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ost conspicuous specialization of segments is in the head. The four major groups of arthropods – Chelicerata (sea spiders, horseshoe crabs and arachnids), Myriapoda (symphylan, pauropods, millipedes and centipedes), Crustacea (oligostracans, copepods, malacostracans, branchiopods, hexapods, etc.), and the extinct Trilobita – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.[23] Despite myriapods and hexapods both having similar head combinations, hexapods are deeply nested within crustacea while myriapods are not, so these traits are believed to have evolved separately. In addition, some extinct arthropods, such as Marrella, belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages.[35] Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "the arthropod head problem".[36] In 1960, R. E. Snodgrass even hoped it would not be solved, as he found trying to work out solutions to be fun.[Note 2]





Exoskeleton Main article: Arthropod exoskeleton Illustration of an idealized arthropod exoskeleton. Arthropod exoskeletons are made of cuticle, a non-cellular material secreted by the epidermis.[23] Their cuticles vary in the details of their structure, but generally consist of three main layers: the epicuticle, a thin outer waxy coat that moisture-proofs the other layers and gives them some protection; the exocuticle, which consists of chitin and chemically hardened proteins; and the endocuticle, which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as the procuticle.[38] Each body segment and limb section is encased in hardened cuticle. The joints between body segments and between limb sections are covered by flexible cuticle.[23] The exoskeletons of most aquatic crustac