Acanthocephalan worms are parasitic throughout their life cycles and the environments they experience are to be found in the bodies of vertebrates, in which they achieve sexual maturity and in the tissues of arthropods, in which they develop. This monograph, first published in 1970, aims to relate present knowledge of acanthocephalan physiology to the physical and biotic factors occurring in the environments occupied at different stages of the life cycle. This theme is illustrated by considering feeding, metabolism and reproduction of adult worms, certain aspects of the physiology of the eggs and developmental stages and the processes of infection of the hosts. The author emphasises the necessity of culturing acanthocephalans in vitro if their biology is to be understood in detail.