Arthropods (Phylum Arthropoda) are the largest phylum of animals and include the insects, arachnids, crustaceans, and other similar creatures. Over four out of five extant (living today) animal species are arthropods, with over a million modern species described and a fossil record reaching back to the early Cambrian. Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (~0.25 mm) up to forms several metres long.
Basic arthropod structure
The success of the arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defense, and locomotion.
Arthropods respire (breathe) through a tracheal system; a potential difficulty considering that the skeletal structure is external and covers nearly all of the body. Aquatic arthropods use gills to exchange gases. These gills are specialized with an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialized for gas exchange. The insects have tracheal systems: air sacs leading into the body from pores, called spiracles,in the cuticle.
Arthropods have an open circulatory system: hemolymph* or blood is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are protostomes. There is a coelom, but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a hemocoel, filled with hemolymph which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a presegmental acron which usually supports compound and simple eyes and a postsegmental telson. These are grouped into distinct, specialized body regions called tagmata. Each segment at least primitively supports a pair of appendages.
The cuticle in arthropods forms a rigid exoskeleton, composed mainly of chitin, which is periodically shed as the animal grows. They contain a inner zone (procuticle) which is made of protein and chitin (a polysaccharide) and is resposible for the strength of the exoskeleton. The outer zone (epicuticle) lies on the surface of the procuticle. It is nonchitinous and is a complex of proteins and lipids. It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called sclerites on the segments, plus rings on the appendages that divide them into segments separated by joints. This is in fact what gives arthropods their name—joint feet—and separates them from their very close relatives, the Onychophora and Tardigrada. The skeletons 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, molting, is expensive in energy consumption. During the molting period, an arthropod is vulnerable. How do arthropods grow? Once their cuticle hardens they can't grow ever again. Their cuticles slowly expand as they increase in mass. They breakdown (digest) their cuticle every now and then when they need to grow. Their cuticle hardens at their adult size and they slowly grow to fill it up. NOTE: the blood of some arthropods does not contain iron based hemoglobin but copper based hemocyanin.
At one point it was considered 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. However, this is rejected by most workers, and is contradicted by genetic studies.
Traditionally the Annelida have been considered the closest relatives of these three phyla, on account of their common segmentation. More recently, however, this has been considered convergent evolution, and the arthropods and allies may be closer related to certain pseudocoelomates such as roundworms that share with them growth by molting, or ecdysis. These two possible lineages have been termed the Articulata and Ecdysozoa.
The classification of the arthropods varies somewhat from source to source. There are five main subgroups: the Trilobita, Chelicerata, Myriapoda, Hexapoda, and Crustacea, which may be variously ranked from subphyla to classes, with various other taxa introduced above or below them and corresponding changes in the ranks of their subgroups. Here we have followed a "splitting" taxonomy, containing only generally accepted groups and assigning them higher ranks.
Aside from these major groups, there are also a number of fossil forms, 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.
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