Many workers have considered the possibility of hydroids being potentially somatically immortal, largely because of the continued production of interstitial cells which can differentiate and migrate to replace cells in other parts of the hydroid (Schmid, Schmid & Tardent, 1974). In addition, many thecate hydroids show an orderly regression and replacement of their hydranths (see review by Toth, 1969) and this led Crowell (1953) to suggest that by this means hydroids might avoid senescence. Such considerations are undoubtedly fostered by the lack of evidence for senescence of hydroids which, although sometimes showing a cycle of degeneration followed by regrowth from a dormant stolon, may live for many years in the laboratory (Strehler, 1961; Brock, 1974). Only recently has the senescent death of a hydroid (Corymorpha nutants (L.)) been recorded (Svoboda, 1973).