Water 



65 



land. (See Pearsei**^ for a discussion of migrations to land.) In the vertebrates, 

 protection against surface drying is afforded by horny scales, feathers, and 

 hair. All of the successful land animals also have protection against excessive 

 water loss from their respiratory surfaces. Lack of such protection keeps sf)me 

 groups (Onychophora, land Isopoda, Amphibia) restricted to a moist en- 

 vironment. 



All of the really successful land dwellers seem to be able to retain water 

 by excreting a concentrated urine. This is functionally comparable to salt 

 secretion by the gills of marine animals, but probably no marine animals except 

 m.ammals excrete a hypertonic urine. Many land insects excrete nitrogenous 

 wastes in solid or semisolid form and probably reabsorb water in the posterior 

 digestive tract. The kidney tubules and cloaca of birds and the tubules of 

 mammals reabsorb water. Water reabsorption as a kidnev function occurs in 

 many aquatic animals; water reabsorption accounts for urea concentration bv 

 the frog kidney, yet is insufficient to permit the frog to live in a dry environ- 

 ment. The importance of quantitative rather than qualitative differences in 

 responses to osmotic stress is illustrated by the animals which live in atypical 

 habitats. In Java there are soil sipunculids and land nereid polvchaetes."' 

 In Java, Florida, and elsewhere there are land-dwelling hermit crabs. In Italy 

 a typical brachyuran crab, Telphusa, lives in fresh water. Insect larvae and 

 one adult Chironomns live in ocean waters near Samoa. There are desert 

 amphibia, some of them ovoviviparous.''^' ^'^'' Closely related snails dwell in 

 salt, brackish, and fresh water and on land. The fact that many animal groups 

 have representatives in atypical environments demonstrates the labilit)' of 

 osmotic characters. 



No particular osmotic character is exclusively associated with life in any 

 particular habitat. The functional superiority by which one group succeeeds 

 in an environment which is difficult osmotically, whereas another group is 

 restricted to an easier environment, is quantitative. Sometimes new structures 

 take over a given function; sometimes old structures become obsolete, but in 

 each major environment some animals exist with the kind of capacities used 

 especially in another environment. Whether a given group will succeed in a 

 new environment depends on its ability to respond quantitatively to environ- 

 mental stresses in making use of functions which it already possesses, rather 

 than in developing totally new functions. 



Within a species it is likely that individuals show certain quantitative 

 differences. Some specimens of Carciniis excrete an isotonic, some a hyper- 

 tonic, and some a hypotonic urine. Would those individuals of Carcimis 

 which excrete a hypotonic urine be more likely to push up estuaries toward 

 fresh water? Keys ^^~ noted that some of a group of marine killifish (Fiin- 

 dulus parvipinnis) readily become acclimated to fresh water, whereas others 

 in the group perish. Combined field and laboratory observations of animals 

 living in a long estuary showed individual and species differences in tolerance 

 of sea-water dilution. -^'^ Gammanis from open sea (Ao=l-79) tolerated 

 dilutions down to 8 per cent sea water, whereas individuals from brackish water 

 (Ao=0.978) survived dilutions down to 2 per cent sea water. Mya from the 

 open sea were limited in tolerance to 52 per cent sea water, but Mya from 

 brackish water tolerated sea-water dilution down to 36 per cent. The experi- 

 mental data on osmotic regulation agree well with distributional data. A 



