EXCRETION AND WATER BALANCE 51 



to inhibit the bacteria there was a fall in the ammonia content of 

 the urine. Presumably the fluid entering the nephridial vesicles 

 contains a fair amount of urea and other complex nitrogenous 

 compounds which are acted on by the bacteria to give a urine 

 containing mostly ammonia. 



2. Water and Salt Balance 



The nephridia of leeches, in addition to their function of 

 excreting nitrogenous waste, perform the task of maintaining water 

 balance. There is no detailed study of this process in leeches but it 

 is certain that the osmotic pressure of the body fluids is consider- 

 ably higher than that of the water in which they live so that there 

 must be a constant inward flow of water through the integument. 

 The function of the nephridia is to remove the excess water while 

 retaining as many as possible of the inorganic ions other than 

 ammonia. It is unlikely that they are completely efficient, any 

 more than are the nephridia of earthworms (Table 1), so there is 

 a steady drain on the salt content of the coelomic fluid which must 

 be replaced. A certain amount of mineral salts will be taken in 

 with the food but Krogh (1939) showed that Haemopis has a salt 

 uptake mechanism in the epidermis. He first exposed the leeches 

 to a current of distilled water for two weeks during which time 

 there was no opportunity to replenish the stock of salts lost in the 

 urine. He then placed them in frog Ringer diluted to 1/100 and 

 found that they took up chloride ions at a rate of 048/xM/g/h. In 

 order to maintain electrostatic equilibrium these ions would have 

 to be accompanied by ions of opposite sign or exchanged against 

 ions of the same sign. When the experiment was repeated using 

 other salt solutions it was found that the leeches were able to take 

 up sodium ions from sodium bicarbonate solution and chloride 

 ions from ammonium chloride solution. Krogh therefore con- 

 cluded that the leeches had separate mechanisms for the active 

 uptake of sodium and chloride ions and that the two were 

 normally taken up together. 



Although leeches frequently leave the water to feed or to breed, 

 they appear to have no very effective mechanism for controlling 

 water loss. When exposed to dry air they secrete abundant mucus 



