Comparative Physiology: Invertebrate Excretory Organs

Publisher Website
Google Scholar
Abstract
Filtration kidneys appear to be widespread among invertebrates. Their occurrence among a number of mollusks and decapod crustaceans is now as well established as in the vertebrates. The status of the annelid metanephrl-dium is more equivocal. Freshwater invertebrates form filtrates at rates comparable to vertebrates, and there does not seem to be any marked reduction in marine forms. This may be accomplished at lower arterial blood pressures because filtration membranes are more porous. The tubular fluid is modified by selective reabsorption of organic nutrients and of some electrolytes. Certain solutes are added to the tubular fluid by inward secretion. A distal segment is found solely in the kidneys of freshwater forms. It is relatively water impermeable, but reabsorbs electrolytes and is therefore responsible for the production of a dilute urine. Where tubular mechanisms have been studied they appear to resemble those found in the vertebrate kidney. The excretory system in insects operates on a fundamentally different plan. The Malpighian tubules form a fluid approximately isotonlc with the hemolymph but with a high K/Na in all cases examined. Fluid secretion is dependent on active inward potassium transport. Rates of fluid secretion have been measured in a few cases, and they may be suprlslngly high (5-10 ml kg-1 hr-1). Nitrogenous compounds such as uric acid or allantoln are secreted by the tubules in most, but not all insects. Fine homeostatlc control of ionic, osmotic, and possibly acid-base balance is exerted through transport mechanisms in the posterior alimentary canal, especially in the rectum. Water is reabsorbed against large osmotic gradients in the rectum of some insects.