The two "primary germinal layers" which the gastraea theory has shown to be the first foundation in the construction of the body are found in this simplest form throughout life only in animals of the lowest grade--in the gastraeads, olynthus (the stem-form of the sponges), hydra, and similar very simple animals. In all the other animals new strata of cells are formed subsequently between these two primary body-layers, and these are generally comprehended under the title of the middle layer, or mesoderm. As a rule, the various products of this middle layer afterwards constitute the great bulk of the animal frame, while the original entoderm, or internal germinal layer, is restricted to the clothing of the alimentary canal and its glandular appendages; and, on the other hand, the ectoderm, or external germinal layer, furnishes the outer clothing of the body, the skin and nervous system. In some large groups of the lower animals, such as the sponges, corals, and flat-worms, the middle germinal layer remains a single connected mass, and most of the body is developed from it; these have been called the three-layered metazoa, in opposition to the two-layered animals described. Like the two-layered animals, they have no body-cavity--that is to say, no cavity distinct from the alimentary system. On the other hand, all the higher animals have this real body-cavity (coeloma), and so are called coelomaria. In all these we can distinguish four secondary germinal layers, which develop from the two primary layers. To the same class belong all true vermalia (excepting the platodes), and also the higher typical animal stems that have been evolved from them--molluscs, echinoderms, articulates, tunicates, and vertebrates. (FIGURES 1.74 AND 1.75. Diagram of the four secondary germinal layers, transverse section through the metazoic embryo: Figure 1.74 of an annelid, Figure 1.75 of a vermalian. a primitive gut, dd ventral glandular layer, df ventral fibre-layer, hm skin-fibre-layer, hs skin-sense-layer, u beginning of the rudimentary kidneys, n beginning of the nerve-plates.) The body-cavity (coeloma) is therefore a new acquisition of the animal body, much younger than the alimentary system, and of great importance. I first pointed out this fundamental significance of the coelom in my Monograph on the Sponges (1872), in the section which draws a distinction between the body-cavity and the gut-cavity, and which follows immediately on the germ-layer theory and the ancestral tree of the animal kingdom (the first sketch of the gastraea theory). Up to that time these two principal cavities of the animal body had been confused, or very imperfectly distinguished; chiefly because Leuckart, the founder of the coelenterata group (1848), has attributed a body-cavity, but not a gut-cavity, to these lowest metazoa. In reality, the truth is just the other way about.