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Segmentation and cuticle

The Chelicerata are arthropods as they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins; heads that are composed of several segments that fuse during the development of the embryo; a much reduced coelom; a hemocoel through which the blood circulates, driven by a tube-like heart. Chelicerates' bodies consist of two tagmata, sets of segments that serve similar functions: the foremost one, called the prosoma or cephalothorax, and the rear tagma is called the opisthosoma or abdomen. However, in the Acari (mites and ticks) there is no visible division between these sections.

The prosoma is formed in the embryo by fusion of the ocular somite (referred as "acron" in previous literatures), which carries the eyes and labrum, with six post-ocular segments (somite 1 to 6), which all have paired appendages. It was previously thought that chelicerates had lost the antennae-bearing somite 1, but later investigations reveal that it is retained and corresponds to a pair of chelicerae or chelifores, small appendages that often form pincers. Somite 2 has a pair of pedipalps that in most sub-groups perform sensory functions, while the remaining four cephalothorax segments (somite 4 to 6) have pairs of legs. In basal forms the ocular somite has a pair of compound eyes on the sides and four pigment-cup ocelli ("little eyes") in the middle. The mouth is between somite 1 and 2 (chelicerae and pedipalps).

The opisthosoma consists of thirteen or fewer segments, may or may not end with a telson. In some taxa such as scorpion and eurypterid the opisthosoma divided into two groups, mesosoma and metasoma. The abdominal appendages of modern chelicerates are missing or heavily modified – for example in spiders the remaining appendages form spinnerets that extrude silk, while those of horseshoe crabs (Xiphosura) form gills.

Like all arthropods, chelicerates' bodies and appendages are covered with a tough cuticle made mainly of chitin and chemically hardened proteins. Since this cannot stretch, the animals must molt to grow. In other words, they grow new but still soft cuticles, then cast off the old one and wait for the new one to harden. Until the new cuticle hardens the animals are defenseless and almost immobilized.