Multicellular animals in which there is an ectoderm (=epi- or hypo-dermis) and an endoderm with tissues of various kinds, - sensory, nervous, muscular, connective and reproductive, derived either solely from them, or, with the exception of the two tissues first-named, from an independent source. A Gastrula stage occurs in the life-history of the individual. There are two subdivisions, the Coelomata and Coelenterata.
Metazoa, in which an intermediate layer of cells, the mesoblast or mesoderm, developed in different ways (pp. xxviii-ix) intervenes between the epi- and hypo-blast of the embryo. It gives rise to the muscular and connective tissues, to the lymph or blood, to the nephridial (renal) and with perhaps few exceptions to the reproductive cells. A cavity or a system of cavities or channels, known as the coelome, is typically present; it lies within the mesoblast and is not homologous throughout the Coelomata (pp. xxix-xxx). In a few instances (most Turbellaria, the Trematoda, and Cestoda), it is represented by intercellular spaces. The principal axis of the Gastrula passing through the blastopore, never persists as the main axis of the body. Bilateral symmetry is always established. A shorter anterior region or head which is preoral, and a longer postoral region, the body, are readily distinguishable in most instances. The Echinodermata, however, acquire a secondary radial symmetry. An anus is typically present.
The Coelomata include the phyla Chordata, Mollusca, Arthropoda, Echinodermata, together with Vermes, as well as a few groups, the zoological position of which is uncertain, e. g. Brachiopoda and Polyzoa.