The animal inhabitant of the Pinna Marina is a blind slug or snail, which has many enemies, the worst of which is the cuttle-fish. No sooner does the pinna open his bivalve shell, than the cuttle-fish rushes in. How can such a blind defenseless creature secure food and protect himself?

Luckily there is a kind of crab-fish, with keen sight, who is a constant companion of the pinna. They live together in the shell that belongs to the pinna. When the latter is hungry, he opens his valves, and sends out his faithful companion to secure food. If any enemy approaches, the watchful crab dashes back to his blind protector, who quickly closes the valves as soon as his friend is inside.

On the other hand, when the crab has returned with food and no foe is about, he makes a gentle noise at the opening of the shell, which is closed during his absence. When admitted, the two friends then feast together on the fruits of the crab’s industry.