Polymorphism of helminths: from morphology to genetics.

Morphological polymorphism is a common phenomenon among several groups of nematodes. Morphs are readily distinguished from species as they differ only by few minor morphological features and are associated in a characteristic manner in their distribution within hosts. Morphological polymorphism has not been recorded in cestodes; strains of Echinococcus are not morphs. The study of eco-ethological polymorphism is still in its infancy but may prove of interest, in particular in trematodes.