ABSTRACT
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. 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. Class Hirudinea usually frequent freshwater environments, but some species may be marine or terrestrial. The clitellum is a reproductive structure composed of modified segments that are unique to Oligochaetes and Hirudinea, hence their classification together as Clitellata. The diversity of habitats occupied by annelids makes them ideal subjects to study osmoregulation and the process of adaptation to diverse environments from marine, freshwater, and terrestrial environments.
