Putting, the Mesozoa aside, the vast majority of multicellular animals may be classified as Metazoa. The growth of the individual is complicated by the formation of tissues and systems of organs. . Sensory and nervous tissue, contractile tissue, supporting or connective tissue, localised reproductive tissue, are differentiated in connection either with the epi-blast s. ectoderm, and hyboblast s. endoderm of the Gastrula, or in part independently of them. Two main divisions of Metazoa are recognisable, the Coelenterata and Coelomata.

The typical characters of the Coelenterata are as follows. The fundamental symmetry of the Gastrula is as a rule retained; the vertical axis passing through the blastopore persists; if an anterior and posterior extremity are distinguishable they are equal; and the same is true of a right and left side. The blastopore may close, or not be developed; the archenteron, except in Porifera, opens by a single principal aperture, which is either a perforation of the two embryonic layers, or a distinct involution of the epiblast known as stomodaeum, which may assume a digestive function (Ctenophora). There appears between the ectoderm and endoderm a gelatinous lamella - the mesoglaea, which may be structureless, partly fibrillate, or invaded by cells derived from one of the two epithelia. Sensory and nervous cells are epithelial or sub-epithelial; the same is true of the contractile cells, which may, however, become imbedded in the mesoglaea. The generative products are sub-epithelial and localised. But a differentiation of the mesoglaeal cells may occur, principally in Porifera. Reproduction by division of the organism is rare; by buds or outgrowths of the ecto- and endoderm jointly, common; by the division of a single mesoglaeal cell, or the growth of mesoglaeal cells plus endoderm cells confined to Porifera. See pp. 713-6; 804.

1 This may be a very primitive condition; cf. Sedgwick, Q. J. M. xxvii. 1886, pp. 515-30.

The typical characters of the Coelomata contrast with those of the Coelenterata as follows. The fundamental symmetry of the Gastrula and the vertical axis passing through the blastopore do not persist. As a rule equal right and left sides are distinguishable; but the anterior and posterior parts of the body, if the permanent mouth, as is most natural, is taken as a point of reference, are not equal, the former being relatively small, and constituting a more or less distinct head. The archenteron, the mesenteron of the adult, communicates with the exterior by a mouth, and as a rule by an anus. The blastopore may become obliterated in its centre, and its two ends may coincide with the position of the future mouth and anus as in Peripatus; it may close from behind forwards, or vice versa, and then the mouth or anus correspond respectively to the part left open; it may close and leave no trace; or it may never be found at all as in Insecta. The permanent mouth and anus of the adult are generally, perhaps always, formed by a more or less pronounced ingrowth of ectoderm, either at the open part of the blastopore, or independent of it.

In the latter case it is a question how far the ingrowth coincides with the obliterated part of the blastopore, or the spot where it might be expected to be. To the oral and anal ingrowths, the terms stomodaeum and proc-todaeum are applied. Instead of a mesoglaea, there is a cellular meso-blast or mesoderm. Its cells in all cases lie or come to lie between the epi- and hypoblast. They are formed at an early period in the ontogeny, and as a matter of fact are derived in several ways the mutual connection of which is disputed. They may have a single or a double source. As to the former, the cells arise (1) as immigrants (mesenchyme cells), from the walls of the blastosphere, or from its endodermal pole as in Nemertea; (2) from the walls of the archenteron close to the blastopore, e. g. many Crustacea; (3) from cells specialised at an early period at the blastopore, e. g. the pole-cells of Chaetopoda; (4) from the primitive streak behind the blastopore in Peripatus, the same streak in Insecta or Spiders, which may be partial or complete homologues of the streak in Peripatus 1; (5) from the walls of diverticula of the archenteron or enterocoelic pouches as in Balanoglossus, Sagitta and Brachiopoda. When it has a double source it may be (1), supra, combined with enterocoelic pouches as in Echinoder-mata, or with pole-cells as in Thalassema; (3) supra, combined with enterocoelic pouches as in Amphioxus; (2) or (4), supra, combined with cells derived from the hypoblast as in most Vertebrata. Whatever significance these facts may have, the mesoblast gives rise to the muscular, connective and skeletal, blood and lymph tissues, and very generally to the genital and excretory cells.

In Vertebrata, Cephalochorda, Arthropoda, some Vermes, e. g. Chaetopoda, it is broken up into a series of paired segments, sometimes, as in many Crustacea, obscurely marked, giving rise in the adult to a metamerism or serial segmentation, the primitive origin of which is a matter of doubt. In connection with the mesoblast or its segments is the cavity, or series of cavities, known as body cavities or coelome, which are not homologous throughout the Coelomata. In some Vermes a coelome is absent, or represented by irregular spaces or gaps in the mesoblastic tissues, as in Turbellaria, Trematoda, and Cestoda, but in other Coelomata it is probable that it falls under one of the following heads. (I) It is an archicoele, or remnant of the blastocoele as in the vascular system of Nemertea, the head cavities of some segmented Vermes, e. g. Polygordius, some Chaetopoda, the body cavity of Rotifera, and Dinophilus (?). (2) It is a system of channels and spaces excavated in the mesoblast secondarily, e. g. the principal portions of the coelome, or the vascular spaces in Peripatus, and perhaps in Mollusca, or the whole of it in Arthropoda in general.