• Webster's Revised Unabridged Dictionary
    • n Pneumatocyst (Zoöl) A cyst or sac of a siphonophore, containing air, and serving as a float, as in Physalia.
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Century Dictionary and Cyclopedia
    • n pneumatocyst The air-sac, float, or pneumatophore of an oceanic hydrozoan or siphonophorous hydromedu-san; one of the several appendages of the stem of these organisms, serving to buoy them in the water. See cuts under Athorybia and Hydrozoa. When pneumatocysts are wanting, they may be replaced by a general inflation of the stem, called then a somatocyst.
    • n pneumatocyst In ornithology, an air-sac or air-space; one of the cavities in a bird's body filled with air.
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Webster's Revised Unabridged Dictionary
Pneumato-, + cyst,