The Ianthinæ, or purple Sea-Snails, carry under their foot a vesicular organ like a congeries of foam-bubbles, that prevents creeping, but serves as a buoy to support them at the surface of the water.
When the sea is quiet, these little creatures,
Like little wanton boys that swim on bladders,
appear in vast shoals on the surface, but as soon as the wind ruffles the ocean, or an enemy approaches, they at once empty their air-cells, contract their float, and sink to the bottom, pouring out at the same time a darkened fluid like that of the Aplysia or the Murex, which no doubt serves them as a defence against their foes, and, according to Lesson, furnished the celebrated purple of the ancients. The Ianthinæ inhabit the Mediterranean and the warmer regions of the Atlantic, but especially towards the close of summer they are frequently drifted by the Gulf Stream to the west coast of Ireland.
While the vast majority of the gasteropods either creep or swim, some are doomed to the sedentary life of the oyster, and remain for ever fixed to the spot where they first attached themselves as small free-swimming larvæ. Thus the _Magilus antiquus_, which in its young state presents all the characters of a regular spiral univalve, establishes itself in the excavations of madrepores, and as the coral increases around it, the Magilus is obliged, in order to have its aperture on a level with the surrounding surface, to construct a tube, lengthening with the growth of the coral. As the tube goes on increasing, the animal abandons the spiral for the tubular part of the shell, and in the operation it leaves behind no partitions, but secretes a compact calcareous matter which reaches to the very summit of the spiral part, so that in an old specimen the posterior part of the shell presents a solid mass.
The Siliquariæ are generally found embedded in a similar manner in sponges or other soft bodies, while the Vermetus, or Worm-Shell, usually attaches itself, like the Serpulæ, to rocks, coral-reefs, or shells.
In these genera, which have been arranged by Cuvier in a separate order (Tubulibranchiata), the foot is naturally reduced to the state of an adhesive organ, its chief functions consisting in opening and closing the lid.