• (n.) Any siphonophore which has an air sac for a float, as the Physalia.
Phys'o·grade noun [ Greek ... a bellows + Latin gradi to walk, go.] (Zoology) Any siphonophore which has an air sac for a float, as the Physalia. 
physograde, physogradous Moving by means of a hollow vesicular float or buoy; of or pertaining to the Physograda, a group of oceanic hydrozoa furnished with such floating organs; a member of this group.