From ZooTerms (Dictionary of Invertebrate Zoology)
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atrium (noun; plural atria; Latin atrium, vestibule): 1. A cavity, division, entrance or passageway of various invertebrates.

2. (Annelida) a. A diverticulum of the spermatheca; a tubular or capsular prostate. b. Male reproductive organ in leeches, consisting of a thin-walled bursa, a thick-walled glandular and muscular chamber and a pair of atrial cornua opening into the muscular median chamber.

3. (Arthropoda: Insecta) A specialized area of the trachea; a. A spiracular atrium. b. The preoral cavity in certain larvae.

4. (Porifera) A cavity into which many exhalant systems empty and conduct the contents to one or more usually terminal ocules.