Identifier: introductiontozo00dave Title: Introduction to zoology; a guide to the study of animals, for the use of secondary schools; Year: 1900 (1900s) Authors: Davenport, Charles Benedict, 1866-1944 Davenport, Gertrude Anna Crotty, 1866- Subjects: Zoology Publisher: New York, Macmillan company London, Macmillian and co., ltd. Contributing Library: MBLWHOI Library Digitizing Sponsor: MBLWHOI Library

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Text Appearing Before Image: ettling organs of some sort.Hydra belongs to this group. There are two common species of Hydra ; the one is ofa green color {Hydra* viridis5)^ and the other is flesh-colored {If. fuscaQ~). They are found in standing or slow-running water, attached to submerged plants, sticks, andstones. Throughout the winter they live at the bottomof ponds, below the ice. The body of Hydra is soft andhighly contractile, so that, when first drawn from thewater, it appears like a speck of jelly. Left undisturbed,the animal expands, and its five to eight tentacles wave s, hollow; evrepov, intestine.nettle. 3 A key to the principal subdivisions of the Cnidaria and especiallyof the Hydromedusse will be found in the Appendix to this Chapter. 4 vdpa, a mythological monster, capable of regenerating its head. 5 Green. 6 Brown. 205 206 ZOOLOGY slowly about in search of food. The tentacles are richlysupplied with nettling cells, each of which contains afluid-filled capsule, in which is coiled a thread-like tube.

Text Appearing After Image: FIG. 102. — Sycon gelatinosum. A portion slightly magnified: one cylinder(that to the right) bisected longitudinally to show the central stomachcavity opening on the exterior by the osculum, and the position of the incur-rent and radial canals; the former indicated by the black bands, the latterdotted, ip marks the position of three of the groups of inhalant pores atthe outer ends of the incurrent canals; o, osculum. When stimulated by contact with some foreign body thetentacle closes around it, while from each capsule thelasso-thread rolls out as the finger of a glove is rolledinside out, and discharges the irritating poison .through THE HYDRA AND ITS ALLIES 207 the lumen of the thread. If a small animal has been theirritant, it is instantly caught in the thread, paralyzed bythe poison, and soon conveyed to the Hydras mouth.The principal food of Hydra is small worms and thesmaller Crustacea, such as Daphnia and Cyclops. a

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