. Animal biology. Biology; Zoology; Physiology. 100 ANIMAL BIOLOGY while those of the ectoderm are variously modified for protection, and the other relations of the individual to its surroundings, as well as for reproduction. In short, in the organization of Hydra the primary tissues (ecto- derm and endoderm) have not become differentiated into secondary. Ectoderm Mesogloea Endoderm Enteric cavity Fig. 58. — Hydra. Transverse section highly magnified. specialized tissues (muscular tissue, nerve tissue, etc.) for one function or another — the simple life processes of the animal are adequately p
. Animal biology. Biology; Zoology; Physiology. 100 ANIMAL BIOLOGY while those of the ectoderm are variously modified for protection, and the other relations of the individual to its surroundings, as well as for reproduction. In short, in the organization of Hydra the primary tissues (ecto- derm and endoderm) have not become differentiated into secondary. Ectoderm Mesogloea Endoderm Enteric cavity Fig. 58. — Hydra. Transverse section highly magnified. specialized tissues (muscular tissue, nerve tissue, etc.) for one function or another — the simple life processes of the animal are adequately provided for by the specialization of isolated cells or small cell groups within ectoderm and endoderm. (Fig. 58.) B. Earthworm The bodies of all animals above the Coelenterates are built of three primary layers, which, as development of the individual proceeds, give rise to the secondary tissues and thereby form a relatively complex body. This third primary layer (tissue), the mesoderm, typically is developed, as we have described earlier, from the endoderm and comes to occupy the position held by the mesogloea of Hydra; that is, between the ectoderm and the endo- derm. The development of the mesoderm is the key to the advance in body organization of higher animals, because it makes possible a radical change in plan that involves the establishment of a body. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Woodruff, Lorande Loss, 1879-1947. New York The Macmillan company
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