Page:Natural History, Mollusca.djvu/182

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170
PECTINIBRANCHIATA.—CYPRÆADÆ.

Some naturalists have supposed that the cowries, precluded, as it appears, (beyond a certain point) from enlarging their shells in the usual manner by the increase of the last whorl, have the power of forsaking their shells, and of forming new ones of larger size, as a crab or lobster sloughs its crust. Others believe that a process of gradual absorption and deposition will meet the necessities of the case, which, however, it must be confessed, presents considerable difficulty.

The earliest stage of life in these animals, as, we believe, in all the Gasteropoda, however diverse their adult condition may be, appears under the form of a nautilus-like shell, the inhabitant of which is furnished with two large-winged lobes, by which it is able to swim freely. Mr. Arthur Adams thus describes the young of one of the cowries:—"While staying at Singapore, I had an opportunity, in conjunction with Dr. Trail of that place, of observing the fry of Cypræa annulus, the species being then in spawn. Several specimens collected by us at low water, were seen to have conglomerated masses of minute transparent shells, adhering to the mantle and other parts of the animal, which masses, when placed in a watch-glass of salt water, under the microscope, became disintegrated, and detached individuals were perceived quitting the rest, and moving in rapid gyrations, with abrupt jerking movements, by means of two rounded flattened alar membranous expansions, reminding one of the motions of some of the Pteropods. When at rest, they joined the principal mass, or adhered, by means of their dilated expansions, to the surface of the watch-glass."[1]

  1. Zool. of Samarang, Part III. p. 23.