Arthropoda

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The largest phylum of Animalia, including the insects, spiders, crustaceans, and other similar creatures. Over four out of five animal species are arthropods. They are common throughout marine, terrestrial, and even aerial environments, and range in size from microscopic planton (~.25 mm) up to forms several metres long.

Arthropods are triploblastic 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.

The monophyly of the Arthropoda has been questioned, and some workers have proposed separate origins for the major subphyla including possibly paraphyly to the Onychophorans, but this is rejected by many workers. Traditionally the Annelida have been held up as the closest kin of these three phyla on account of their common segmentation, but new work suggests this is convergent evolution, and the Arthropods and their allies may be closer related to pseudocoelomates (Nematoda, etc) that share with them growth by successive molting.

The arthropods are divided into four subphylums: the extinct Trilobitomorpha, the Cheliceriformes, the Crustacea, and the Uniramia. There are also a number of fossil forms, mostly from the Cambrian, with no clear affinity to any of these groups (or clear affinity to more than one).

Eventually I plan to put a brief survey here, plus some more on how arthropods are built. Please feel free to beat me to it.