. Text-book of zoology for junior students. Zoology. DIVISIONS OF THE HYDROZOA. 59 also contains numerous green granules, apparently identical with " chlorophyll," or the green colouring-matter of plants. At the base or proximal extremity of the cylindrical body is a kind of disc- shaped sucker or hydrorhiza, by means of which the animal can attach itself at will to any foi-eign body. Its favourite position ap- pears to be that of hanging head-downwards, suspend- ed from the stem of some water-jjlant. It is not, how- ever, permanently fixed, but it can detach itself and change its ])
. Text-book of zoology for junior students. Zoology. DIVISIONS OF THE HYDROZOA. 59 also contains numerous green granules, apparently identical with " chlorophyll," or the green colouring-matter of plants. At the base or proximal extremity of the cylindrical body is a kind of disc- shaped sucker or hydrorhiza, by means of which the animal can attach itself at will to any foi-eign body. Its favourite position ap- pears to be that of hanging head-downwards, suspend- ed from the stem of some water-jjlant. It is not, how- ever, permanently fixed, but it can detach itself and change its ])lace at will. At the opposite or distal extremity uf the body is placed the apertm-e of the mouth, surrounded by a circle of from five to fifteen small tubular filaments, which are termed the " tentacles" (fig. 30, a). Each tentacle consists of a tubular prolongation of both ectoderm and ento- derm, and encloses a canal which opens at its base into the general cavity of the body. The ectoderm is richly furinshed with thread - cells, which are especially abundant in the tentacles. On the inside of the ectoderm, between it and the endoderm, exists also a peculiar fibrous layer, wliich has been generally regarded as partly of a ner- vous and partly of a muscular character. The tentacles exhibit the most extraordinary contractility, being capable of retraction till they appear as nothing more than so many little warts or tubercles, and of being extended to a length which is in some species many times longer than the body itself. They are the organs by means of which the Hydra obtains its food, consisting chiefly of minute aquatic oigan- isras, such as small worms, insects, Criishtcea and Itotifent. These are seized by the tentacles and gradually drawn into the mouth; but in. Fig. ,'jO.—The Green Fresh-water PoTyjie (Hydra viridis), suspended liead-downwards iViuii apiece stem of an afjuatic plant, enlaiged, <' One of i of tlie tentacles; h Testis or sperniariiini, wi
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1885
