Tracheate or branchiate Arthropoda with a cephalothorax bearing 4-6 pairs of ambulatory limbs, and an abdomen either segmented or unsegmented and sometimes divisible into two distinct regions, an anterior mesosoma, and a posterior metasoma, with a post-anal telson or spine.

The head is rarely distinct from the thorax (some Acarina), or the thorax broken up into three clearly separate somites (Solifugae). In tracheate forms the abdomen may be soft, unsegmented, and separated by a constriction from the cephalothorax (Araneidae), or else united to it (Acarina). It is scarcely represented in Tardigrada. It possesses six somites in Phalangidae, in the remaining groups nine to twelve. Of these the first seven in Scorpionidae or nine in Thelyphonus (Pedipalpi) are broad, while the other five (Scorpionidae) or three (Thelyphonus) are contracted. Among branchiate forms Limulus (Xiphosura) has the mesosoma and metasoma fused and the original segmentation lost; the Eurypterina possess twelve distinct somites, whilst the Trilobita have a mesosoma containing 1-26 somites, and a metasoma or pygidium with somites undifferentiated. The telson is present in Xiphosura and Eurypterina, and in the Scorpionidae, where it forms the 'sting' and incloses a poison gland. In Thelyphonus it is represented by a long jointed filament, and in Araneidae by a small caudal lobe or anal valve.

The Linguatidina differ remarkably from other Arachnida, and have an elongated ringed body, sometimes flattened, with an anterior pair of sensory papillae, and two pairs of chitinoid hooks placed anteriorly close to the mouth.

A well-developed chitinoid piece - the camerostome (the hypostoma so-called, in Trilobita ?) - overhangs the mouth anteriorly. The sternal region in the thorax is variously developed or suppressed in the different groups. The posterior part or metasternite developes in Limulus into the two lobes or chilaria which lie behind the last pair of cephalothoracic limbs.

In the tracheate forms the appendages of the cephalothorax are as follows: - a pair of fakes or chelicerae post-oral in development, prae-oral in the adult, composed of 1 to 3 joints, and terminated by a claw or pincer-like chela (Scorpio, etc.) or else represented by a pair of stilets (some Acarind); a pair of palpi (pedipalpi) or chelae usually large, the basal joints of which are well developed, inclosing the mouth (and stilets when present), and terminated either by a claw or chela, or by a copulatory apparatus in the male Araneid 1; four pairs of limbs, generally elongated, ambulatory, and terminated by claws, to which adhesive lobes or discs may be added in some Acarina 2. The number of joints in the limbs is variable, usually 6-J; and there may be a well-formed tarsus (Solifugae). The coxal joints of the two first pairs of ambulatory limbs are in relation with the mouth in Scorpionidae and Phalangida. The abdomen has appendages only in Scorpionidae, the first pair forming a minute genital operculum, the second the pectines or combs, organs probably of touch. The remaining six pairs of abdominal limbs present in the embryo Scorpion abort.

Abdominal limbs are present to the number of four pairs in the embryo Spider, but abort at an early period.

Among branchiate forms Limulus has a pair of 3-jointed prae-oral chelicerae, the Eurypterina homologous appendages which are sometimes non-chelate. Limulus has five pairs of post-oral cephalothoracic limbs, the Eurypterina and Trilobita four pairs, of which the last are the most powerful. The coxae of these limbs surround the oral depression, and are therefore masticatory. Limulus has six pairs of limbs on the abdominal mesosoma, of which the first pair fuse to form a genital operculum, as in Scorpio; whilst the remaining five are lamellate and carry externally on their posterior surfaces a series of close-set branchial folds. The two first abdominal somites in Enrypterina are covered by a flap (? genital operculum), and probably bore branchiae. In Trilobita the somites of the mesosoma and the unsegmented metasoma (pygidium) have slender jointed limbs furnished with a short external jointed epi- or exo-podite. Two straight or spirally coiled branchial filaments are attached externally to the base of the limb, and appear to resemble closely the branchiae of certain species of Cyamus (Crustacea Amphipoda).

1 During the development of some Acarina the embryonic chelicerae and pedipalpi fuse into a proboscis. At a later stage, after an ecdysis, the stilets and palpi grow out from the proboscis, and are presumed to be the homologues respectively of the chelicerae (or mandibles as they are sometimes termed in the Acarina) and pedipalpi. Haller states that the Acarina possess four pairs of mouth-parts, a pair of mandibles, a first and a second pair of maxillae, the second often rudimentary, and a labium usually palpate. As an adult Mite also possesses four pairs of ambulatory limbs, the last of which Haller regards as abdominal, it has two pairs of limbs in addition to those present in a Spider. A Spider however has embryonic limbs, lost in development; and rudiments of two extra pairs are present in the Scorpion, as the operculum and pectinated appendages. See pp. 496-7 ante, and table, pp. 174-5.

2 Kramer holds, as do some other authorities, that the two last pairs of limbs belong to the abdomen in Acarina. See his paper on 'Segmentation of the Mites,' A. N. 48 (I), 1882 (A. N. H. (5), x). The abdomen in the Mite in question (Alycus roscus ?) has nine somites.

The chitinoid cuticle is thick and dense in Limtdus, Scorpio, etc, delicate in Linguatalina. There are cutaneous glands in Linguatidina and some Acarina. The Solifugae are covered with thick-set hairs, and in Acarina cuticular hairs or processes may attain a great development. The Araneidae and Pseudoscorpionidae possess silk glands. In the former group they are very numerous, and open on the apices of 4-6 spinnerets placed in front of the anus: their secretion is used to form the web and attach the ova. In the latter they open near the genital outlet on the second abdominal somite 1