Virtually all animal cells maintain an intracellular content of solutes which differs from the medium surrounding them. More complex animals usually possess a second extra-cellular compartment of body fluid which partially isolates the cells from the external environment. The body fluids may have a similar osmotic pressure to the surroundings (isosmotic) but frequently contain either more solutes (hyperosmotic) or sometimes less solutes (hypo-osmotic) than the external medium. A number of animals can tolerate quite large variations in their internal osmotic pressure following a change in the external osmolarity (osmoconformers). Many animals that are able to tolerate wider environmental fluctuations are capable of regulating the water or ionic content of their body fluids (osmoregulators). There is no rigid division between these responses to osmotic stress; some animals are osmoconformers in one concentration range but regulate in another.1 64 Nematodes are likely to possess some of these abilities, particularly those species that normally experience variations in the osmotic pressure of their aquatic habitats.
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