1. Certain aspects of the general morphology ofPolymorphus minutusare described together with a detailed description of the body wall.2. An additional layer of the body wall, the epicuticle, has been demonstrated. It appears to consist of acid mucopolysaccharide and may have a function of protecting the parasite from the enzymes of its host.3. A histochemical investigation has been made of the layers of the body wall and it is concluded that lipoprotein is one of the main structural components.4. The distribution of the activity of the two enzymes, non-specific esterase and alkaline phosphatase, has been studied throughout the animal and the activity of a third enzyme, leucine aminopeptidase, has been detected in the body wall.5. It is suggested that all the layers of the body wall, with the exception of the cuticle and epicuticle, are of metabolic importance. The striped layer may be connected with absorption and the felt and radial layers may be involved in the further metabolism of absorbed compounds.6. The results obtained are used to formulate a possible structure of the surface of the parasite which would facilitate the absorption of nutrient substances through the body wall.I am grateful to Dr P. Tate for advice and encouragement during this work, Dr R. J. Tatchell for helpful discussions, and Dr D. L. Lee for criticising the manuscript and Mr T. M. Warwick, Department of Zoology, University of Edinburgh for providing material.
Sex, age and examination data with special reference to serum alkaline phosphatase.
