Water 



51 



Most of the osmotic concentration is made up by retention of huge quantities 

 of urea in all body tissues and fluids. Thus the elasmobranch can maintain 

 the same osmotic gradient in sea water as in fresh water, and continues to 

 excrete a hypotonic urine. It was formerly believed that the kidney unit had 

 a special segment for absorbing urea, but this is not correct (Fig. 20). '-'• In 

 the dogfish there is a long neck segment, a wide proximal tubule with scattered 

 cilia, then a narrow proximal tubule followed by a long distal tubule which 

 winds around the neck segment. Smith estimates that an average daily filtra- 

 tion of 80 cc./kg. is necessary to the formation of 20 cc. urine/kg. The skin 



TABLE 2. OSMOTIC CONCENTRATION AND UREA IN BODY FLUIDS 

 IN ELASMOBRANCHS (FROM SMITH'"). 



and gills are relatively impermeable to urea and the young are provided with 

 a store of urea, either by viviparity or by placement in an impermeable egg case 

 containing urea until they develop their own renal mechanism for conserv- 

 ing it. 



Smith studied elasmobranchs which live in fresh and brackish water in 

 southeastern Asia. Table 2 shows that for a 100 per cent increase in osmotic 

 pressure of the blood on going from fresh-water species to marine species the 

 urea increases by nearly 100 per cent and the serum Cl~ by only 60 per cent. 

 The urine output is low in marine elasmobranchs, and the urine is concen- 

 trated but always remains hypotonic to the blood. Magnesium, phosphate, and 

 sulfate are excreted by the kidneys, other salts extrarenally. 



The abihty of individual rays and doghsh to adapt to changed tonicity of 

 medium is not great. The dogfish Scyllium increased in weight by 10 per cent 

 in 4 hours after transfer to 80 per cent sea water. "'' The limits for Scylliinn 

 are said to be 46 to 102 per cent sea water. '''- 



Osmotic regulation in cyclostomes is not understood. Blood of the marine 

 Myodne glutinosa is reported to be hypertonic --^ or slightly hypotonic to sea 

 water. ^'^ Another marine cyclostome Polistotrema stotiti may be slightly 

 hypertonic. "^ The chloride concentration of the blood is lower than that 

 of sea water, ^'^ and the urea concentration is between that of elasmobranchs 

 and that of the teleosts. ^^ Petromyzon fluviatilis from fresh water shows a 

 serum freezing point of -0.48° (Dekhuyzen, from Smith---*). When P. 

 fluviatilis was put into 33 per cent sea water, "^^ the blood concentrations rose 

 from A 1=0.46 to Ai=0.52 after 22 hours, but irregular respiration indicated 



