The Collaborative International Dictionary of English v.0.48:

Somatocyst \So"ma*to*cyst\, n. [Gr. sw^ma, sw`matos, body +
   ky`stis a bladder.] (Zool.)
   A cavity in the primary nectocalyx of certain Siphonophora.
   See Illust. under Nectocalyx.
   [1913 Webster]