Arachnids belong to the phylum Arthropoda, and as such have the basic characteristics of this assemblage. These characteristics include having jointed appendages, an exoskeleton, an open circulatory system, and a ventral nerve cord. The Arachnida are a subgroup within the subphylum Cheliceriformes, those arthropods with a pair of primitively pincerlike chelicerae mouthparts. Arachnids are considered to have an aquatic origin, but most present-day forms are terrestrial in nature. Arachnids all have the following characteristics: a body that is divided primitively into a prosoma and an opisthosoma; a pair of chelicerae used as mouth parts, a pair of pedipalps that often end in pincerlike claws and are modified for prey manipulation and sperm transfer; four pairs of walking legs in the adult (juveniles may have three pairs) originating from the prosoma; absence of antennae; simple eyes in most; coxial glands at base of the legs and malpighian tubules extending between the hemocoels and gut tube used for excretion and osmoregulation; gut tube with diverticula; breathing accomplished by book gills (aquatic forms), book lungs, or tracheal tubes; and a dorsal heart. The following orders comprise the Arachnida: Scorpiones (scorpions), Uropygi (whip-tailed scorpions), Schizomida (schizomids), Amblypygi (whip spiders), Palpigradi, Araneae (spiders), Ricinuleids, Pseudoscorpionida (false scorpions), Solpugida (wind scorpions), Opiliones (daddy longlegs), and Acari (ticks and mites).
Geographical location: All continents
Habitat: Mainly terrestrial, with some few freshwater forms found among spiders
Gestational period: Extremely variable depending upon group or individual species
Life span: Variable, but some members may live up to twenty-five years
Special anatomy: All have eight legs as adults; pincerlike mouthparts that may be modified into fangs in some groups; most members are free-living but parasites are found among the Acarina; includes some of the most venomous animals of the world, such as the African fat-tailed scorpion, the American black widow spider, and the Australian red back and funnel web spiders
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