The Phylum Arthropoda is an enormous large and diverse group of invertebrate animals with a both internally and externally segmented body and segmented appendages. Most groups within the arthropods display a high degree of regional body specialisation, leading to groups of segments called tagmata (for instance, head, thorax, abdomen). Arthropods have an exoskeleton, generally with thick sclerotised plates (sclerites), consisting of dorsal tergites, lateral pleurites and ventral sternites (in most groups). Primitively each arthropod segment bears a pair of segmented appendages, but in the process of specialisation (tagmosis), some of these appendages may have disappeared. Usually a pair of compound eyes is present, but sometimes also one or more simple median eyes is present, or there is no eye at all. The exoskeleton is formed by the chitinous cuticle, and is calcified in many groups. Growth is discontinuous, incremental, and happens by the process of moulting (ecdysis). Most arthropod species are dioecious, with direct, indirect or mixed development. The recent Arthropoda are divided in three sybphyla:
¥ Sybphylum Cheliceriformes — scorpions, spiders, sea spiders (Class Pycnogonida)
¥ Subphylum Uniramia — insects and myriapods
¥ Subphylum Crustacea — many different groups
In particular crustacean arthropods are well represented in the marine environment.
[After Brusca and Brusca, 1990]