The Arthropods are invertebrates protostome coelom gathered in the phylum Arthropoda (Latreille 1829), includes about 5 / 6 species previously classified (2005). They show a remarkable affinity with the anatomical Annelids. The fact that they have been described more than a million species of arthropods shows that their basic structure is versatile and adaptable to different ways of life.
Morphology
Certain characteristics distinguish arthropods. A cuticle is an exoskeleton that sclerificata low density, elastic and stiff enough to protect and support the body. It is composed of chitin, a complex organic material produced by the cells immediately below. The exoskeleton does not contain cells so it is not able to grow with the animal and therefore the increase is to mute. The exoskeleton is not continuous but consists of plates (sclerites) joined by sections of the cuticle not sclerificata (pleura) that make up the joints (so that the movements would be impeded by a rigid exoskeleton continuous). A heteronomous metamerism that involves the whole body, which is formed by segments (also known as somites or somites). In more primitive forms, the metamerism omonoma tends to be (as in the Annelida) in the course of evolution the various segments are always grouped together to form more diverse regions of the body or Tagma (heteronomous metamerism, the rule for arthropods). In particular, there is a division of the body in at least two regions: the prosoma and opistosoma Chelicerata, head and trunk in the mandible (the set of Crustacea, Myriapoda and Hexapoda) more primitive than in the mandible has evolved is a division into three regions: head, thorax and abdomen (for differentiation of the trunk, in the three regions are called crustaceans Cephalon, pereion and Pleon). Of jointed appendages, from which the name of the phylum. The appendices do not always contain muscles, they are often moved by tendons attached to muscles that are found in the trunk. The most primitive of appendages are the legs, which in more primitive forms are present in all segments. With differentiation, the legs tend to remain only in the thoracic segments. The appendices of the other areas of the body disappear or transform themselves to carry out other functions. The cavities are modified to form a emocele. This cavity is bounded by its walls and located just around the gut, as the typical body cavities, but tends to reach many parts of the body. Drift during embryonic development, in part by coelomic pockets that form and in part to a remnant of the blastocoel to flaking of celoteli and mixing of the two chambers: This type of cavity is said Sinceloma meaning mixed or Mixocele. The liquid dell'emocele, called hemolymph, is the same that flows through the circulatory system, which is open, and performs all the functions of transport. The respiratory system is very efficient, allowing energetically costly activities such as flying. Aquatic respiration in arthropods occurs through the gills, which protrude outward. In the terrestrial ones, there are two main types of systems through small branches called tracheae, which protrude outwards through tiny pores called spiracles (like insects), or through special structures called "book lungs", consisting of sheets folded to 'inside the body to increase the area devoted to gas exchange (like spiders).
Subphyla: subphylum Trilobitomorpha (extinct)-subphylum subphylum Crustacea-Trace-subphylum Chelicerata
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