somatocyst

from The Century Dictionary.

• noun The inflated stem or body of some siphonophorans, or oceanic hydrozoans, serving as a pneumatocyst or air-sac to float or buoy these organisms, as in the case of the Portuguese man-of-war. See Calycophora, Siphonophora, and cuts under Diphyidæ and Physalia.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) A cavity in the primary nectocalyx of certain Siphonophora. See Illust. under nectocalyx.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology A cavity in the primary nectocalyx of certain Siphonophora.