Metazoa in which a gelatinoid substance, the supporting lamina or mesoglaea 1, intervenes between the epi- and hypo-blast of the embryo or larva, and persists throughout life, holding the position of the absent meso-blast or intermediate cell layer of Coelomata. This mesoglaea varies in amount and consistency: it is homogeneous, sometimes partially fibrillate, and may be invaded by cells derived from the ecto- or endo-derm, or from both. The cells of the ectoderm may be uni- or multi-laminar. They may be differentiated into epithelium or covering cells, epithelio-muscle cells, muscle cells, pigment cells, gland cells, cnidoblasts, skeletogenous cells, sense with supporting cells, ganglion cells, and genital cells. The endo-derm is unilaminar, rarely multilaminar and then, as a rule, only in limited spots. Its cells may be differentiated, like the cells of the ectoderm, with the addition of skeletal cells, but skeletogenous cells, ganglion cells, and sense cells, with the exception of otolith cells, are rarely met with.

Both ecto- and endo-derm are primitively unilaminar in the larva.

1 The term 'mesoglaea' is due to Mr. G. C. Bourne (Q. J. M. xxvii. pt. 3, 1887). It seems unadvisable to apply either of the now synonymous terms 'mesoblast' and 'mesoderm' to the sup

A persistence of the vertical axis passing through the Gastrula mouth as the long axis of the body, and a symmetry of form round this axis are typical of Coelenterata. When bilateral symmetry is established, or a lateral elongation of the body in a vertical plane takes place, an even balance with reference to the vertical axis just named is, as a rule, maintained1. Irregular growth, however, occurs in most Sponges.

There are four classes, Ctenopkora, Anthozoa, Hydrozoa, and Porifera. The three first named possess peculiar offensive structures, either adhesive cells or cnidoblasts containing nematocysts, the former in the Ctenophora porting lamella of Coelenterata, for the following reasons: (1) It appears typically as a clear layer between the epi- and hypo-blast of the embryo some time after their differentiation; (2) not only does it contain no cells at first, but may remain permanently in this condition; (3) when cells enter it, they never form a defined layer either sub-ectodermic or sub-endodermic; (4) it is frequently specialised in density and fibrillation; (5) though its cells, when present, may assume particular functions, the supporting lamina as a whole never gives rise to tissues in the same way as does the mesoblast. Apparent exceptions are (1) the early developed group of cells said by Metschnikoff to be derived from the endoderm in embryo Ctenophores, the source of mesoglaeal cells later on (see note 2, p. 720); and (2) the cellular mass filling the planula of certain Sponges. But according to Chun, mesoglaeal cells are derived in Ctenophores throughout life from the superficial and stomachal ectoderm; and the cells of the Sponge larvae in question appear to be the common source of both endoderm and mesoglaeal cells.

A gelatinous substance fills the blastocoele of Echinoderm larvae, the Nemertean Pitidhim, etc.; but it appears before the epi- and hypo-blast are differentiated, and is, therefore, probably not homologous with a supporting lamella. Cells enter it either from the walls of the blastula or of the archenteron. They subsequently give rise to continuous cell-layers, and the jelly disappears at the same time. The cells referred to are therefore destined to a development never attained in any Coelenterate.

It is not likely that Coelomate forms are derived from Coelenterate. The latter are specialised from a simple gastrula type, from which the Coelomata have also sprung, but in a different direction. The great complexity often acquired by the Coelenterate ecto- and endo-derm points to the same conclusion and a few Hydrozoa, the latter in one Ctenophore, in the Anthozoa and Hydrozoa. All are aquatic, and with few exceptions marine. The larva is ciliated and, except in Ctenophora set free, as a rule, at an early stage of development.

1The converse of these three propositions appears to be true of the Coelomata. The principal axis of the Gastrula never persists as the principal axis of the body. Bilateral symmetry is always established, but may be disguised or lost; it is balanced with reference to the Gastrula axis, but this is not the case with the elongation of the body. Cp. p. 584, on the Trochosphere, etc. For points connected with the mesoblast and coelome, see General Introduction.

With the absence of a mesoblast is correlated the absence of coelomic spaces of all kinds, of a circulatory, specialised respiratory and nephridial systems. The genital cells are sub-epithelial, or in Porifera mesoglaeal.

The mesoglaea is a proteid substance, sometimes as dense as hyaline cartilage, sometimes excessively soft and watery, e. g. it contains 95.392 °/0 of water in Rhizo-stoma Cuvieri (=Pilema pulmd). This percentage may be exceeded when the sea-water is less saline than usual; e. g. Aurelia aurita from Kiel has 97.90 %. Fibrillae are very commonly present. They vary in character, size, mode of disposition, whether reticulate, tangential, or vertical to the surface. They are probably derived by condensation of the jelly: but in some instances very delicate fibrils have been traced into continuity with the ecto- and endo-derm cells. As to mesoglaeal cells, they may be wanting as in Craspedote Medusae: when present either simple or, as in Ctenophora and Porifera, specialised in various ways. They may be derived from the ecto- or endo-derm, or from both, and are sometimes set apart at an early period as in Ctenophora (note 2, p. 720). The mesoglaea itself is probably derived in some instances from the ectoderm, in others from the endoderm, e. g. in the taeniolae of Acraspeda.

An epithelium cell is one that is solely protective: an epithelio-muscular cell is an epithelium cell with a basal muscular process, whereas a muscle cell is subepithelial in position. Muscle cells may become inclosed within the mesoglaea by the growth of the latter, and are then termed by Hertwig 'mesodermal muscles/ The term 'neuro-muscular' cell (Kleinenberg) has been applied to an epithelio-muscular cell, but there is no reason to ascribe any special nervous function to the cell portion of such a unit, any more than to the undifferentiated cell portion or muscle-corpuscle of a striated muscle-fibre. The term has recently been used by Korotneff in connection with certain remarkable branched ectoderm cells, the processes of which are in continuity with the longitudinal ectodermic musculature of some Siphonophora (Mitth. Zool. Stat. Naples, v. p. 235, PI. 14, Fig. 13). The muscle substance is highly refractile, and is transversely striated in the subumbrellar muscles of Medusae.