Evolution of invertebrate homeostasis: Osmotic and ionic regulation

Abstract

1. 1. Homeostasis, or the maintenance of a constant internal environment, in invertebrate organisms decreases the dependence of these organisms on the vagaries of the external environment. 2. 2. The evolution of physiological processes influencing homeostatic conditions begins with the organism being totally dependent upon the external environment, passing through a series of intermediate stages during which fluctuation in a given internal parameter occur, proceeding finally to a condition where the internal environment is constant and independent of the external environment. 3. 3. A hypothetical scheme for the possible evolutionary pathway of osmotic and ionic regulation in aquatic invertebrates was developed in an attempt to follow the process of homeostasis in these organisms. 4. 4. Primitive marine cells developed mechanisms to regulate the ionic composition of the cytoplasm probably in association with the need for volume regulation. 5. 5. The development of a body wall separated an internal body fluid from the external sea-water and the ionic composition of the body fluids was initially maintained slightly different from that of the sea-water by essentially passive means. 6. 6. The addition of excretory organs, which arose initially through an inpushing of the body wall, brought about the ability for osmoregulation. 7. 7. Homeostatic mechanisms must be in place before an organism can move from one environment to another.