The phylum Arthropoda (Gr., arthros – jointed + podos = foot) is the largest group of the animal kingdom. Phylum Arthropoda surpasses all others both in number of individuals and in the diversity of their ecological distribution.
They have undergone an adaptive radiation for aerial, aquatic, terrestrial, and parasitic environments filling every conceivable ecological niche. More than 8, 00,000 species have been described, thus, Arthropoda are approximately 80 per cent of all known animals.
Phylum Arthropoda are bilaterally symmetrical, metamerically segmented Metazoa. Their metameres are not alike but are specialised, and their number is generally fixed. Either all or some segments bear paired appendages which are jointed, at least one pair of appendages function as jaws. The head is well developed.
The schizocoelic coelom is much reduced and replaced by a perivisceral haemocoel which is filled with blood. Circulatory system is of the open type. Coelom is replaced by a haemocoel because a hydraulic turgescence of the body wall is no longer needed, and a consequence of reduction of coelom is the loss of nephridia.
True nephridia are absent but coelomoducts act as gonoducts and often as excretory organs. Muscles are mostly striped, they are separate, unlike the continuous muscle layers of Annelida.
There are no cilia in the body. There is an exoskeleton made of chitinous cuticle which is often thick and hard, but in places on the trunk and limbs it is flexible to provide movable joints. The thick cuticle has brought about several changes, it causes a necessity of joints in limbs. In order to move the hard piece of exoskeleton the muscles are separate.
Thick cuticle prevents a loss of water, this has enabled Arthropoda to invade land, they are the only invertebrates which have adapted to living on land on a highly successful scale, hence, there are more terrestrial forms than in any other invertebrate phylum.
But the presence of a hard exoskeletal covering of cuticle has created two problems which had to be overcome in the evolution of arthropods, they are the problems of growth and locomotion.
Problem of growth has been solved by periodic moultings so that the animal can increase in size before the new cuticle becomes hardened.
The problem of locomotion was removed by division of the cuticle of the body and appendages to form separate plates or sclerites, with thin, soft and flexible articular membranes between any two sclerites which permit bending and locomotion. Arthropoda have evolved from annelids or at least from some common ancestor.