Figs. 74 and 75. Diagram of the four secondary terminal layers. Figs. 74 and 75—Diagram of the four secondary germinal layers, transverse section through the metazoic embryo: Fig. 74 of an annelid, Fig. 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 (_cœloma_) 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 cœlom 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 gastræa 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 cœlenterata 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.
The ventral cavity, the original organ of nutrition in the multicellular animal-body, is the oldest and most important organ of all the metazoa, and, together with the primitive mouth, is formed in every case in the gastrula as the primitive gut; it is only at a much later stage that the body-cavity, which is entirely wanting in the cœlenterata, is developed in some of the metazoa between the ventral and the body wall. The two cavities are entirely different in content and purport. The alimentary cavity (_enteron_) serves the purpose of digestion; it contains water and food taken from without, as well as the pulp (chymus) formed from this by digestion. On the other hand, the body-cavity, quite distinct from the gut and closed externally, has nothing to do with digestion; it encloses the gut itself and its glandular appendages, and also contains the sexual products and a certain amount of blood or lymph, a fluid that is transuded through the ventral wall.
As soon as the body-cavity appears, the ventral wall is found to be separated from the enclosing body-wall, but the two continue to be directly connected at various points. We can also then always distinguish a number of different layers of tissue in both walls—at least two in each. These tissue-layers are formed originally from four different simple cell-layers, which are the much-discussed four secondary germinal layers. The outermost of these, the skin-sense-layer (Figs. 74, 75 _hs_), and the innermost, the gut-gland-layer (_dd_), remain at first simple epithelia or covering-layers. The one covers the outer surface of the body, the other the inner surface of the ventral wall; hence they are called confining or limiting layers. Between them are the two middle-layers, or mesoblasts, which enclose the body-cavity.