The porifera and coelentera . sac-like body, and also the tentacles andperistome are always composed of three layers of tissue, of whichtwo, the external layer or ectoderm, and the internal layer orendoderm, are always cellular, and are coextensive and identicalwith the epiblast and hypoblast of the embryo. The third layer, lying between the ectoderm and endoderm,varies considerably in structure and importance in different groujisof the Coelentera. Typically, it is not a cellular layer, but is ofgelatinous consistency, and is formed as a sort of secretion fromthe ectoderm; in some cases the en
The porifera and coelentera . sac-like body, and also the tentacles andperistome are always composed of three layers of tissue, of whichtwo, the external layer or ectoderm, and the internal layer orendoderm, are always cellular, and are coextensive and identicalwith the epiblast and hypoblast of the embryo. The third layer, lying between the ectoderm and endoderm,varies considerably in structure and importance in different groujisof the Coelentera. Typically, it is not a cellular layer, but is ofgelatinous consistency, and is formed as a sort of secretion fromthe ectoderm; in some cases the endoderm also takes a share inits formation. After treatment with reagents, the middle layer THE ANTHOZOA may show a fibrillar structure, which, in many cases, is undoubtedlyan artifact. It may be homogeneous and devoid of all trace ofstructure, or it may contain numerous cells, which are eitherbranched, nucleated, so-called connective tissue cells ; nerve cellsand fibres, muscular fibres, or cells in which calcareous skeletal. Fio. I. 1.—Diagrammatic longitudinal section through a typical Anthozoan zooid. v, IkhIj- wall;js, peristonie ; h, base; t, tentacles; »t, stoniodaeutn ; wi, mesentery. 2.—Dia};raniniatic transverse section throuj;h a tyi)ical Anthozoan zooid in the region of thestomodaeuMi. <c, eotoilenn ; en, endo<lerm ; ni;/, meso^loea ; «?, sulcus ; .</, sulculus. 3a.—Nematocyst of Vorj/nactis viridis, fully everted. 36. The same, before eversiou. same, partly everted. 4.—Section through a typical Anthozoan mesentery with its mesenterial fllament. en,endotlerui; nui, mesogloea ; )».«•, muscle banner with supporting plications of the niesogloea. 5.—Portion of the muscular layer of A lumonia sulcata showing the nerve plexus and ganglioncells. (1-4 original; 5 after O. and R. Hertwig.) spicules are developed. All these cells or cell-products are in-trusive, and are derived from one or other of the two primarylimiting layers comparatively la
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