Page:Natural History, Mollusca.djvu/60

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the power of forming threads of silky substance much stronger and more durable than those of our pond snails. The Common Mussel (Mytilus edulis) is one of these marine silk-worms; and we have a good many others. The bundle of threads, familiar to many of my readers as the beard of the shell-fish, is the substance in question, termed by naturalists byssus, a Greek word originally signifying silk; and the use to which it is applied by the animal itself is that of a cable to moor itself to the solid and immovable rock, that it may not be washed away by the violence of the waves. The mode in which the threads are formed, and the organ by which they are secreted, are thus described by Professor Rymer Jones:—

"The foot in the Mussel is of small dimensions, being useless as an instrument of progression. By its inferior aspect it gives attachment to the horny threads of the byssus, which are individually about half an inch in length, or as long as the foot itself, by which, in fact, they are formed, in a manner quite peculiar to certain families of Conchifera; no other animals presenting a secreting apparatus at all analogous, either in structure or office, to that with which these creatures are provided. The manner in which the manufacture of the byssus is accomplished is as follows: A deep groove runs along the under surface of the foot, at the bottom of which thin horny filaments are formed by an exudation of a peculiar substance, that soon hardens and assumes the requisite tenacity and firmness. While still soft, the Mussel, by means of its foot, applies the extremity of the filament, which is dilated into a kind of little sucker, to the foreign substance whereunto it wishes to adhere,