Arachnida

noun plural
Arach·ni·da | \ə-ˈrak-nəd-ə \

Medical Definition of Arachnida 

: a large class of arthropods that are mostly air-breathing by means of trachea or book lungs, that include the spiders and scorpions, mites, and ticks, and that have a segmented body divided into two regions of which the anterior bears four pairs of legs but no antennae

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