The biology of the external surfaces of helminth parasites

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Abstract
The biological properties of the surface of helminth parasites are examined under three headings, morphology, digestive-absorptive function and defence. The concept of the host-parasite interface is explored.Platyhelminth parasites are covered by a syncytial tegument with sunken parenchymal cell bodies. The surface is either microvillous or deeply folded, presenting a large surface area relative to volume. The acanthocephalan surface is structurally unique, possessing a branching pore-canal system. The majority of nematodes are surrounded by a thick cuticle composed of distinct layers; exceptions include the entomophilic nematodes.The absorptive properties of the surface of helminth parasites are discussed and the published data on transtegumental transport of nutrient molecules examined. Most nematodes are impermeable to small molecules, but all other helminths utilise surface transport mechanisms. Cestodes and acanthocephalans lack a gut and the tegument becomes the major site of nutrient uptake.Surface-bound enzymes have been described in several helminths and indicate the possible extent of surface digestion by parasites. Binding of host digestive enzymes to the helminth surface may also serve a protective as well as nutritional function.Surface defence mechanisms are described whereby physiological interactions between host and parasite at the helminth surface effect evasion of the host's defences. The essence of these mechanisms is surface disguise.