anomura

from The Century Dictionary.

• A suborder of podophthalmous decapodous crustaceans, intermediate between the macrurous and the brachyurous groups, and embracing forms, such as the hermit-crabs, which have the tail soft and unfitted for swimming or otherwise anomalous.

from the GNU version of the Collaborative International Dictionary of English.

• noun plural (Zoöl.) A group of decapod Crustacea, of which the hermit crab in an example.