in zoology; a genus belonging to the order of vermes testacea. See MYTILUS, no. 6. The animal is a slug. The shell is bivalve, fragile, and furnished with a beard; gapes at one end; the valves hinge without a tooth. They inhabit the coasts of Provence, Italy, and the Indian ocean. The largest and most remarkable species inhabits the Mediterranean. It is blind, as are all of the genus; but furnished with very strong calcareous valves. The scuttle-fish (sepia), an inhabitant of the same sea, is a deadly foe to this animal; as soon as the pinna opens its shell, he rushes upon her like a lion; and would always devour her, but for another animal whom she protects within her shell, and from whom in return she receives very important services. It is an animal of the crab kind (see CANCER, no. 15.), naked like the hermit, and very quick-fighted. This cancer or crab the pinna receives into her covering; and when she opens her valves in quest of food, lets him out to look for prey. During this the scuttle-fish approaches; the crab returns with the utmost speed and anxiety to his hostess, who being thus warned of the danger shuts her doors, and keeps out the enemy. That very sagacious observer Dr Hasselquist, in his voyage towards Pinna-Palestine, beheld this curious phenomenon, which tho' well known to the ancients had escaped the moderns. Aristotle (Hist. lib. 5. c. 15.) relates, that the pinna kept a guard to watch for her: That there grew to the mouth of the pinna a small animal, having claws, and serving as a caterer, which was like a crab, and was called the pinnothylax. Pliny (lib. 9. 51.) says, the smallest of all the kinds is called the pinnoteres, and therefore liable to injury; this has the prudence to hide itself in the shells of oysters. Again, lib. 9. 66. he says, the pinna is of the genus of shell-fish; it is produced in muddy waters, always erect, nor ever without a companion, which some call the pinnoteres, others the pinnothylax. This sometimes is a small squill, sometimes a crab, that follows the pinna for the sake of food. The pinna, upon opening its shell, exposes itself as a prey to the smallest kind of fishes; for they immediately assault her, and, growing bolder upon finding no resistance, venture in. The guard watching its time gives notice by a bite; upon which the pinna, closing its shell, shuts in, kills, and gives part of whatever happens to be there to its companion.

The pinna and the crab together dwell, For mutual succour, in one common shell. They both to gain a livelihood combine; That takes the prey, when this has given the sign. From hence this crab, above his fellows fam'd, By ancient Greeks was pinnoteres nam'd.—OPPIAN.

The pinnae marinæ differ less from muscles in the size of their shells than in the fineness and number of certain brown threads which attach them to the rocks, hold them in a fixed situation, secure them from the rolling of the waves, especially in tempests, and assist them in laying hold of flume. See MYTILUS, p. 611. note (B). These threads, says Rondelet, are as fine, compared with those of muscles, as the finest flax is compared with tow. M. de Reaumur says, that these threads are nearly as fine and beautiful as silk from the silk-worm, and hence he calls them the silk-worms of the sea. Stuffs, and several kinds of beautiful manufacture, are made of these threads at Palermo; in many places they are the chief object of fishing, and become a silk proper for many purposes. It requires a considerable number of the pinnae marinæ for one pair of stockings. Nothing can equal the delicacy of this singular thread. It is so fine, that a pair of stockings made of it can be easily contained in a snuff-box of an ordinary size. In 1754, a pair of gloves or stockings of these materials was presented to Pope Benedict XIV. which, notwithstanding their extreme fineness, secured the leg both from cold and heat. A robe of the same singular materials was the gift of the Roman emperor to the Satraps of Armenia. See Procopius de Edif. lib. 3. c. 1. A great many manufacturers are employed in manufacturing these threads into various stuffs at Palermo and other places.

The men who are employed in fishing up the pinna marina, inform us, that it is necessary to break the tuft of threads. They are fished up at Toulon, from the depth of 15, 20, and sometimes more than 30 feet, with an instrument called a cramp. This is a kind of fork of iron, of which the prongs are perpendicular with respect to the handle. Each of them is PINNA about eight feet in length, and there is a space between them of about six inches; the length of the handle is in proportion to the depth of the water; the pinnae are seized, separated from the rock, and raised to the surface by means of this instrument. The tuft of silk issues directly from the body of the animal; it comes from the shell at the place where it opens, about four or five inches from the summit or point in the large pinna.

M. de Renumur, Mem. de l'Acad. des Sciences, 1711, page 216, and 1717, page 177, considers the pinna as the most proper of all shell-fish to elucidate the formation of pearls. It produces many of them of different colours, as grey or lead-coloured, red, and some of a blackish colour, and in the form of a pear.

M. d'Argenville distinguishes three kinds of the pinnae: 1st, The large kind, which are red within, and which have reddish mother-of-pearl, similar to the substance of the shell itself. There are of those shells which weigh near 15 pounds. This is the affluence of the Venetians.

2d, The smaller kind. Some of these are slender, paperyaceous, of the colour of horn, a little shaded with pale red.

3d, The kind called perna. These are adorned with points in the channels of their shell; but what is very singular, the edges of the shell are thicker at the openings than at the joining of the valves.

The animal which lodges in the pinna marina rarely shows itself, because the valves are seldom opened. Its head is below, its largest extremity opposite; it is kept in the shell by four vigorous muscles, placed at the extremities of the valves; the shell has no hinges, but a flat and blackish ligament, which is equal in length to one-half of the shell. See PINNOCERUS and PEARL.