Pea Crabs.
The fact that these small crabs take up their abode within the shells of mollusks was well known to the ancients, and gave rise to many curious fables. A species is very common in the pinnæ (mollusks) of the Mediterranean, and was imagined to render important services to its host in return for its lodging, keeping a lookout for approaching dangers, against which the blind pinna itself could not guard, and particularly apprising it, that it might close its shell when the cuttle-fish came near. It is curious to find this repeated by Hasselquist, in the middle of the last century, as a piece of genuine natural history. Whether the pea crab lives at the expense of the mollusk, and sucks its juices, is uncertain. It is certain, however, that the flesh of such mollusks is palatable to pea crabs, as they eat it greedily in the aquarium.