Parameter Tree:
EARTH SCIENCE > BIOLOGICAL CLASSIFICATION > ANIMALS/INVERTEBRATES > ARTHROPODS > CHELICERATES > ARACHNIDS

Definition:
Arachnids are arthropods with a body divided into an anterior and
posterior part. The posterior section contains four pairs of legs.
Arachnids are almost entirely terrestrial and mainly free living.  


Reference:
Burns, David M. Animal World. 1980. Octopus Books Ltd. London.