Cordylophora

Cordylophora is a very convenient type of the compound Hydroid Zoophytes, as it lives in fresh or brackish water and is common in English canals and docks, where it usually occurs attached to floating timber. It consists of a branched plant-like colony composed of two elements, the trophosome and the gonosome: the latter is the reproductive portion of the colony, and consists of a number of blunt, egg-shaped sacs (gonophores) borne at the base of the terminal branehes. The trophosome is the main part of the colony, and consists of the long-branched stem (or hydrocaulus) and of the polypes (or hydranths) at the ends of the branches. The whole stem is covered by a thin membrane known as the perisarc, which extends over the gonophores but not over the polypes. Cordylophora reproduces by the formation of eggs on the central axis of the gonophore, which develop into free-swimming larvae known as "planulae" (q.v.); these become fixed by one end and grow into the adult form. The development is, therefore, said to be direct, as there is no "medusa" or jellyfish stage as occurs in many of the Hydroids. Cordylophora was once a marine and estuarine form, and has only spread into fresh water during the present century.