Any enclosed body cavity or chamber.
An upper chamber of the heart that receives blood from the veins and forces it into a ventricle. In higher vertebrates, the right atrium receives blood from the superior vena cava and inferior vena cava, and the left atrium receives blood from the left and right pulmonary veins.
A cavity inside a porate aperture of a pollen grain formed by the separation of the sexine and nexine layers, widening toward the interior of the grain.
A cavity, entrance, or passage.
A microscopic air sac within a pulmonary alveolus.
A central room or space in ancient Roman homes, open to the sky in the middle; a similar space in other buildings.
A square hall lit by daylight from above, into which rooms open at one or more levels.
A chamber in the mammalian kidney through which urine passes.
The outermost whorl of flower parts, comprising the sepals, which covers and protects the petals as they develop.
The crown containing the viscera of crinoids and similar echinoderms, entoprocts, and the polyps of some cnidarians.
A flattened cap of neuropil in the brain of insects.
A funnel-shaped expansion of the vas deferens or oviduct of insects.