. Biology . wo-layered animals, as dis-tinguished from the triploblastic or three-layered higher animalsmade up of ectoderm, endoderm and intermediate layer, themesoderm. The different types of cells of Hydra perform their functionsfor the good of the entire organism, and represent, morpholog-ically, the incipient stages of organ systems in more highly dif-ferentiated animals. A. Ectoderm Cells.—Six types of cells are present in theectoderm: (i) epithelio-muscle (neuro-muscle) cells; (2) nettle orstinging cells; (3) nerve cells; (4) sensory cells; (5) germ cells;(6) formative or interstitial c
. Biology . wo-layered animals, as dis-tinguished from the triploblastic or three-layered higher animalsmade up of ectoderm, endoderm and intermediate layer, themesoderm. The different types of cells of Hydra perform their functionsfor the good of the entire organism, and represent, morpholog-ically, the incipient stages of organ systems in more highly dif-ferentiated animals. A. Ectoderm Cells.—Six types of cells are present in theectoderm: (i) epithelio-muscle (neuro-muscle) cells; (2) nettle orstinging cells; (3) nerve cells; (4) sensory cells; (5) germ cells;(6) formative or interstitial cells. The bulk of the body cover- 84 ORGANISMS OF TISSUES ing is made up of the epithelio-muscle cells, while sensory cellsare rare and limited to the regions about the mouth and thepedal disc. The nettle or stinging cells are superficially placedon the epithelial cells and partly embedded in them. The nerveand interstitial cells lie between the bases of the epithelial cellsand upon the supporting Fig. 34.—Hydra fusca as seen in optical section through the enteric cavity;two testes are shown just below the tentacles and an ovary farther down; onthe opposite side a well-developed bud. (Modified after Marshall and Hurst.) I. The Epithelio-muscle Cells.—There is no muscular systemin Hydra, the covering, or epithelial cells, possessing contractileprocesses which take the part of muscles in higher cells are much elongated in the gonad region and on thepedal disc where the cells are loaded with granules of secretion HISTOLOGY OF HYDRA 85 (Fig. 35). On the tentacles they are much flattened, while inother regions of the body their size is intermediate. The trunkepithelial cells are more or less vacuolated. All forms of theseepithelial cells are characterized by the presence of muscularfibers (myofibrils) in the basal parts of the cells. These fibersrun up and down the body, thus forming a complete longitudinalmuscular investment for the entire animal, givi
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectbiology
