Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget (1834) (14755906436)

Identifier: animalandvegetab01roge (find matches)
Title: Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget ..
Year: 1834 (1830s)
Authors: Roget, Peter Mark, 1779-1869
Subjects: Biology Physiology Plant physiology Natural theology
Publisher: London : W. Pickering
Contributing Library: University of California Libraries
Digitizing Sponsor: MSN



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ables: but farther observation convinced himthat they were endowed with powers of spon-taneous motion, and that they preyed upon smallinsects: and he, therefore, could no longerdoubt their animal nature. He found that theyalways placed themselves on the side of theglass next to the light; and by w atching theirchanges of position, he discovered the mode inwhich they eflbct their progressive motions. Ifthe hydra be standing in the erect position, itsfoot being applied to the bottom of the glass(Fig. 73), it slowly bends the body in the direc-tion in which it intends to advance till its headtouches the vessel, as shown in Fig. 74. It thenadheres to the surface by the mouth, or by oneor two of its tentacula, and, detaching the foot,bends the body into a curve, at the same timeslightly retracting it, so that the foot is broughtnear the head (Fig. 75). The foot is then againfixed, preparatory to a ncAv step, which it takesby detaching the head and projecting it forwardsas before (Fig. 70).
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The progress made by these successive effortsis but slow: for the hydra often pauses in themidst of a step, as if deliberating whether it HYDRA. 179 should proceed ; so that the traversing a distanceof seven or eight inches is to these animals avery good days journey, even in summer. Buta mode of travelling rather more expeditiousthan this is occasionally resorted to. It con-sists of a succession of somersets: the hydra,while adhering firmly by the mouth, detachesits foot, and, making it describe a semi-circle, throws it over its head, and places itforemost in the line of progression. Havingattained this situation, the foot is then fixed,and a similar semi-revolution is performed bythe head, the body continuing all the whileelongated. By these and other manoeuvres these animalscontrive to vralk vrith equal facility in anydirection, either on the bottom or sides of thevessel, or along the stems of aquatic plants, towhich they are most frequently found attached.The position in which the