Siphonophora

Siphonophora, an order of Craspedote (q.v.) Hydrozoa, including a number of forms which live on the surface of the seas, mostly in the tropics. They are colonial in habit, and the colonies are characterized by very marked polymorphism, i.e. the different zooids are specialised to serve different functions. They are all free, and as a rule the stem is unbranched, but is often expanded into a float or pneumatophore. The order is divided into four sub-orders:- (1) The Physophorida (q.v.), including Physophora and others with flask-shaped floats; (2) Physalida, including the Portuguese Man-of-War or Physalia; (3) Discoidea, with disc-shaped floats, such as Velella and Porpita, and (4) Calycophorida, in which the zooids are placed on an elongated, tubular coenosarc, as in Diphyes, Abyla, etc.