Entry 1 sense Webster, 1913 Somatocyst · So·ma·to·cyst · IPA 01 n. A cavity in the primary nectocalyx of certain Siphonophora. See Illust. under Nectocalyx. 1. A cavity in the primary nectocalyx of certain Siphonophora. See Illust. under Nectocalyx.(Zool.) See: Nectocalyx