A manual of the Mollusca, or, A rudimentary treatise of recent and fossil shells . shell, and hence the/rj^ are apt to be mistaken for distinct speciesfrom their parents. In cymha (fig. 20) the nucleus is large and irregular; mfususantiquus it is cylindrical; in the pyramidellidce it is oblique; andit is spiral in carinaria, atlanta, and many limpets, which aresymmetrical when adult. The rudimentary shell of the nudibrancTis is shed at an early * In its most reduced form the shell is only a hollow cone, or plate, pro-tecting the breathing organ and heart, as in Umax, testacella, carinaria. Its
A manual of the Mollusca, or, A rudimentary treatise of recent and fossil shells . shell, and hence the/rj^ are apt to be mistaken for distinct speciesfrom their parents. In cymha (fig. 20) the nucleus is large and irregular; mfususantiquus it is cylindrical; in the pyramidellidce it is oblique; andit is spiral in carinaria, atlanta, and many limpets, which aresymmetrical when adult. The rudimentary shell of the nudibrancTis is shed at an early * In its most reduced form the shell is only a hollow cone, or plate, pro-tecting the breathing organ and heart, as in Umax, testacella, carinaria. Itspeculiar features always relate to the condition of the breathing-organ; and interehratula ani pelonaia it becomes identified with the gill. In the nudi-brauchs the vascular mantle perfomis wholly or in part the respiratory the cephalopods the shell becomes complicated by the addition of a distinct,internal, chambered portion (phrapnocone), which is properly a visceralskeleton; in spirula the shell is reduced to this part. STEUCTURE AND PHYSIOLOGY OF THE MOLLUSC A. 37. age, and never replaced. In this respect tliemolluscan shell differs entirely from the shellof the crab and other articulate animals,which is periodically cast oif and renewed. In the bivalves the embryonic shell formsthe umbo of each valve ; it is often very unlikethe after-growth, as in unio pidorum, cyclashenslowiana and pecten piisio. In attachedshells like the oyster and anomia the umbo fre-quently presents an exact imitation of the sur-face to which the young shell originally adhered. Shells are composed of carbonate of lime,with a small proportion of animal source of this lime is to be looked ^^S- 20. Cymha*for in their food. Modern inquiries into organic chemistry haveshown that vegetables derive their elements from the mineralkingdom (air, water, and the soil), and animals theirs from thevegetable. The sea-weed filters the salt-water, and separateslime as well as organic elements; an
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectmollusks, bookyear185
