File:Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget (1834) (14775799001).jpg

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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ting sur-face, which are in the interior of the tube, theinternal space is gradually filled up by thesedeposits; the process of the mantle retiring tomake way for their advance towards the axis ofthe tube. In the course of time, every part ofthe cavity is obliterated, the process of the shellbecoming entirely solid. Such is the origin ofthe many curious projecting cones or spineswhich several shells exhibit, and which havearisen periodically during their growth fromtheir outer surface. In the Murex these pro- 246 THE MECHANICAL FUNCTIONS. cesses are often exceedingly numerous, and occurat regular intervals, frequently shooting out intovarious anomalous forms. In many shells ofthe genus Strombus these spines are of greatlength, and are arranged round the circumfe-rence of the base, being at first tubular, andafterwards solid, according to the period ofgrowth. This is exemplified in the PteroceraScorpio (Lamarck) of which Fig. 110 shows theearly, and Fig. Ill the later period of growth.
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A limit has been assigned by nature to thegrowth of molluscous animals, and to the shellswhich they form : and there is a certain epochof their existence, when considerable changestake place in the disposition of the mantle,and in its powers of secretion. Often we findit suddenly expanding into a broad surface,adding to the shell what may be termed a largelip. Sometimes no sooner has this been accom- FORMATION OF SHELLS. 247 plished than the same part again shrinks, andthe mantle retires a little way within the shell,still continuing to deposit calcareous layers,which give greater thickness to the adjacentpart of the shell: and at the same time narrowits aperture, and materially alter its generalshape and aspect. Thus it happens that theshells of the young and of the old individuals ofthe same species are very different, and wouldnot be recognised as belonging to the same tribeof mollusca. This is remarkably the case withthe shell of the Cyprcea, or Cowrie, which in theearly stage of

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