The distinguishing feature of arthropods, and one to which many other evolutionary innovations are related, is the chitinous exoskeleton or cuticle. This covers the entire body and is divided into inter-connecting plates which permit movement. Growth is facilitated by periodic moulting.
The phylum consists of insects, spiders, and crustaceans; their existence is threatened by such factors as: fishing; damage to natural habitats; agricultural, commercial and residential development; exploitation for biomedical use; and the demand for unusual and exotic pets.