. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. end ect. Planula, without t)rifice, formed by direct growth. (Optical section.) Planula with orifice, which has broken through. (Surface view.) layers, the inner of which bounds a cavity which subsequently, by a breaking through at one pole, communicates with the exterior. In either case the result is an organic form charac- terized by being constructed of two layers of cells, the inner of which lines a cavity opening to the exterior. This cavity is the primitive gastric cavit
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. end ect. Planula, without t)rifice, formed by direct growth. (Optical section.) Planula with orifice, which has broken through. (Surface view.) layers, the inner of which bounds a cavity which subsequently, by a breaking through at one pole, communicates with the exterior. In either case the result is an organic form charac- terized by being constructed of two layers of cells, the inner of which lines a cavity opening to the exterior. This cavity is the primitive gastric cavity; and the organic form thus characterized may be known as the Planula'^. The production of such a Planula^ recognizable under extreme modifications of non-essential general shape (one of the most common causes of which is the admixture of a large mass of secondary yelk with the original egg-cell), is comvion to the developmental history of all animals ahove the Protozoa. But after this there is a divergence; for whilst there is a further development of primitive cells in the Vermes, Mollusks, Echi- noderms. Arthropods, and Vertebrates, in the Coelenterata (in- cluding herein the Sponges) these two layers of cells, the endoderm and ectoderm, remain throughout life as the basis of further histological diifereutiation, even though in the larger forms the ectoderm may largely develop deep layers of a special muscular or skeletal nature. The series of forms thus branching off from the genealogical tree may be termed Diploblastica. The endoderm and ectoderm of the polypes and corals was recognized first by Professor Huxley, who at the same time * It may be advantageous to use the term Gadrulu for that condition of the rianula when the orifice is present, as Hiickel has proposed since the above scheme was drawn Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrati
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