. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 246 N. S. RUSTUM MALUF Because glucose, xylose, and inulin are excreted by the kidney of the crayfish and not by the vertebrate aglomerular kidney, it at first seemed that filtration occurs in the former. This might imply that the hypotonicity of crayfish-urine is produced as in the Amphibia, namely, by the formation of a protein-free filtrate at the proximal end of the nephron and by subsequent resorption of relatively more salts than water by the tubule. On the other hand, other important data (see Discussion) centra-indic
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 246 N. S. RUSTUM MALUF Because glucose, xylose, and inulin are excreted by the kidney of the crayfish and not by the vertebrate aglomerular kidney, it at first seemed that filtration occurs in the former. This might imply that the hypotonicity of crayfish-urine is produced as in the Amphibia, namely, by the formation of a protein-free filtrate at the proximal end of the nephron and by subsequent resorption of relatively more salts than water by the tubule. On the other hand, other important data (see Discussion) centra-indicate filtration. Because inulin is neither secreted by nor passively resorbed through the vertebrate nephron, the inulin-clearance in this phylum is an unvary- ing function of the concentration of inulin in the plasma. This is true. o o° 20O 3OO 40O 5OO 60O TOO BOO 90O COO IIOO COO OOO MOO 60O BCD TOO BX> FIG. 6. The renal clearance of inulin, xylose, and creatinine in cc. per hour (ordinate) as a function of the concentration of these compounds in the plasma in mg. per cent (abscissa). Solid circles, inulin-clearances with direct measurement of urinary flow and nephropores sealed; open circles, inulin-clearances without di- rect measurement of urinary flow and nephropores not sealed; triangles, xylose- clearances with direct measurement of urinary flow and nephropores sealed; crosses, creatinine-clearances with direct measurement of urinary flow and nephropores sealed. Each point stands for a single separate animal. even at the low plasma-concentrations (Miller, Alving, and Rubin, 1940). To find whether secretion can account for the marked occur- rence of inulin in the urine of the crayfish, a study was made of the inulin-clearance at various levels of inulin in the plasma. Variation of the renal clearance with the plasma-concentration would demonstrate secretion. Parenthetically, even if the renal clearance of a substance does not vary with its plasma-concentration, secret
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