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CHAPTER IV

POLYPS (CUPLIKE ANIMALS)

The Hydra, or Fresh Water Polyp

Suggestions.—Except in the drier regions of the United States, the hydra can usually be found by careful search in fresh water ponds not too stagnant. It is found attached to stones, sticks, or leaves, and has a slender, cylindrical body from a quarter to half an inch long, varying in thickness from that of a fine needle to that of a common pin. The green hydra and the brown hydra, both very small, are common species, though hydras are often white or colorless. They should be kept in a large glass dish filled with water. They may be distinguished by the naked eye but are not studied satisfactorily without a magnifying glass or microscope. Place a living specimen attached to a bit of wood in a watch crystal filled with water, or on a hollowed slip, or on a slip with a bit of weed to support the cover glass, and examine with hand lens or lowest power of microscope. Prepared microscopical sections, both transverse and longitudinal, may be bought of dealers in microscopic supplies. One is shown in Fig. 39.

Fig. 33.—A Hydra.

Fig. 34.—Forms assumed by Hydra.

Is the hydra's body round or two-sided? (Fig. 35.) What is its general shape? Does one individual keep the same shape? (Fig. 34.) How does the length of the thread-