Anarthropodis aninvertebrateanimalhaving anexoskeleton(externalskeleton), a segmented body, and jointedappendages. Arthropods are members of thephylumArthropoda(fromGreekἄρθρονárthron, "joint", andποδόςpodós"leg", which together mean "jointed leg"), and include theinsects,arachnids, andcrustaceans. Arthropods are characterized by their jointed limbs andcuticles, which are mainly made of α-chitin; the cuticles of crustaceans are alsobiomineralizedwithcalcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically bymoulting. The arthropodbody planconsists of repeatedsegments, each with a pair ofappendages. They are so versatile that they have been compared toSwiss Army knives, and it has enabled them to become the most species-rich members of allecological guildsin most environments. They have over a million described species, making up more than 80% of all described living animal species, and are one of only two animal groups that are very successful in dry environments – the other being theamniotes. They range in size from microscopicplanktonup to forms a few meters long.
Arthropods' primary internal cavity is ahemocoel, which accommodates their internalorgansand through which theirbloodcirculates; they haveopen circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Theirnervous systemis "ladder-like", with pairedventralnerve cordsrunning through all segments and forming pairedgangliain each segment. Their heads are formed by fusion of varying numbers of segments, and theirbrainsare formed by fusion of the ganglia of these segments and encircle theesophagus. Therespiratoryandexcretorysystems of arthropods vary, depending as much on their environment as on thesubphylumto which they belong.
Their vision relies on various combinations ofcompound eyesand pigment-pitocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes ofspidersare ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the manysetae(bristles) that project through their cuticles.
Arthropods' methods of reproduction and development are diverse; all terrestrial species useinternal 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 orexternal fertilization. Almost all arthropods lay eggs, butscorpionsgive birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs andcaterpillarsthat lack jointed limbs and eventually undergo a totalmetamorphosisto produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions.
The versatility of the arthropod modular body plan has made it difficult forzoologistsandpaleontologiststo classify them and work out their evolutionary ancestry, which dates back to theCambrianperiod. From the late 1950s to late 1970s, it was thought that arthropods werepolyphyletic, that is, there was no single arthropod ancestor. Now they are generally regarded asmonophyletic. Historically, the closest evolutionary relatives of arthropods were considered to beannelidworms, as both groups have segmented bodies. This hypothesis is now largely rejected, with annelids andmolluscsforming the superphylumLophotrochozoa. Many analyses support a placement of arthropods withcycloneuralians(or their constituent clades) in a superphylumEcdysozoa. Overall however, thebasalrelationships of Metazoa are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated.
Arthropods contribute to the human food supply both directly as food, and more importantly aspollinatorsof crops. Some specific species are known to spread severe disease to
