134 PHYSIOLOGICAL EEGULATIONS 



The garter snake appears to maintain its water content (body 

 weight) as accurately as the other small vertebrates that have been 

 studied (CA ± 1.1 at 24-hour intervals). The maintenance is partly 

 by holding on to most of the water it has, day after day; but in 

 addition water is drunk nearly every day. Contrasted with mam- 

 mals, the turnover is small and is more largely influenced by the 

 humidity and temperature of the environment. The small turn- 

 over is said to correspond to the excretion of most non-aqueous ma- 

 terials in solid form and to the extreme resistance to passage of 

 water through the skin. Thamnophis and some other reptiles might 

 be able to supply ordinary water requirements (turnover) by oxi- 

 dative gain alone, even when only dry food is available ; but this has 

 not been demonstrated. 



Ingestion into the alimentary tract accurately adjusts the 

 snake's deficits; slow excretion of water through the urinary 

 (cloacal) tract adjusts excesses. Recovery of water content after 

 disturbance is as accurate as in mammals, and there are no recog- 

 nized points in which adjustment is incomplete. Without knowing 

 how a reptile is inconvenienced by a water load, the fact is plain 

 that no load is permitted to remain. 



§ 45. LiMAx 



Though cogent ''reasons" are often advanced why mammals, or 

 vertebrates, might value a constant water content, it is often im- 

 plied that to invertebrate animals such uniformity does not matter. 

 Is it true that these organisms are more indifferent about water, or 

 more slovenly in their operations toward water? Not having those 

 kinds of organs by which vertebrates exchange water, do they still 

 have special equipment for handling water 1 Are they slower about 

 adjusting it? 



Among molluscs, terrestrial snails were studied in relation to 

 water by Kiinkel ( '16). In four species of the "naked" pulmonate 

 slug Limax, desiccation is allowed to proceed for a few days, then 

 water is presented (fig. 82) . Net gains are recorded only in 24-hour 

 periods, but other of Kiinkel's observations indicate that most of 

 the intake, which is probably by drinking through the mouth, is 

 immediate. In general the amounts taken nearly compensate the 

 deficits existing. The accuracy of recovery is somewhat less than 

 that in dog or in snake. 



