There are far more species of arthropods than species in all other phyla combined, and the number of undescribed species in the largest assemblage of arthropods, the insects, probably numbers in the tens of millions. Members of the phylum have been responsible for the most devastating plagues and famines mankind has known. Yet other species of arthropods are essential for our existence, directly or indirectly providing us with food, clothing, medicines, and protection from harmful organisms. The systematic relationships of arthropod groups is not fully understood, which is not surprising given the size and diversity of the phylum. A number of important characteristics are shared by most members of this phylum.
Arthropods are bilaterally symmetrical protostomes with strongly segmented bodies.
Segmentation affects both external and internal structure. Arthropods generally grow by molting their exoskeletons in a process called ecdysis. Movement of appendages is controlled primarily by a complex muscular system, divided into smooth and striated components as in chordates. Cilia are not present. Arthropods are eucoelomate with the coelom formed by schizocoely, but the volume of the coelom is much reduced and usually restricted to portions of the reproductive and excretory systems.