258 INVERTEBRATE PHYSIOLOGY 



mental period, to cut through a leg at the femur and to stop the bleeding 

 by compressing the stump with a hot forceps. In some experimental ani- 

 mals handling produces a shock effect on urine formation, so that this 

 method of blood collection may not be ideal. It does not seem possible to 

 argue that there was any large effect, because reference to Table 1 shows 

 no evidence that there was any suppression of urine formation in com- 

 parison with other fresh-water decapods. In his discussion of the shock 

 effect Wikgren (1953) states that no marked shock effect was demon- 

 strable in the crayfish. The result of all these comparisons is, then, that 

 the observations of Maluf are essentially correct. 



It does not follow that the interpretation is correct, and the alternatives 

 are worth a brief examination. Maluf prefers to interpret the concentra- 

 tion curve of inulin as showing that at low blood-inulin levels the secretory 

 process for inulin is more rapid than that for water, but that saturation of 

 the mechanism begins to take place at higher blood concentrations. The 

 data may equally be interpreted as the result of a filtration followed by re- 

 absorption of water. The reabsorption of water from the kidney of a fresh- 

 M^ater animal seems at first glance to be highly improbable. Yet it may be 

 essential that water accompany the very large quantity of salt that is re- 

 absorbed by the kidneys of these animals, and that this reabsorption is 

 related to the very low rates of urine flow which characterize the repre- 

 sentatives of this group of fresh-water animals in contrast to the fresh- 

 water and terrestrial molluscs and worms, as has been illustrated in 

 Table 1. 



Another result reported by Maluf seems to the writer rather to favor 

 the view of water reabsorption, though it does not provide crucial evidence 

 on the point. Why, we may ask, should one carbohydrate — inulin — be 

 secreted, while another carbohydrate — xylose — is being reabsorbed. From 

 experience with xylose in other animals it would appear that it is only a 

 stereotype for glucose and that its reabsorption indicates an active glucose- 

 reabsorbing mechanism. But if all the components of the urine are secreted, 

 as in the aglomerular fish, why a specific reabsorbing mechanism for either 

 glucose or xylose ? The fact that the xylose U/B ratio curve rises to a value 

 of approximately one at the higher blood concentrations receives a labored 

 explanation by Maluf ; but by the above interpretation this may carry the 

 normal significance, simply that a transport mechanism has reached its 

 maximum rate. It may be predicted on the new interpretation that phlorizin 

 treatment will result in a curve for xylose like those for inulin and 

 creatinine. 



The creatinine curve, finally, does not contribute much towards making 

 a choice of these alternatives. Most kidneys which have been studied are 

 not indifferent to this substance, with both reabsorption and secretion 



