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).
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