| Summary: | 1. Physical and chemical parameters were measured in the Japanese oyster Crassostrea gigas to investigate whether the first step of urine formation in the lamellibranchs could be an ultrafiltration and to give a quantitative description. 2. The effective filtration pressure was not constant, but a function of time, oscillating between 31.7 mmH 2 O and -3.8mmH 2 O. During the filtration, a separation of proteins took place: the protein concentration in the haemolymph was 17 µmoll-1 and the average molecular weight was 141 000. In the filtrate, the protein concentration was 2 µmoll-1 and the average molecular weight was 45 000. The marker substance inulin, applied via the gills, appeared successively within the haemolymph and the pericardial fluid. These findings establish the idea that the pericardial fluid is formed by ultrafiltration from the haemolymph. 3. The rate of filtration was found to be 0.4 µg-1 min-1 by quantitative analysis of the transport of the inulin. The coefficient of filtration was 4.5x10-6 mls-1 cm-2 mmHg-1.
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