EXCRETION 299 



blood and urine is 9-6 cm, leaving a surplus pressure to produce filtration 

 into the coelomic sac of the kidney. Inorganic crystalloids, however, are 

 differentially treated by the kidney as part of the mechanism of ionic regula- 

 tion. Crustacean blood tends to have low levels of Mg^ h and S0 4 =, and 

 the kidneys act so as to eliminate these substances from the blood, and 

 conserve Na + and K>. In dilute media the rate of urine production is 

 markedly increased {Carcinus, Palaemonetes). 



Ammonia is the principal constituent of total excretory-N in crusta- 

 ceans, forming some 60-85% of the total. Most of the excretory-N is 

 eliminated extrarenally. The gills of Eriocheir are highly permeable to 

 ammonia, which is lost at the rate of about 40 mg NH 3 -N/kg/day, and 



Bladder 



Coelomic Bladder- labyrinth 



sac opening 



Labyrinth 



Bladdi 



Nephridiopore 



Fig. 7.3. Excretory Organ of Palinurus. 

 (From Winter stein 's Handbuch) 



there is little change in ammonia excretion when the antennary glands are 

 closed. Nitrogen excretion in Carcinus maenas occurs at the rate of 44 mg 

 total N or 38 mg NH 3 -N/kg/day, and it is increased by injury and high 

 protein diet. Amino-acids are excreted in amounts which constitute 

 10-20% of total excretory-N in some species. In Maia squinado urine is 

 produced at the rate of 30 c.c./kg/day, and contains only 5-4-10 mg non- 

 protein-N/100 c.c. These nitrogen concentrations are about twice those in 

 the blood, and the kidney, accordingly, appears to concentrate excretory-N. 

 In some crabs {Cancer, Carcinus) ammonia represents a large part of non- 

 protein nitrogen in the urine; in Maia, however, it forms only some 10% 

 of the total, leaving a substantial fraction unaccounted for (8, 27, 30, 32#, 

 32/?, 36, 37, 38, 40, 43). 



Marine Fish. Most of the waste nitrogen in bony fishes is excreted across 

 the gills, probably by diffusion, only a small amount escaping in the urine. 

 Homer Smith was able to separate the excretory output of the gills and the 

 kidney by means of an experiment in which a fish was encircled with a 

 rubber dam, whereby the fluid in the anterior branchial region could be 



