Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . ile in the Star-fish, and Echinus, nothing inpoint of situation was definite, excepting theupper and the lower surface, and there was noside which could be exclusively denominated ANNELIDA. 271 either the right or the left side, and no end thatcould be properly said to be the front, or theback, in Articulated, as well as in Vertebratedanimals, all these distinctions are clearly markedand easily defined. In all the Annelida the firmest parts of thebody, or those which give mechanical support
Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . ile in the Star-fish, and Echinus, nothing inpoint of situation was definite, excepting theupper and the lower surface, and there was noside which could be exclusively denominated ANNELIDA. 271 either the right or the left side, and no end thatcould be properly said to be the front, or theback, in Articulated, as well as in Vertebratedanimals, all these distinctions are clearly markedand easily defined. In all the Annelida the firmest parts of thebody, or those which give mechanical support tothe rest, are external, and may be regarded eitheras appendages to the integuments, or as modifi-cations of the integuments themselves. Theyconsist of a frame-work, composed of a series ofhorny bands or rings: their assemblage havingmore or less of a lengthened cylindric shape,and constituting a kind of external skeleton,which encloses all the other organs. This isexemplified in the earth-worm ; in the Pont-ohdella (Fig. 128), which is a species of leech ;and in the Nereis (Fig. 129). These rings. give rise to the division of the body into asmany different segments. In some cases, how-ever, we find all these rings compressed into 272 THE MECHANICAL FUNCTIONS. the form of a flat oval disk. This is the case inthe EryohdeUa, of which Fig. 130 is an enlargedrepresentation. In general, the first of the segments intowhich the body is divided, contains the prin-cipal organs of sense, and is sufficiently distinctfrom those which follow to entitle it to the ap-pellation of the head; while the lengthened pro-longation of the opposite extremity, when suclia form is present, may be denominated thetail. The rings which encircle the body are con-nected laterally by a looser and more flexibleportion of integument, and also by layers ofmuscular fibres, curiously collected into muscular flesh of insects, and other animalsof this class, differs much from that of the largeranimals, b
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