arthropod - Basic arthropod structure, Classification of arthropods, Evolution

A member of the largest and most diverse phylum of animals (Arthropoda), characterized by jointed limbs and an external chitinous skeleton. Arthropods have segmented bodies, most segments carrying a pair of limbs variously modified for locomotion, feeding, respiration, or reproduction. The external skeleton (cuticle) is moulted periodically to permit growth. Size ranges from 80 µm to 3·6 m/11¾ ft. Arthropods have a long fossil history from the Cambrian to the present, and are the most abundant animals on Earth, including the insects, arachnids, crustaceans, trilobites, centipedes, millipedes and several minor groups.

iArthropods
Mexican redknee tarantula Brachypelma smithi
Scientific classification
Kingdom: Animalia Superphylum: Ecdysozoa Phylum: Arthropoda Latreille, 1829
Subphyla and Classes
Subphylum Trilobitomorpha Trilobita - trilobites (extinct) Subphylum Chelicerata Arachnida - spiders,scorpions, etc.

Arthropods (Phylum Arthropoda, from the Greek ἀρθρον, meaning joint and πούς/ποδός, meaning foot) are the largest phylum of animals and include the insects, arachnids, crustaceans, and others. Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms.

Arthropods are characterised by the possession of a segmented body with appendages on each segment. All arthropods are covered by a hard exoskeleton made of chitin, a polysaccharide, which provides physical protection and resistance to desiccation. Periodically, an arthropod sheds this covering when it moults.

Basic arthropod structure

The success of arthropods is related to their hard exoskeleton, segmentation, and jointed appendages.

Aquatic arthropods use gills to exchange gases. Terrestrial arthropods have internal surfaces that are specialised for gas exchange.

Arthropods have an open circulatory system. Arthropods are protostomes. The arthropod body is divided into a series of distinct segments, plus a pre-segmental acron which usually supports compound and simple eyes and a post-segmental telson.

The cuticle in arthropods forms a rigid exoskeleton, composed mainly of chitin, which is periodically shed as the animal grows. This is in fact what gives arthropods their name — jointed feet — and separates them from their relatives, the Onychophora and Tardigrada, also called Lobopoda (and which is sometimes included in a group called Panarthropoda that also includes arthropods). The exoskeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one. This process, ecdysis, is expensive in terms of energy, and during the moulting period, an arthropod is vulnerable.

Classification of arthropods

Arthropods are typically classified into five subphyla, four of which are extant :

Trilobites are a group of formerly numerous marine animals that died in the mass extinction at the end of the Permian-Triassic extinction event.

Aside from these major groups, there are also a number of fossil forms including anomalocarids and euthycarcinoids , mostly from the lower Cambrian, which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them.

The phylogeny of the arthropods has been an area of considerable interest and dispute. The validity of many of the arthropod groups suggested by earlier authors is being questioned by recent studies;

Since the International Code of Zoological Nomenclature recognises no priority above the rank of family, many of the higher groups can be referred to by a variety of different names .

Evolution

Arthropods are thought to have evolved from segmented worms during the Pre-Cambrian era. The common ancestral arthropod, though, apparently happened to be one who had evolved not just chitinous mouthparts like other segmented worms, but also a chitinous structure all over its body; with all arthropods, the segments have become distinct (at least in larvae), each covered with one or more plate, and with legs, or limbs, one pair per segment.

At one point, it was believed that the different subphyla of arthropods had separate origins from segmented worms, and in particular that the Uniramia were closer to the Onychophora than to other arthropods. More recently, however, this has been considered convergent evolution, and the arthropods and allies may be more closely related to certain pseudocoelomates such as roundworms that share with them growth by moulting, or ecdysis.