ABSTRACT
Nematodes reside in diverse environments, including fresh and salt water, soil, plant roots and various animal tissues, especially the intestinal tract of vertebrates. Indeed, a species of nematode is adapted for every environment on earth where organisms can be surrounded by at least a film of water (Wharton, 1986). Aside from moisture, each habitat poses particular challenges for nematode survival. These challenges include physicochemical stressors, such as osmotic strength, ion concentration, pH, and O2 tension that can exceed the levels typically associated with metazoan existence. Nematodes are also subjected to hostile reactions, such as immune responses and digestive enzymes, from host animals. Considering how successful they are as a phylum of organisms, nematodes have obviously evolved ways to overcome a wide variety of environmental challenges. In this chapter, we discuss the functional, and where available, molecular biology of the processes and tissues that help maintain homeostasis in nematodes, defined here as a balance of electrochemical and osmotic (including hydrodynamic) gradients between the pseudocoelomic fluid and the external environment. Our discussion includes a consideration of the structures and tissues that establish and maintain these gradients. In addition, we distinguish processes of excretion, in which indigestible material and waste products are removed from internal tissue compartments, from those of secretion, the release of bioactive molecules that affect the environment. Nematodes secrete molecules that, among other effects, contribute to immune evasion, extracorporal digestion and loca-lised paralysis of the host intestine. We also briefly consider the anatomical structures from which such molecules are secreted, including the tubular system, epidermal gland cells, pharyngeal glands, rectal glands and stichosomes. Detailed descriptions of each of these structures are provided in Chapter 2.
