Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an asymmetric abdomen concealed by a snug-fitting shell. Hermit crabs' soft (non-calcified) abdominal exoskeleton means they must occupy shelter produced by other organisms or risk being defenseless.
Hermit crab
A hermit crab from Chabahar, Iran
Four hermit crabs in an aquarium
A hermit crab emerges from its shell, Coenobita perlatus
Anomura is a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans include the word crab, all true crabs are in the sister group to the Anomura, the Brachyura.
Anomura
Image: Aegla sp. from Nahuel Huapi Lake, Bariloche, Argentina
Image: Eumunida picta
Image: Munidopsis tridentata