The body wall of Polymorphus minutus has been studied with the electron microscope and the structure of the various layers has been described.The layers are the same in number as those seen with the light microscope, and pores have been found which penetrate the cuticle. Thus, the structure of the surface layers is such as would facilitate the absorption of nutrients.It has been found that the cuticle and striped layer extend over the trunk spines, a feature which increases the area of the absorptive surface of the parasite.The structure of the striped layer of the praesoma supports the theory that the praesoma body wall and lemnisci are involved in the absorption of fat.Mitochondria have been detected in the felt and radial layers of the body wall and in the circular and longitudinal muscles.The body wall of this acanthocephalan worm is entirely different from the body wall of trematodes, cestodes and parasitic nematodes.We are grateful to Dr P. Tate for helpful discussions, Dr R. J. Skaer for criticism of the manuscript and to Professor J. D. Boyd for permission to use the electron microscope in the Department of Anatomy. Thanks are also due to Mr A. J. Page for technical assistance.

1. The distribution of non-specific esterase and of cholinesterase in Ascaris lumbricoides has been determined using histochemical methods.2. Non-specific esterase has been shown to be present in the cuticle, in the walls of the excretory canals, in the innervation processes of the muscles, in the coelomocytes, in the oesophageal glands, in the intestine, in the rectal glands, in the male and female reproductive systems, in parts of the nervous system and as 'caps' of enzyme on fat globules in the hypodermis and muscles.3. Cholinesterase has been detected in the innervation processes and sheath of the muscles, in the sphincter muscles, in the intestine, in the sensory papillae and amphids, and in parts of the nervous system, including a nerve in the spicules.4. Parts of the nervous system and the innervation processes of the muscles contain an esterase which is not inhibited by cholinesterase inhibitors and it is suggested that there may be esterase, distinct from cholinesterase, involved in nerve transmission.5. Merocrine secretion of esterase has been observed in the intestine.

1. The exrectory system of Anoplocephala perfoliata has been re-described and shown to consist of a dorsal ‘network’ system of vessels to which the flame cells are connected and a ventral pair of longitudinal vessels linked only by a transverse vessel in each proglottid.2. The two excretory systems unite only in the apical ramifying vessels of the scolex.3. Alkaline phosphatase activity was demonstrated in the cuticular microvilli, the subcuticular region and in the excretory vessels.4. Non-specific esterase, insensitive to 10−4m E 600, was located in the lappets and embryonated eggs and esterases, inhibited by 10−4m E600, in parts of the cuticle and subcuticular region and parenchyma.5. Cholinesterases were found in the nervous system and in the reproductive ducts.6. Aminopeptidase was found in the neck region, in the undifferentiated proglottids, the active testes and ovaries, the male reproductive ducts and in the region of high metabolic activity in the fringe of the proglottid.