Water 9 



Several of these mechanisms require energy, hence work is clone in osmotic 

 regulation. The relative importance ol' each mechanism differs with the 

 animal. 



Animals tend to maintain an "optimum" osmotic concentration for a given 

 environment. Adolph ' has shown that in many species, upon return to 

 normal environment after a period of dehydration, water is taken up and, after 

 a period of hydration, water is lost, until the osmo-conccntration reaches the 

 "optimum" for the particular animal. 



There are numerous variations in osmotic properties of animals with season, 

 age, nutrition, reproductive and molting cycles, and geographic races. In the 

 following account an attempt is made to select comparable material but it must 

 be emphasized that measured values for a given species may differ with condi- 

 tions. There may be acclimatization of individuals to osmotic condition and 

 there may be genetic acclimatization which operates by selection. 



A careful summary of the mechanisms of osmotic response in each phylum 

 of animals which has been studied is given by Krogh. ^^-^ We shall, therefore, 

 confine our discussion to a search for phylogenetic and ecological implications 

 and applications of the subject. Ecologically, few factors in the environment 

 so limit the distribution of animals as does the availability of water. Phylo- 

 geneticallv, so many separate exits have been made from the ocean to fresh 

 water, and from water to land, and so many separate reinvasions of water have 

 been made that evolution with respect to water is a tree of many trunks. 

 Nature is carrying out experim.ents in osmotic adaptation which we can 

 observe at the present time, and species may be determined by the range of 

 osmotic labihty of a population. 



The ocean is the ancestral animal home. The composition of salts in sea 

 water differs in different parts of the w^orld and has changed considerably 

 during biological history. The total osmotically effective concentration of the 

 ocean has increased since the earliest appearance of life. '1 he water of mid- 

 ocean is concentrated, smaller seas and bays are diluted by inflow of fresh 

 water, and in estuaries and tributary mouths brackish water merges with fresh 

 water. In seas such as the Mediterranean, where the evaporation is high, the 

 salinity exceeds that of the open ocean. Fresh-water dilutions of ocean water 

 are often expressed in per cent sea water. As an average figure for reference 

 we can consider sea water as equivalent to a 3.4 per cent sodium chloride 

 solution, i.e., it has a salinity of 3.4 per cent or a chlorinity of 1.9 per cent. 

 Sea water freezes at approximately -1.8°C. and is, therefore, nearly molal in 

 its osmotic concentration. Compare this with a fresh-water pond which may 

 show a freezing point depression of 0.02^C. or less, or with salt lakes where 

 phytoflagellates swim in water which freezes at -7.5 "C. 



All ph}la and a majorit>' of the classes of animals have representatives 

 which have made the adjustments necessary for marine life: some have 

 remained in the ancestral home, others ha\e returned to it. Animals from 

 fewer phvla have ventured into brackish water and only a few ha\e sufficient 

 osmoregulation for life in fresh water. Some animals have invaded land 

 directlv from the ocean, others through the a\enues of estuarinc and fresh 

 water. The parasitic habit has been assumed by marine, fresh-water, and soil- 

 dwelling groups. Evidence regarding the osmotic limitation to distribution of a 

 group of animals can be obtained by observing the responses to osmotic stress. 



