Page:Natural History, Mollusca.djvu/197
becoming thinner and more convex. He found that so long a time as four months elapsed before the vesicle opened, and then the included whelklings did not quit their cradle all at once, but took their time in coming out, according to their individual dispositions; doubtless, the quick-minded and more curious commencing their travels first, whilst those of slow and studious constitutions would remain as long as a fortnight before resolving to see the world, which with young Purpuræ is no very dangerous adventure, since the neighbouring barnacles enable them to look about with safety, before making a long journey from their birthplace.
A small and unimportant group is indicated by this name, represented in Britain by two genera, each consisting of two species. They have a shell, the aperture of which is very broad and open, and the spire minute; in texture it is thin, sometimes pellucid, and sometimes even membranaceous. In one genus it is entirely included within the substance of the mantle, as in Pleurobranchus: in the other it is external, but partially invested by the edges of the mantle, and covered with a skin (epidermis).
The animal is large, with a short broad head, furnished with two tentacles, and eyes at the exterior of their bases. There are two gill-plumes. The operculum is wanting.Our most common species is Velutina lævigata, reckoned by Linnæus among the snails, and long supposed to be a fresh-water mollusk; it is, how-
- Cited in Forbes and Hanley, iii.384.