The phylum of arthropods has the largest number of species known on earth today. It is the phylum of the ubiquitous insects and even trilobites. Arthropods belong to the superphylum Ecdysozoa. They are protostomians. They are bilaterally symmetrical animals with segmentation called somites. They have paired limbs, chitinous cuticle as an exoskeletal covering and a reduced coelom which extends into a haemocoele. They exhibit tagmatosis which is the formation of tagmata (divisions) of the limbs. There is also conspicuous cephalization.
They are usually divided into six classes:
a) Onychophora e.g. Peripatus
b) Trilobites which are fossil arthropods e.g. Olenus
c) Crustacea e.g. Astacus
d) Myriapoda e.g. Lulus
e) Insecta e.g. Aedes aegypti
f) Arachnida e.g. Spider Araneus diadematus