a junior course of practical zoology

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Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: Chapter II. HYDRA. Hydra is a small freshwater auimal, abundant in ponds and ditches and in slowly moving streams. It consists of a tubular body, about a quarter of an inch in length, one end of which is closed, while the opposite end is open and forms the mouth, a short distance below which is a circle of tentacles, usually six to eight in number. Both the tentacles and the body are extremely contractile. The former, when the animal is disturbed, can be almost completely withdrawn, and the latter may shrink up so as to become a mere knob. Hydra is usually attached by its closed basal end or foot to water-weeds or other bodies. By contractions of the foot it can crawl along slowly, and it can also progress more rapidly by fixing itself alternately by the mouth and foot, arching the body with a looping movement like a caterpillar. It is carnivorous, and by means of the nematocysts with which its tentacles are studded can paralyse and kill animals nearly as big as itself and of active habits. Hydra receives its name from its remarkable power of recovery from injury. A specimen may be cut into two or more pieces, either transversely or longitudinally, and each fragment will not only survive, but within a short time will become a complete Hydra. The entire animal may even be regenerated from a single tentacle. At least three species of Hydra are described as occurring commonly in this country. Of these H. viridis is distinguished by its green colour and its smaller size, H.fusca is brown in colour, and H. vulgaris almost colourless. It is uncertain, however, how far these species are really distinct from one another, and the following description will apply to any of them. Fin 12.—Hydra. A diagrammatic Jongitndinal section of a specimen with ripe reproductiv...
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