272 INVERTEBRATE PHYSIOLOGY 



largest amount. In the distal coil of the crayfish kidney Maluf described 

 relatively large columnar cells with distinct mitochondriona and large 

 clear apical granules which bulge into the lumen of the tubule. The evi- 

 dence that water is moving outward is that, when the animal is placed 

 in a medium of greater salinity, the rate of urinary flow falls and the 

 osmotic pressure of the urine simultaneously rises. But the constancy in 

 body weight indicated that the total volume of water in the animal remained 

 about the same up to 272 mM NaCl per liter, an amount of salt which gave 

 a fluid initially hypertonic to the blood. Maluf argued that this indicated 

 that the decrease in urinary flow, with rising external salinity, was not 

 due to a decrease in hemocoelic pressure which, assuming that filtration 

 does occur, might cause a decrease in the rate of filtration. In his view, with 

 the rate of urinary secretion depressed, the apical vacuoles of the tubule 

 should disappear. Maluf showed that the vacuoles do indeed become 

 markedly reduced in number after even 24 hours of this saline treatment, 

 and often disappeared entirely after a few days of the treatment. 



But Peters, who had demonstrated the hypotonicity of the urine to the 

 blood in 1935, had interpreted his data to indicate that filtration occurs in 

 this animal followed by reabsorption of salts. The apical vacuoles were 

 taken by Peters to indicate an active absorption of salt from lumen to 

 blood. We do not have at hand the critical evidence for a choice between 

 these two views. There appear to the writer no good reasons for rejecting 

 Peters' views, since it would seem that water might well accompany the 

 reabsorption of salts, as it does in the vertebrates, for example. If less salt 

 is reabsorbed, less water would be taken up and so the disappearance of 

 the vacuoles may be interpreted as equally favorable to either point of 

 view. 



Burger (1955a) has carried out studies of the time course of excretion 

 of substances injected into Honiarus, including phenol red, bromsulfalein, 

 and para-amino hippuric acid (PAH). Both PAH and phenol red were 

 concentrated by the kidney, with the U/B ratios falling towards one as the 

 blood concentration was increased. It was interesting that secretion into 

 the gut appeared to be even more active than that into the urine ; but, 

 since the dye was then reabsorbed, this route played little active part in the 

 excretion. The observation indicates simply that a transport mechanism 

 useful in some other way to the animal is active in the hepatopancreas and 

 perhaps epithelial cells of the gut. 



Although insects are specifically excluded from this review, an obser- 

 vation made by Ramsay (1954) on the secretion of phenol red by the 

 Malpighian tubule of the stick insect should be mentioned for its general 

 interest. Since secretion appears to be the chief means of excretion in the 

 insects, it is no surprise that phenol red should be secreted. What is sur- 



