in zoology, a genus belonging to the order of vermes tectacea. The animal is a slug. The shell is bivalve, fragile, and furnished with a beard; fig. 2, gapes at one end; the valves hinge without a tooth. 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 Haffelquist, in his voyage towards 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 pinnoterex, 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 pinnoterex, others the pinnothylax. This sometimes is a small eel, sometimes a crab, that follows the pinna for the sake of food. The pinna is blind; and when, upon opening its shell, it exposes itself as a prey to the smallest kind of fishes, these 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 pinnoterex nam'd.