Known as wood lice, sow bugs, or pillbugs, these small animals can be found under the bark of trees, beneath stones and rocks, and in other damp places. When disturbed they curl up armadillolike, withdrawing into the exoskeleton. The free-swimming larva characteristic of crustaceans, called a nauplius larva, has an unsegmented body, a median eye, and three pairs of appendages. In the majority of larger crustaceans the head and thorax are fused into a cephalothorax, which is protected by a large shieldlike area of the exoskeleton called the carapace. Crustacean appendages have undergone extensive adaptation for various tasks such as swimming, sensory reception, and walking.
Many species have the first pair of thoracic appendages modified into claws and pincers.
The gills are generally attached at the bases of the thoracic appendages, and the beating of the appendages creates a flow of water over the gills that facilitates respiration. Reproduction is sexual, and in most forms the sexes are separate. In many species the eggs are brooded beneath the abdominal segments of the female. Crustaceans constitute the subphylum Crustacea of the phylum Arthropoda .