Crustaceans (Crustacea) - class invertebrates type arthropods (Artliropoda), uniting about 20 thousand species and divided into two subclasses: lower cancers (Entomostraca) and higher cancers (Malacostraca). Crustaceans - mainly water organisms inhabiting as the bottom of water basins and the water column. A ground-dwelling way of life of crustaceans adapted only woodlice, "sea fleas" and some tropical decapods.
Crustaceans are intermediate hosts of a number of worms. Thus, from the lower crustaceans intermediate hosts the worm (Dracunculus medinensis) and wide lentetsa (Diphyllobothrium latum) serve as Cyclops and Diaptomus. From the highest cancers medical importance decapods (Decapoda). Freshwater crab (Eriocheir japonicus) is the intermediate host lung Fluke (Paragonimus westermani).
Some parasitic crustaceans harm the fish.
Many RV used for human food (lobsters, lobsters, crabs, shrimp, crayfish). Some R., accounting for the bulk of plankton, serve as food for fishes. R. facilitate purification of water bodies as eat the corpses of dead animals.