Arachnida

Arachnida, a class of Arthropoda, the members of which breathe by tracheae, a series of air-tubes running through the body; they have eight legs, no jointed limbs on the abdomen, nor antennae; the head and thorax may be united. Many authors include the Trilobites, Limulus (King-Crab) , etc. (for which see Arthrogastra). The class is sometimes united with the Crustacea as the Acerata. As here defined, the class includes seven orders, viz. Linguatulida (worm-like parasites), Acarina (ticks), Tardigrada (water-bears), Araneida (spiders), Phalangidae (harvest-men), Pedipalpi, Scorpionidae, Pseudoscorpionidae and Solifuges. Representatives of the class occur first in the Silurian period.