PHYSICOCHEMICAL AND CHEMICAL PHASES 



169 



the amphibians and insects, have been al- 

 most unable successfully to invade marine 

 environments. Animals adapted to life in 

 fresh water overcome the constant tendency 

 for a strong inflow of water in one or more 

 of the following ways: 



1. Most commonly there is an increased 

 activity of the excretory system, and the 

 surplus water is ejected. The contractile 

 vacuoles of fresh-water protozoans have 

 this as their main function. Marine Protozoa 

 may, or may not, have a contractile vacuole; 

 if present, its rate of pulsation is slow and 

 serves to eliminate water engulfed with 

 food. The nephridial systems of fresh-water 

 metazoans, whether Hame cells or nephidia 

 proper, are all active in eliminating excess 

 water. The mechanism whereby fresh-water 

 sponges and coelenterates rid themselves of 

 water that enters osmotically is unknown 

 (Krogh, 1939, p. 31). 



2. Eury saline animals have some power 

 of adjusting skin permeability in keeping 

 with the concentration of the surrounding 

 medium. Calcium reduces permeability of 

 membranes, as is shown almost diagrammat- 

 ically by exposing the marine planarian Pro- 

 cerodes (= Gunda) to fresh water with 

 and without calcium (Oesting and Alice, 

 1935). The presence of calcium in quantity 

 is important for the invasion of brackish 

 water by marine organisms (Breder, 1934). 



3. The possession of naturally imperme- 

 able body walls reaches a logical extreme in 

 the water spider, Argyroneta, which carries 

 its own supply of oxygen below the surface 

 and avoids all osmotic exchange with the 

 fresh water in which it hves. Air-breathing 

 insects, like the dytiscid beetles, are simi- 

 larly independent of osmosis. Small aquatic 

 arthropods, whether crustaceans, arachnids, 

 or insects, if the adults are small, or in the 

 egg stage, or in early instars of larger forms 

 may carry on respiration by exchange 

 through an outer membrane impermeable 

 to water. The same exoskeleton that, as a 

 water-conserving mechanism, makes life 

 possible for insects in a dry atmosphere, 

 permits them to invade fresh water habitats 

 and allows some to invade the waters of 

 salt lakes. Larger insect nymphs with gills 

 excrete water that enters by osmosis, as do 

 many other animals of the fresh water. 



4. A vitelline membrane, impermeable to 

 water, has been described for trout eggs and 

 is probably common among eggs of fresh- 



water animals. The eggs of Hydra and ot 

 crayfish have a dense covering, and fresh- 

 water planarian eggs, among others, are 

 enclosed in a thick-walled case. 



5. A heavy coating of mucous appar- 

 ently slows down the ingress of water into 

 many aquatic plants and animals, including 

 the eel (Hesse, Alice, and Schmidt, 1937, 

 p. 35). 



Bony fishes from fresh water have a 

 concentration of body fluids which is much 

 higher than that of their environment and 

 approaches the concentration in marine 

 fishes. The exposed membranes, including 

 gills, skin, and mucous membranes, are per- 

 meable to water, and there is a large inflow 

 of water through these structures. Even 

 fresh- water fishes drink water (Alice and 

 Frank, 1948). Osmotic balance is main- 

 tained, as with invertebrates, largely by the 

 active excretion of dilute urine; water in- 

 take and excretion are more nearly propor- 

 tional to surface area than to weight. 



Marine fishes have evolved two methods 

 of meeting the osmotic situation presented 

 by the salt concentration found in sea water. 

 Elasmobranch fishes, the sharks and rays, 

 are nearly isotonic with their environment 

 as a result of the urea found in their blood 

 and body fluids. Actually, the osmotically 

 active internal concentration is somewha* 

 higher than that of sea water in the open 

 ocean or in the higher salinity of the Medi- 

 terranean. The internal concentration does 

 not fall to a point approaching equilibrium 

 when these fishes invade fresh water habi- 

 tats. The usual slight inward flow is in- 

 creased among the invaders of fresh waters. 

 In both instances, the excess is eliminated 

 by the kidneys; the fresh-water forms have 

 a large output of quite dilute urine (Smith, 

 1936). 



The bony fishes of the sea have a lower 

 concentration of osmotically active sub- 

 stances in their body fluids than that of sea 

 water; hence, unlike fresh-water animals 

 they face a steady loss of water by osmosis 

 unless their integument and gills are imper- 

 meable to water. This is usually not the 

 case. Actually, marine fishes drink large 

 quantities of sea water, and both water and 

 salts are absorbed from the alimentary tract, 

 the latter somewhat selectively (Smith, 

 1930), and excess ions are disposed of ap- 

 parently by extrarenal excretion. 



Anadromous and catadromous fishes pro- 



