298 



THE BIOLOGY OF MARINE ANIMALS 



of cephalopods contains up to 140 mg non-protein-N per 100 c.c., of 

 which from one-third to two-thirds is NH 3 -N. The relative proportions of 

 ammonia are higher in the water surrounding the animal than in the urine, 

 indicating that some ammonia is lost extrarenally. However, there is a 

 possibility of bacterial action affecting the ammonia concentration of the 

 surrounding sea water during the lengthy periods in which the animals 

 were immersed. Concentrations of non-protein-N, especially NH 3 , are 

 always greater in the urine than in the blood, whereas NH 2 -N is less con- 

 centrated in the urine. It would appear, therefore, that ammonia is con- 



Auricle 



Ren o-peri cardial 

 canal 



Gut 



Ventricle 



Pericardium 



Gonad and 



genital 



duct 



Ventricle 

 Auricle 



583 



Pericardium 



Opening into 

 /pericardium 



(b) 



Reno-pencardial canal 



CO 



Fig. 7.2. (a) Diagrammatic Representation of the Renal Organs of a 



Lamellibranch; (b) Pericardial Glands of Mytilus; (c) Reno-pericardial 



Aperture of Patella, ((a), (c) after Goodrich ; (b) from Fretter.) 



centrated and amino-acids selectively retained by the kidney sac. Inorganic 

 constituents are also differentially treated by the kidney, for resorption of 

 K + , Ca ++ , Mg ++ and Cl~ takes place, while S0 4 = and Na + are secreted 

 into the renal fluid. These ionic displacements form part of the mechanism 

 of ionic regulation (p. 71) (8, 43). 



The urine of marine crustaceans appears to be formed as a blood ultra- 

 filtrate. In the lobster (Homarus), levels of injected inulin, creatinine and 

 glucose are the same in blood and urine, thus indicating that these par- 

 ticular substances, at least, are neither secreted nor absorbed in the kidney. 

 When the animals are retained in normal sea water, the urine formed is 

 isotonic with the blood. The hydrostatic pressure of the blood is about 13 

 cm H 2 in Carcinus, and the difference in colloid osmotic pressure between 



