The Pinna is a bivalve[183] shell-fish, which, when full grown, is 18 inches long, and 6 wide at its broad end. It is found near the shores of South Italy, Sicily, Corsica, and Sardinia; also in the Bay of Smyrna, and in the Indian Ocean. It does not fasten itself to rocks in the same position as the muscle, but sticks its sharp end into the mud or sand, while the rest of the shell is at liberty to open in the water. In common with the muscle, it has the power of spinning a viscid matter from its body, conformably with that of the spider and caterpillar. Although the pinna is vastly larger than the muscle, its shell being sometimes found two feet long, the threads which it produces are more delicate and slender than those of the muscle, being in fineness and beauty scarcely inferior to the single filament of the comparatively minute silk-worm. Threads so delicately thin, as may readily be imagined, do not singly possess much strength; but the little power of each is made up by the aggregate of the almost infinite number which each fish puts forth to secure itself in a fixed situation, and preserve it against the rolling of the waves. The threads are, however, similar in their nature to those of the muscle, differing only in their superior fineness and greater length. These fish have, therefore, been distinguished by some naturalists, the one as the silk-worm, the other as caterpillar of the sea. [183] An animal having two valves, or a shell consisting of two parts which open and shut. It has been from a very remote period well known, that muscles have the power of affixing themselves either to rocks or the shells of one another, in a very firm manner; yet their method of effecting this was not understood until explained by the accurate observations of M. Réaumur, the _first_ naturalist who ascertained that if, by any accident, the animals were torn from their hold, they possessed the power of substituting other threads for those which had been broken or injured. It was found by him, that if muscles, detached from each other, were placed in any kind of vessel and then plunged into the sea, they contrived in a very short time to fasten themselves both to the vessel’s side and one another’s shells: in this process, the extremity of each thread seemed to perform the office of a hand in seizing upon the body to which it would attach itself.