The organisms that constitute the phylum Annelida, comprised of at least 20,000 species, are remarkably diverse and occupy habitats from open ocean, to estuaries, to freshwater streams and lakes, to soil in terrestrial environments. 3,81 A suite of biological adaptations is necessary for survival in each of these specific niches, but one of the most important adaptations is certainly the capacity to regulate internal osmotic pressure and the composition of cellular and tissue osmolytes. Freshwater species generally maintain comparatively high internal osmotic pressures (150 to 250 mOsm; see below) and must compensate for osmotic water gain. Terrestrial species face transient freshwater challenges and desiccation stress. Saltwater species generally are in near osmotic equilibrium with seawater but of necessity must regulate intracellular composition (as do virtually all living cells). Brackish water species and those saltwater species that migrate into estuarine habitats may regulate or resist osmotic challenge but in most cases are capable of osmotolerance. In addition, parasitic, mutualistic, and commensal species also exist. 6,81 Although some reviews in the area of epithelial transport and osmoregulation have been published, it appears that a review from a more general perspective has not been undertaken for some time. 11,96 This broad approach seeks to honor in some small way the spirit of the classic volume of Potts and Parry that this series commemorates. 66