The Collaborative International Dictionary of English v.0.48:

Nectocalyx \Nec`to*ca"lyx\, n.; pl. Nectocalyces. [NL., fr.
   Gr. nhkto`s swimming + ? a calyx.] (Zool.)
   (a) The swimming bell or umbrella of a jellyfish of medusa.
   (b) One of the zooids of certain Siphonophora, having
       somewhat the form, and the essential structure, of the
       bell of a jellyfish, and acting as a swimming organ.
       [1913 Webster] Nectosac