Page:Natural History, Mollusca.djvu/152

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140
TECTIBRANCHIATA.—APLYSIADÆ.

large vessel which collects and receives the venous blood from all parts of the system, and carries it forward to the gills, was believed to have numerous perforations in its walls, through which a free communication existed with the general cavity of the abdomen, so that the fluids contained in the one could readily permeate the other.[1] But it has been since proved that these supposed perforations are merely depressions, and that the lining membrane of this great blood-vessel is entire, as in other animals.[2]

The Mollusca of this Order undergo a metamorphosis exactly similar to that already described in the Nudibranchs; there is, in fact, scarcely any appreciable difference in the form of the newly-hatched young in either of these Orders, in that of the Pectinibranchs, and in the Class Pteropoda. How long the infant animal remains in this, its first condition, is not yet ascertained. Arrived at the second stage, we find it still enclosed in its transparent and nautilus-like shell; but the mantle has become detached, and covers tightly the mass of the viscera. The foot is so enlarged, that it forms a considerable projection beyond the margin of its operculum; and the veils have also grown in size, while the eyes have altered to a violet colour. The head has now two short, conical, ciliated tentacles, and the little animal swims with surprising quickness.

In the third stage the shell has fallen off, and the general shape is that of the parent, but the veils still remain. In the fourth stage, the creature begins to crawl in the gasteropod fashion, and the branchiæ and cæca begin to sprout. There are now,