Hermit Crab, inhabiting a whelk shell.
Hermit Crabs found on the beach are soft-bellied marine anthropoda which live in empty snail shells. (There are actually hundreds of different species of hermit crabs.) The crabs choose to live in “previously-owned homes” because, since only their head and claws are calcified, they need protection. The last two pairs of a Hermit Crab's legs are modified so that it can hold itself inside its adopted shell. In fact, if you wanted to pull a Hermit Crab out of a shell, you probably couldn't because they hold on so tightly.
The crab has two pairs of “walking feet” which enable it to drag around a house shell five times the size of its body. The front claws are unequal in size, one being larger than the other. For protection, the Hermit can fold its big claws, making a barrier, or “door” to the shell which it inhabits. These large claws are also handy for tearing apart the Hermit's food.
As a Hermit grows larger, he needs a bigger house. He will leave his old home and look for a suitable new house of the right size. To find a house shell he likes, he probes empty shells with his long tentacles, even cleaning up by whisking out debris. He'll go in and try it out — if the shell is unsuitable (too heavy, for example), he will continue his search.
In spite of their name, Hermit Crabs are social animals, and typically live in colonies of more than a hundred.
Omnivorous Hermit Crabs are beach “cleaner-uppers,” scavenging up dead animals. They also are predators of small marine creatures, including other Hermit Crabs when those Hermits are between shells. They frequent the ocean, of course, since they are marine crabs. (Hermit Crabs sold in pet stores are usually land-oriented animals, different species from the Hermit Crabs seen in the surf's edge.)
Hermit Crabs are fit for human consumption.