Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . 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
Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . 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. 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. 114. ANNELIDA. 2^7 only, the Scrpnla (Fig. 133), forms for itself anexternal shell, which is shaped into a spiral , 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 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
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