Page:Natural History, Mollusca.djvu/315

 matter are illuminated while the back-ground is dark, and these by their motion clearly reveal the currents of the fluid in which they are suspended. A few moments of practice will enable even an unaccustomed eye to perceive the atoms converging from all points around, with an even but increasing velocity, towards the principal tube, down which they disappear like the streams of passengers and traffic in the neighbourhood of a great city, converging towards it as to a common centre of attraction by a hundred different routes. The current of the expelling tube is even still more marked in its character; a forcible jet of water is periodically ejected from this orifice, which draws the surrounding particles into its vortex, and shoots them forward to a distance of many inches. It is by the expulsive force of this anal current, chiefly, that the passage is kept free from the deposit of mud and other substances, which would otherwise soon choke it up.

A fresh, supply of water for respiration, and its dismissal when no longer fit for use, are efficiently provided for by this contrivance. But since many particles of matter float in the water, which from their form or other qualities might be hurtful to the delicate tissues of the viscera to be traversed, how is the entrance of these to be guarded against in an indiscriminating current? A beautiful contrivance is provided for this necessity. The margin of the entering siphon, and sometimes, though more rarely, of the ejecting one, is set round with a number of short tentacular processes, varying indeed in their length, but the longest scarcely more than equalling half the diameter of the mouth of the tube. In Saxicava rugosa, which bores