Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget (1834) (14592272750)

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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guments. The headis then applied to the ground, and made thefixed point, and the segments next to it, whichhad been elongated, are now contracted bythe action of their longitudinal muscles; indoing which, equal portions of the succeedingsegments are necessarily elongated : these arenext contracted; and so on, in succession, tillthe whole is brought forwards to the head : afterwhich the same series of actions is repeated,beginning with the advance of the head. * Home; Lectures, &c. Vol. i. p. 115. 276 THE MECHANICAL FUNCTIONS. Worms often reverse this motion, and are thusenabled to move backwards, or with the tailforemost. * Great variety exists in the forms of the ani-mals referable to the type of Annelida. TheGordius, or hair-worm (Fig. 132) is that whichexhibits the greatest developement in lengthcompared with the breadth of the body. It hasthe form of a very long and slender thread : theannular structure being indicated only by veryslight transverse folds of the integuments. No

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external members, nor even tentacula, havebeen given to this simplest of vermiform ani-mals. Many of the animals of this class being softand defenceless, are obliged to consult theirsafety by retreating into holes and recesses, orby burrowing in the sand or mud. One genus * See Home; Lectures on Comparative Anatomy, Vol. i.p. 114. ANNELIDA. 2^7 only, the Scrpnla (Fig. 133), forms for itself anexternal shell, which is shaped into a spiral tube.Others, as the Sabella and the Tcrebella, accom-plish the same object by collecting grains ofsand, or fragments of decayed shells, or othersubstances, which they agglutinate together bymeans of a viscid exudation, so as to form afirm defensive covering, like a coat of mail.Fig. 134 shows this rude architecture in theTerebella conchilega. These coverings, however,composed as they are of extraneous materials,and not being organic productions of the animalsthemselves, are structures wholly foreign to theirsystems. These inhabitants of tubes, the Tii

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1834

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