The chief external feature of Arachnida is the division of the body into two parts, properly to be called the prosoma and the opisthosoma. The former is often called the cephalothorax, the latter is known as the abdomen. The objection to the use of these terms is that they are also in use for other animals, in which their constitution is not the same as in arachnids. The prosoma [pro] appears be made up of six somites, and each somite carries a pair of appendages, which are present in all Arachnida without exception. The dorsal plates or tergites of the prosoma are usually fused to form a carapace or epistome [epi]. The opisthosoma a maximum of 12 somites is recognizable. It is typically divided into a mesosoma [mes] and metasoma [met], although this is only clearly visible in scorpions, and in some orders, the abdominal sections are completely fused. In general the opisthosoma carries no appendages, but mesosoma of certain fossil arachnids bears obvious gills [zp]. In many arachnids the prosoma and opisthosoma are broadly joined, while in others they are joined by a narrow stalklike pedicel (modified 7th somite).

The first pair of appendages, the chelicerae [ch], serve in feeding and defense. The chelicerae consist of two or three segments. In Pseudoscorpiones they contain silk gland and in Araneae, poison. The next pair of appendages, the pedipalps [ped], have been adapted for feeding, locomotion, and/or reproductive functions. The pedipalpi consist of six segments. The limbs themselves may be simple tactile organs outwardly resembling the legs (as in spiders), or chelate weapons of great size (scorpions and false-scorpions). They may be specialized in different ways, as in spiders where they act as accessory male organ, and in Solifugae, where they terminate in suckers. The legs are of six segments – coxa, trochanter, femur, patella, tibia and tarsus. They may be all alike (and this is the general rule), but in some orders, notably Amblypygi and Solifugae, the first pair are not used for walking but are carried aloft and directed forwards as tactile organs.

After approximately 50 years of discussion most morphologists, palaeontologists and molecular taxonomists agree that the Arachnida are a monophylum. Arachnids possess a deep and extensive fossil record (Garwood & Dunlop 2014). Arachnid monophyly is supported by at least 11 synapomorphies (Coddington et al. 2004). Inside the arachnids, Xiphosura, Ricinulei, and Scorpiones possess the primitive characters. Almost all studies support the monophyly of Tetrapulmonata. The positions of Palpigradi, Opiliones, Ricinulei, and Acari are poorly resolved. Previous molecular studies confirm the non-monophyletic origin of mites (there are two orders Parasitiformes and Acariformes). There are some systematic conflicts in the phylogenetic information, particularly affecting the orders Acariformes, Parasitiformes and Pseudoscorpiones, which have had much faster evolutionary rates (Sharma et al. 2014). Idealized hypothetical arachnid phylogenetic tree and glossary of arachnid phylogenetic clades:

Acaromorpha = Ricinulei + Acari
Arachnopulmonata = Scorpiones + Tetrapulmonata
Camarostomata = Thelyphonida + Schizomida
Cephalosomata = Palpigradi + Solifugae + Acariformes
Cryptoperculata = Opiliones + Ricinulei + Acari
Dromopoda = Opiliones + Scorpiones + Haplocnemata
Euchelicerata = Merostomata + Arachnida
Haplocnemata / Apatella = Pseudoscorpiones + Solifugae
Labellata = Araneae + Amblypygi
Megoperculata = Uropygi + Amblypygi + Araneae
Merostomata = Xiphosurida + † Eurypterida
Metastomata = † Eurypterida + Arachnida
Pantetrapulmonata = † Trigonotarbida + Tetrapulmonata
Pedipalpi = Amblypygi + Thelyphonida + Schizomida
Poecilophysidea = Solifugae + Acariformes
Schizotarsata = Haptopoda + Pedipalpi
Serikodiastida = † Uraraneida + Araneae
Stomothecata = Scorpiones + Opiliones
Tetrapulmonata = Araneae + Pedipalpi