The comment of the poster to my request for clarification cites “The formation of ammonia itself requires… large quantities of water to dilute it out of a biological system” I conclude that he assumes that the problem with animals eliminating ammonia as such is one of solubility in water. This is not the case. The solubility of ammonia is such that saturated solutions exceed a concentration of 10 mol/L.
The problem is that ammonia is extremely toxic at low concentrations, so that decreasing its concentration to non-toxic levels would require a large volume of water. The writer of the cited text is, in my reading of it, presenting this as a hypothetical solution, and then rejecting it because the demands of supplying this volume of water would be prohibitive to the organism.
This is not my field, so the best information I have about the toxicity of ammonia comes from an anonymous web page on the University College London web site. It states:
Ammonia is highly toxic. Normally blood ammonium concentration is < 50 µmol /L, and an increase to only 100 µmol /L can lead to disturbance of consciousness. A blood ammonium concentration of 200 µmol /L is associated with coma and convulsions.
The page goes on to state (in red):
200 µmol /L is far too low a concentration of ammonium to affect plasma pH or the normal transport of sodium and potassium ions across nerve cell membranes.
I know almost no physiology, but my inference of the significance of this is that it would be difficult for the kidney to respond to this concentration of ammonia by filtering it out. Hence it needs to be converted to a non-toxic (or much less toxic) compound — urea, in the case of vertebrates.
Further reading suggests that the problem is that there is indeed a renal system involving the proximal tubule for secreting ammonia into the urine by a mechanism coupled to the cation transport and responsive to pH. In fact this system exists to control blood pH by generating ammonia to neutralize blood acid. This would explain the second part of the quotation above — the organism’s mechanism to control blood acid is inconsistent with a secretion mechanism for blood ammonia.
It would be good if a molecular physiologist could comment on or edit this section.