Hydra tuba, the name of the first stage after the egg in the development of some of the Hydrozoa (q.v.) of the subclass Acraspeda (q.v.). It is a small fixed hydra-like body, but with solid tentacles. By a series of transverse constrictions it is divided into a series of segments, each of which ultimeetely develops a series of lobes and breaks away from the parent; they then swim about as independent jelly-fish. This method of development occurs in the commonest English jelly-fish, Aurelia aurita.
It is found in the Rhizostontecs and many of the Pelaijidw.