PINNA, in Zoology, a genus of shell-fish belonging to the order of vermes testacea. See CONCHOLGY Index.—Pliny, who gives some account, perhaps not very correct, of the history of some of the species of this genus, (lib. ix. 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. ix. 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 pinnophylax. 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, fluts 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.