Page:A Text-book of Animal Physiology.djvu/51

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UNICELLULAR ANIMALS.
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possibly the clear fluid with which they are filled may be a special secretion with solvent action on food.

Figs. 34 to 40.—In the following figures d, denotes disc; p, peristome; vc, contractile vacuole; vf, food-vacuole; vs, vestibule; c, contractile fiber; c, cyst; nc, nucleus; cl, cilium.
Fig. 34.—A group of vorticellse showing the creature in various positions (A. 3).
Fig. 35.—The same, in the extended and in the retracted state. (Surface views.)
Fig. 36.—Shows food-vacuoles; one in the act of ingestion.
Fig. 37.—A vorticella, in which the process of multiplication by fission is begun.
Fig. 38.—The results of fission; the production of two individuals of unequal size.
Fig. 39.—Illustration of reproduction by conjugation.
Fig. 40.—An encysted vorticella.

Situated somewhat centrally is a horseshoe-shaped body, with well-defined edges, which stains more readily than the rest of the cell, indicating a different chemical composition; and, from the prominent part it takes in the reproductive and other functions of the creature, it may be considered the nucleus (endoplast).

Multiplication of the species is either by gemmation or by fission. In the first case the nucleus divides and the fragments are transformed into locomotive germs; in the latter the entire animal, including the nucleus, divides longitudinally, each half becoming a similar complete, independent organism. Still an--