 The arthropod, an arthropod is an invertebrate animal having an exoskeleton external skeleton a segmented body, and paired jointed appendages. Arthropods formed the phylum Uarthropida, which includes insects, arachnids, myriapods, and crustaceans. The term arthropida as originally proposed refers to a proposed grouping of Uarthropods and the phylum Monocophora. Arthropods are characterized by their jointed limbs and cuticle made of chyrton, often mineralized with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. Their versatility has enabled them to become the most species rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living animal species, some of which, unlike most other animals, are very successful in dried environments. Arthropods range in size from the microscopic crustacean stygotin chelus up to the Japanese spider crab. Arthropods' primary internal cavity is a hemocel, which accommodates their internal organs, and through which their hemocleumph, analogue of blood, circulates. They have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system has ladder-like with-paired ventral nerve cords running through all segments and forming paired ganglio in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of a ganglio of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pick oceali. In most species the oceali can only detect the direction from which light is coming. And the compound eyes are the main source of information, but the main eyes of spiders are oceali that can form images and, in a few cases, can swivel to track praise. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many seeding bristles that project through their cuticles. Arthropods' methods of reproduction and development are diverse, altrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lamp jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from non-existent to the prolonged care provided by scorpions. The evolutionary ancestry of arthropods dates back to the Cambrian period. The group is generally regarded as monophytic, and many analyzes support the placement of arthropods with cycloneureliens who their constituent clades in a superphylum that is H.O.A. Overall, however, the basal relationships of metazoa are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. Arthropods contribute to the human food supplied both directly as food, and more importantly indirectly as pollinators of crops. Some species aren't known to spread severe disease to humans, livestock, and crops.