30 



Comparative Animal Physiology 



Chloride concentrations in blood and urine, recalculated into millimols per 

 liter by Krogh, are as follows: 



Blood Cl~ Urine Cl~ 



mM/1. mM/1. 



Astaciis fluviatilis--^ 195 10 



Astacus fluviatilis'"' 196 10.6 



Camharus clarkii^^'^ 117 9.6 



This difference suggests either that salt is reabsorbed into the blood or that 

 water is secreted by the kidney. In Eriocheir in fresh water certain substances 



4 Nephridial canal 



3 a Labyrinth 



5 Nephridial 

 canal 



2 Nephrostome 



36 Labyrinth 



5 Nephridial canal 



I Coeloraic sac 



1 mm. 



6 Urinary bladder 



Fig. 16. Cross section of the kidney of a crayfish, consisting of a much coiled but con- 

 tinuous tube with coelomic sac, labyrinth, nephridial canal, and bladder. From Peters."'* 



such as magnesium are much less concentrated in the urine than in the blood, 

 but chloride content is similar in each fluid, whereas in the crayfish all of the 

 urine salts are lower in concentration than are the blood salts. 



I he crayfish kidney consists first of a coelomic sac penetrated by blood 

 vessels and sinuses and lined by a single epithelial layer (Fig. 16). Then 

 fohows the green tubular labyrinth, a sponge-like structure, and a long (3 cm.) 

 nephridial canal. The canal empties into the urinary bladder. The blood 

 supply is rich to all parts except the bladder. The blood vessels open into 

 hcm(Koelc spaces which surround the kidney tubules. In the lobster, which 

 excretes an isotonic urine, the labyrinth tubule is larger, but the nephridial 

 canal is not well differentiated. ^'-^^ The nephridial tubule in the fresh-water 



