classes can be broken up into well-defined sub-classes and orders. But the essential matter, in the first place, is to be quite clear about the different classes, and to have a distinct knowledge of all the sharply-definable modifications of animal structure which are discernible in the animal kingdom.
These are among the simplest animal forms of which we have any knowledge. They are the inhabitants of the bodies, for the most part, of invertebrate, but also of vertebrate, animals; and they are commonly to be found in abundance in the alimentary canal of the common cockroach, and in earth-worms. They are all microscopic, and any one of them, leaving minor modifications aside, may be said to consist of a sac, composed of a more or less structureless, not very well-defined membrane, containing a soft semi-fluid substance, in the midst, or at one end, of which lies a delicate vesicle; in the centre of the latter is a more solid particle. (Fig. 1, A.) No doubt many persons will be struck with the close resemblance of the structure of this body to that which is possessed by an ovum.
You might take the more solid particle to be the representative of the germinal spot, and the vesicle to be that of the germinal vesicle; while the semi-fluid sarcodic contents might be regarded as the yolk, and the outer membrane as the vitelline membrane. I do not wish to strain the analogy too far, but it is, at any rate, interesting to observe this close morphological resemblance between one of the lowest of animals and that form in which all the higher animals commence their existence. It is a very remarkable characteristic of' this group, that there is no separation of the body into distinct layers, or into cellular elements. The Gregarinida are devoid of mouths and of digestive apparatus, living entirely by imbibition of the juices of the animal in whose intestine, or body cavity, they are contained. The most conspicuous of those phenomena, which we ordinarily regard as signs of life, which they exhibit, is a certain contraction and expansion along different diameters, the body slowly narrowing, and then lengthening, in various directions. Under certain circumstances (though the conditions of the change are not thoroughly understood), it is observed that one of these Gregarinida, whatever its form may be, will convert itself into a well-rounded sac, the outer membrane ceasing to exhibit any longer those movements of which I spoke, and becoming coated by a structureless investment, or "cyst" (Fig. 1, B.).
The substance of the body contained within the cyst next undergoes a singular change. The central nucleus and the vesicle disappear; after a time, the mass breaks up into a series of rounded portions and, then, each of those rounded portions elongates, and, becoming slightly pointed at each end, constitutes a little body which has been called a "Pseudo-navicella," from its resemblance to the Diatomaceous Navicula or Navicella (Fig. 1, C, D). Next, the capsule bursts and the Pseudo-navicellae (Fig. 1, E, F). are scattered and passed out of the body of the animal which they inhabit. Though, of course, a great number of them are destroyed, some, at any rate, are devoured by other animals; and, when that is the case, the little particle of protein substance which is enclosed within the Pseudonavicella is set free from its shell, and exhibits much more lively movements than before, thrusting out processes in various directions, and drawing them in again, and, in fact, closely resembling one of those animalcules which have been called Amoebae (Fig. H). The young Amoebiform Gregarina grows, increases in size, and at length assumes the structure which it had at first. That, in substance, is all that we know of this lowest division of animal life. But it will be observed, there is a hiatus in our knowledge. We cannot say that we know the whole nature and mode of existence of this, or any other animal, until we have traced it to its sexual state; but, at present, we know nothing whatever of this condition among the Gregarinae; so that in reasoning about them we must always exercise a certain reticence, not knowing how far we may have to modify our opinions by the discovery of the sexual state hereafter.
The process of becoming encysted, preceded or accompanied very often by the mutual apposition of two Gregarinae, was formerly imagined to correspond with what is termed among plants "conjugation," --a process which in some cases, at any rate, appears to be of a sexual nature. But the discovery that a single Gregarina may become encysted and break up into Pseudo-navicellae, seems to negative this analogy.
The Centipedes and Millipedes (Fig. 29) have the chitinous integument of the body divided into somites, provided with articulated appendages; and nervous and circulatory organs constructed upon a similar plan to those of the former groups.
The body consists of more than twenty somites, and those which correspond with the abdomen of Arachnida are provided with locomotive limbs.
The head consists of at least five, and probably of six, coalescent and modified somites, and some of the anterior segments of the body are, in many genera, coalescent, and have their appendages specially modified to subserve prehension. The respiratory organs are tracheae, which open by stigmata upon the surface of the body, and the walls of which are strengthened by chitin, so disposed as readily to pull out into a spirally coiled filament.
In this enormous assemblage of animals the respiratory organs are like those of the Myriapoda, with a nervous and a circulatory system disposed essentially as in this and the two preceding classes. But the total number of somites of the body never exceeds twenty. Of these five certainly, and six probably, constitute the head, which possesses a pair of antennae, a pair of mandibles, and two pairs of maxillae; the hinder pair of which are coalescent, and form the organ called the "labium."
Three, or perhaps, in some cases, more, somites unite and become specially modified to form the thorax, to which the three pairs of locomotive limbs, characteristic of perfect insects,* are attached.
Two additional pairs of locomotive organs--the wings--are developed, in most insects, from the tergal walls of the second and third thoracic somites. No locomotive limbs are ever developed from the abdomen of the adult insect, but the ventral portions of the abdominal somites, from the eighth backwards, are often metamorphosed into apparatuses ancillary to the generative function (Figs. 30 and 31).
* The female Stylops is stated to possess no thoracic limbs.