SummaryOligochaete nephridia are now classified into holonephridia and meronephridia; the former are large and occur as a pair per segment, while the latter are small and are numerous in each segment. The meronephridia are derived from holonephridia by a process of branching and multiplication either at an early or a late stage of development. The tufted nephridia are either incompletely branched holonephridia or meronephridia secondarily grouped together. The two kinds of nephridia are further distinguished as open or closed according as they possess or lack an internal funnel, and still further as exonephric or enteronephric according as they discharge their excretory products to the exterior or into the gut. The holonephridia are almost all open and exonephric; closed holonephridia have been found only in one form, and in one form alone are the holonephridia only incipiently enteronephric. But it is the meronephridia that exhibit the greatest variation in number and size as well as form and disposition, leading to a progressive and divergent specialization in the Megascolecidae, the largest family of the Oligochaeta. As many as ten kinds of meronephridia are met with. The enteronephric system in six genera shows various stages of development from simple segmentally separate units to a single unified and elaborate system of the enteronephridia of the whole of the intestinal region. In one genus the meronephridia are known to be incipiently enteronephric.An account of the process of excretion is given. Analyses have been made of the urine, coelomic fluid and blood ofPheretima.The excretory substances are ammonia, urea and creatinine; they are initially formed in the body wall and gut wall, and are thence taken up both by the coelomic fluid and blood, which pass the excretory products to the nephridia for elimination to the exterior. The mechanism of nephridial excretion has been analysed into processes of filtration, reabsorption and tubular secretion. The initial nephridial filtrate consists of blood‐plasma minus the colloids, together with the coelomic plasma, but it is modified during its passage down the nephridial canal, as there is a large scale reabsorption of useful substances. Glucose and amino‐acids are completely reabsorbed, while PO C1, Na, K and creatinine are reabsorbed extensively along with water. The urine is really a protein‐free filtrate, but measurable quantities of protein find their way into it, probably from the mucus secreted by the worms during the process of collection. The brownish granules in the ciliated middle tube of nephridia are coloured by a haem pigment. The role of the nephridia in osmotic regulation has been determined by measurement of the depression of freezing point of blood, coelomic fluid and urine and by a comparison of the concentration of various substances in these fluids. The earthworm maintains a steady state like a freshwater organism. The adaptive significance of the enteronephric system is to enablePhcretimato conserve much of its water which is toagreat extent returned to and reabsorbed by the inte
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