The structure of the nephridiostome of Lumbricus terrestris L. is described, including the anatomical relations of canal, gutter, central, and marginal cells and their cytological characters. The extent and relation of the lower lip to other parts are also described.

An account of the development of the nephridium is given from the stage when the rudiment still bears a single large ‘funnel-cell’ bulging forwards through the septum into the coelom. The whole nephridiostome (excluding the covering of coelomic epithelium and the connective tissue) is shown to arise from the nephridial rudiment, wholely or partly from that part of the funnel-rudiment which is derived from the ‘funnel-cell’. Upper, lateral, and lower lips are all developed from the funnel rudiment in which the lumen becomes pierced. There is no evidence that the coelomic epithelium contributes any part of the true nephridiostome.

The view sometimes put forward that the excretory organ of Lumbricus is a nephromixium is not founded on sound evidence, and is opposed to the simple straightforward interpretation of its morphology which follows most naturally from the facts and a comparison with lower forms.

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