Whenever the pinna ventures to open its shell, it is immediately exposed to the attacks of various of the smaller kinds of fish, which, meeting with no resistance to their first assaults, acquire boldness and venture in. The vigilant guard, by a gentle bite, gives notice of this to his companion, who, upon such a hint, closes her shell, and having thus shut them in makes a prey of those who had come to prey upon her: when thus supplied with food, she never fails to share her booty with so useful an ally. We are told that the sagacious observer, Dr. Hasselquist, in his voyage, (about the middle of the last century,) to Palestine, which he undertook for objects connected with the study of natural history, beheld this curious phenomenon, which, although well known to the ancients, had escaped the attention of the moderns. It is related by Aristotle[185] that the pinna keeps a guard to watch for her, which grows to her mouth, and serves as her caterer: this he calls pinnophylax, and describes as a little fish with claws like a crab. Pliny observes[186], that the smallest species of crab is called the pinnotores, and being from its diminutive size liable to injury, has the prudence to conceal itself in the shells of oysters. In another place he describes the pinna as of the genus of shell-fish, with the further particulars that it is found in muddy waters, always erect, and never without a companion, called by some pinnatores, by others pinnophylax; this being sometimes a small squill, and at others a crab, which remains with the pinna for the sake of food. [185] Hist. lib. v. c. 15. [186] Lib. ix. 51. 66. The description of the pinna by the Greek poet Oppianus, who flourished in the second century, has been thus given in English verse:-- The pinna and the crab together dwell, For mutual succor 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 famed, By ancient Greeks was Pinnotores named. It is said that the pinna fastens itself so strongly to the rocks, that the men employed in fishing for it are obliged to use considerable force to break the tuft of threads by which it is secured fifteen, twenty, and sometimes even thirty feet below the surface of the sea.