Fig. 4.—Section, highly magnified, of a two-layered animal, Hydra (Grade II.). Ec, outer layer of Ectoderm; En, inner layer of Endoderm; l, lamella dividing the two, represented by a line; n, nuclei of the cells; v, thin vacuoles of small interstitial cells; E, the Enteron or digestive cavity.
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 Cœlenterata receive their name, which means "possessing a hollow space only, by way of intestines." The name of Acœlomata, animals without a body-cavity, has therefore been given to the Cœlenterata 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 Cœlenterata and Sponges, but here it is further established. It includes a very thick layer of muscles.
Fig. 5.—Diagrammatic plan of section cut through an Earthworm to show the position of the three body-layers and the body-cavity (Grade IV.). Sk, skin; al, glandular lining of the alimentary canal; w, muscular wall of body; w', muscular wall of intestine, both belonging to the third layer or mesoblast; b.c., body-cavity (shaded); al.c., cavity of alimentary canal (shaded); n, nerve.
Grade IV. The Cœlomata 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.