Page:Natural History, Mollusca.djvu/314

 interior of these tubes is said to be lined with innumerable delicate cilia; by the action of which the surrounding water is drawn towards the entering orifice, and conveyed in a strong current through the tube over the surface of the gills. Then, having been deprived of its oxygen, it is poured through the other tube and expelled in a jet at its extremity, by a similar machinery.

This apparatus of double siphonal tubes is principally developed in those species which burrow, whether in sand, mud, wood or stone. As the burrowing bivalve usually, if not always, dwells in the interior of the passage it has excavated, it is needful that there should be a communication with the external water, and hence a hole is always found extending to the surface of the material bored. The entering and departing currents keep this passage clear, a process which in mud or sand might seem at first not very easy of accomplishment. It is facilitated, however, by the faculty which the boring bivalves have of lengthening the siphonal tubes at will; and the degree to which this may be accomplished depends on the depth of the cavity which the species is accustomed to make.

If we take one of the stone-boring Mollusca, a Pholas or a Saxicava for example, from its excavation, without injuring the animal, and place it in a glass vessel of sea-water, it will not be difficult to detect the currents in question, even with the naked eye; though a lens of moderate power will render them more distinctly appreciable. The vessel should be so placed as that the light may be nearly, but not exactly, opposite to the eye. By this arrangement the minute atoms of floating