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
converted itself into demonstration.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[1] 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. The Protozoa.
Haeckel has shown that, among the Protozoa, there are some which are simpler than the rest, inasmuch as they are devoid of both nuclei and contractile vesicles. To these he applies the name of—
1. Monera.—Among the members of this group, which are at present known, three series are distinguishable, in all of which multiplication is effected by division, preceded, or not preceded, by the assumption of an encysted condition. In one state, each of these Monera is a myxopod,[2] that is, is provided with longer or shorter pseudopodia as locomotive organs, and, in Protamoeba and Protogenes, the result of the process of division is also two or more rnyxopods. But, in Protomonas, the myxopod, after becoming encysted, gives rise by division to bodies provided with long flagelli- form cilia, by which they are propelled, and which may be termed mastigopods ; and in Myxastrum, the encysted body divides into a multitude of oval particles, each en closed in its own coat. These are set free, and each gives rise to a new myxopod of the same character as the parent.
In Protomyxa, the myxopods coalesce into a reticulated plasniodium ; and Vampyrella is parasitic, devouring stalked diatoms, and encysting itself upon the ends of their stalks, the encysted form dividing into new Vam- pyrelloe. Most of these interesting Monera have been made known by Haeckel, so that, in all probability, many others remain to be discovered. It is probable that the Fora- minifera, notwithstanding the complexity of the skeletons, belong to this group, but too little is known of the struc ture of their soft parts to enable any certain conclusion to be drawn respecting them, and the analogy of Gromia leads to the suspicion that they may belong to the next division.
2. Endoplastica.—In these Protozoa a portion of the in terior protoplasmic body is separated from the rest as a dis tinct, more or less rounded, body, which may be termed the endoplast, as a term suggestive of its similarity to the nucleus of a histogenetic cell, without implying its identity therewith. Of such endoplasts there may be one or many, but the protoplasm in which they lie does not give rise to cells, which become metamorphosed into elements of the tissues. Veiy often they possess one or more vacuoles, which rhyth mically dilate and contract, in accordance with the changes in the protoplasm in which they lie, and which are termed contractile vesicles.
In this division of the Protozoa, three groups the Amoebidce, the Flagellata (or flagellate Inf iisoria), and the Gregarinidoe closely repeat the forms and mode of leproduction of the Protamoebidce, Protomonadidce, and Myxastridce among the Monera. Among the rest, the Acinetidoe are distinguished by their pseudopodia being converted into suckers, through which they draw tho juices of their prey. In all these, and in the preceding forms, there is a more or less marked distinction of the pro toplasm constituting the body into a firmer and denser outer layer, the ectosarc, and a more fluid inner substance, the endosarc; and, in some of the Gregarinidce, the ectosarc becomes differentiated into muscular fibres. In the Flagel lata there is a permanent oral aperture ; and in one member of this group, Noctiluca, additional complications of structure, in the form of a ridge-like tooth and a tentacle, occur. In the Radiolaria, the body is still more clearly differentiated into an inner substance, surrounded by a capsule, and containing nuclei and even cells, and a vacuo- lated ectosarc, whence the radiating pseudopodia proceed. Coloured corpuscles, usually yellow, appear in the ectosarc, and have been shown by Haeckel to contain starch and to multiply independently. In the Ciliata (ciliated Infu soria], with which the Catallacta of Haeckel may be included, the differentiation of the protoplasm of the body, without any development of histogenetic cells, goes still further. A permanent mouth and anus may appear, con nected by a permanently softer and more fluid region of the protoplasm (as is plainly to be seen, for example, in Nyctotherus} foreshadowing an intestinal cavity. The ecto sarc may be differentiated into a specially modified cortical layer, and well-marked muscular fibres may be developed. Morever, the endoplast, or " nucleus," becomes an organ of reproduction, the germs of the young being given off by division from it. Very generally, a small body the so-called " nucleolus," but which has, admittedly, nothing to do with the structure so named in a true cell, and may be termed the " endoplastula " is to be found close to the nucleus, and there is some ground for supposing it to be a testis. The Infusoria frequently multiply by fission, which may, or may not, be preceded by encyst- ment ; and in many of them, as in the Gregarinidce, Acinetidce, and some Flagdlata, conjugation has been observed. It is yet disputed how far the conjugation is a necessary antecedent of the process of endogenous germ formation.
which first gave a clear insight into the structure of the ciliated Infusoria, that they were animals of complex structure, possessing, on a minute scale, all the organs characteristic of the higher forms of animal life. In opposi tion to this view, Dujardin started the conception that they are little more than masses of sarcode ( = protoplasm) ; and Von Siebold, modifying this view in accordance with the cell theory, regards them as the equivalents of single cells of the tissues of the higher animals. The result of the long controversy which has been carried on on this subject seems to be, . on the one hand, that Ehrenberg was quite right in vindicating for the Infusoria a far greater com plexity of structure than they had been supposed to possess. It is certain that an Infusorium may possess a distinct integumentary layer, muscles, a permanent oesophagus, a
permanent anal area, and, in some cases, a persistent tract
