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; class Arachnida)

Order of small or very small arachnids most of which have a short, unsegmented abdomen. The body comprises the capitulum and a prosoma. In most forms the prosoma is covered by a single carapace (see cephalothorax). The respiratory organs are tracheae. Many species are parasites of medical and veterinary importance, and many are agricultural pests. The Acarina are regarded by many zoologists as an unnatural, polyphyletic group comprising elements derived independently from different arachnid stocks. There are 20 000 known species, 2800 of which are aquatic, including some that are marine.

Subjects: Zoology and Animal Sciences.

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