The synthesis of excretory urea in certain land planaria

(Campbell, 1965), earthworms (Bishop and Campbell, 1963, 1965) and snails (Campbell and Bishop, 1970; Campbell and Speeg, 1968; Tramell and Campbell, 1972) via the ornithine–urea cycle (OUC) indicates that the usurpation of the basic nutritional pathway of arginine synthesis for ammonia detoxification first took place in invertebrate animals (Campbell, 1973). Osmotic water retention may have been a primary factor in the selection for a high rate of urea synthesis in terrestrial gastropods. As for lower vertebrates, it seems probable that the selective pressures for the integration of the components of OUC for an increased rate of urea synthesis first came about in the marine environment (Campbell, 1973). However, whether this was originally directed towards ammonia detoxification, the synthesis of urea as an osmolyte or both is a moot point. The capacity to synthesize urea during periods of restricted water availability, as seen in the ammonotelic and ureogenic lungfishes, might have preadapted the early vertebrates for their transition to the land. The ammonotelic–ureotelic transition made by 3625 The Journal of Experimental Biology 206, 3625-3633 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00612