The arthropoda constitute a vast assemblage of invertebrates comprising about two million species; several times the number of all other animal species combined. Their adaptive diversity has enabled representatives to survive in virtually every habitat, and in many respects they are the most successful invaders of the land.
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.