A manual of the Mollusca, or, A rudimentary treatise of recent and fossil shells . antle to unite withthose previously formed. Being extra-vascular it has no inherentpower of repair. {Carpenter.) The epidermis and cellular structures are formed by the mar-gin (or collar) of the mantle; the membranous and nacreouslayers, by the thin and transparent portion which contains theviscera; hence we find the pearly texture only as a lininginside the shell, as in the nautilus, and all the aviculida andturbinida. If the margin of a shell is fractured during the life-time of theanimal, the injury will be
A manual of the Mollusca, or, A rudimentary treatise of recent and fossil shells . antle to unite withthose previously formed. Being extra-vascular it has no inherentpower of repair. {Carpenter.) The epidermis and cellular structures are formed by the mar-gin (or collar) of the mantle; the membranous and nacreouslayers, by the thin and transparent portion which contains theviscera; hence we find the pearly texture only as a lininginside the shell, as in the nautilus, and all the aviculida andturbinida. If the margin of a shell is fractured during the life-time of theanimal, the injury will be completely repau-ed by the reproductionboth of the epidermis and of the outer layer of shell with its pro-per colour. But if the apex is destroyed, or a hole made at adistance from the aperture, it will merely be closed with thematerial secreted by the visceral mantle. Such inroads are oftenmade by boring worms and shells, and even by a sponge (cliona)which completely mines the most solid shells. In Mr. Grayscabinet is the section of a cone, in whose apex a colony of lltJiodomi. ¥ig. 25. Section of a cone perforated by hthochmi. STRUCTURE AIND PHYSIOLOGY OF THE MOLLUSC A. A3 had settled, compelling the animal to contract itself, faster eventhan it could form shell to fill up the void. Lines of growth. So long as the animal continues grooving,each new layer of shell extends beyond the one formed before it;and, in consequence, the external smface becomes marked withlines of groictJl. During winter, or the season of rest which cor-responds to it, shells cease to grow; and these periodic resting-places are often indicated by interruptions of the otherwise regu-lar lines of growth and colour, or by still more obvious signs. Itis probable that this pause, or cessation from growth, extendsinto the breeding season ; otherwise there would be two periodsof growth, and two of rest in each year. In many shells thegTOwth is uniform; but in others each stage is finished by thedevelopment of
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectmollusks, bookyear185
