Paguridae and Diogenidae - Hermit Crabs
Hermit crabs are not considered to be "true" crabs. They lack an external shell on their soft abdomen which leaves them vulnerable to predators. To protect themselves they live in abandoned gastropod (snail) shells. Their last 2 pair of legs are small and modified and along with its uropods (appendages at the end of the abdomen) are used to clamp onto the internal whorls of the shell. Trying to remove a hermit crab from his home is difficult without tearing the crab apart. The whole body can be retracted into the shell, when needed. Eventually the hermit crab will outgrow his shell and will search for a new one. Each hermit crab has a pair of claws and 2 pair of walking legs (5 pair of leg appendages in all).
Hermit crabs in the families Paguridae and Diogenidae are all aquatic marine crabs. The terrestrial hermit crabs found in many pet stores belong to another family and can be found in Florida and the Caribbean areas. Paguridae are called the right-handed hermit crabs because the right claw is usually larger than the left claw. Diogenidae are left-handed hermits because the left claw is usually the largest. Unfortunately, these are generalizations and don't always hold true, so it's not really a good way to distinguish families. Most hermit crabs off Texas can be identified by their size, color and shape of their claws.