On some of the abdominal appendages book gills are exposed. Horseshow crabs can swim awkwardly by means of their abdominal plates and can walk on their walking legs . They feed at night on worms and small molluscs and are harmless to humans. Class Arachnida - Arachnids are a numerous and diverse group, with over 50,000 species described so far. Subphylum Crustacea - The 30,000 or more species of Crustacea are mainly marine, and are the only arthropod group that is primarily aquatic.
Crustaceans are also the only arthropods with two pairs of antennae .
In addition to the antennae and mandibles, they have two pairs of maxillae on the head, followed by a pair of appendages on each body segment. Class Branchiopoda - Branchipods have reduced first antennae and second maxillae. Their legs are flattened and leaflike and are the chief respiratory organs. The most important and successful order is the Cladocera, which often forms a large segment of the freshwater zooplankton. The class Malacostraca is the largest class of Crustacea and shows great diversity.