The literature of cyclomorphosis, the seasonal change in morphology within populations, is reviewed and a general theory concerning its adaptive significance is proposed. Several recent studies of cyclomorphic populations suggest that cyclomorphosis may be a means of reducing mortality due to size‐selective predation. Various appurtenances (helmets, horns, or spines) are grown by some prey species as an antilock and key mechanism to foil invertebrate predators. The larger prey species reduce the visible parts of their body to reduce vertebrate predation and elaborate hyaline appurtenances to foil invertebrates. Temporally, maximum development of appurtenances is best correlated with the presence of predator species. The appurtenances are not generally maintained in the absence of predators, presumably because of energy requirements for swimming and reproduction.