Arthropods (Phylum Arthropoda) are the largest phylum of Animals, including the insects, spiders, crustaseans, and other similar creatures. Over four out of five animal species are arthropods, with over a million modern species and a fossil record reaching back to the early Cambrian. They are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms, and range in size from microscopic plankton (~.25 mm) up to forms several metres long.
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 blood 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. 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. 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 then 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 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.