1. Inulin (MW 5000) and two dextrans (MW 15,000-20,000 and 60,000-90,000) appear in the urine of crayfish after injection into the blood. All three compounds are more concentrated in the urine than in blood, indicating that water is reabsorbed from a filtrate formed within the antennal gland.

2. Inulin and the low molecular weight dextran seem to be handled in the same fashion since their urine/blood concentration ratios are about the same. However, the high-molecular-weight dextran is excreted only about 70% as effectively as the other pair. This suggests that filtration of polymers in the MW range 50,000-100,000 is restrained.

3. Excretion of human serum albumin occurs but it is even more severely restricted than the large dextran. Mammalian globulin (MW about 180,000) and crayfish blood protein are not excreted in the urine.

4. Blood protein becomes concentrated in the coelomosac. Localization of fluorescein-labelled mammalian globulin shows that the peritubular cells in the coelomosac are the main sites of protein accumulation. Concentration of blood protein in the coelomosac suggests that filtration occurs in this region, but the intracellular location suggests that the filtration mechanism differs from that in the vertebrate nephron.

This investigation was supported by funds for medical and biological research, State of Washington Initiative Measure 171, and by a grant (G 12471) from the National Science Foundation.