Morphological adaptations of intestinal helminths.

Nematodes, trematodes, cestodes, and acanthocephalans each have become adapted in different ways to the microenvironment of the vertebrate intestine. Life in this specialized habitat affords parasites a reliable source of nutrients, a relatively homeostatic environment, and protection from predators but, in exchange for these advantages, presents the special challenges of exposure to digestive enzymes, normal peristalsis, and host immune response to infection. Logically, the surface of the parasite should be the first part of the organism to encounter such challenges, and, for this reason, any response or reaction by the parasite is expected to be manifested at the parasite-host interface. Morphological adaptations of intestinal helminths to their microenvironment include modification of the tegumental surface that affords protection and increases absorptive surface area, development of specialized attachment organs, and, in some cases, complete loss of their own internal digestive system. Representative examples of such adaptations by helminths are described and discussed in terms of the parasite's nutritional requirements, site selection, and host specificity, and the possibility is suggested that some helminths may have adapted in ways that exploit host defensive mechanisms for their own benefit.