Integument 97 



adaptation" means, but there is no question that primitive marine 

 animals in general have freer interchange between internal and ex- 

 ternal fluids than do more specialized fresh-water and land animals. 

 In fresh water it is necessary that liquids be prevented from passing 

 out of the bodies of animals and thus decreasing essential sub- 

 stances, or water must be rapidly eliminated in order to maintain 

 the concentration of internal fluids. Most fresh-water animals 

 readily permit the passage of water from their bodies. A paramecium 

 eliminates water equal to 31 to 700 times its own volume daily 

 (Hesse, 1920), depending on temperatures. There are two pri- 

 mary mechanisms involved in the active regulation of osmotic con- 

 centration. The kidney concentration of salt and elimination of 

 large amounts of water are characteristic of stenohaline fresh-water 

 fishes; the elimination of excess salt and conservation of the water 

 by the gills appear to be common to the stenohaline marine teleosts 

 and the euryhaline forms like the eel (Keyes, 1933). Chloride- 

 secreting cells in the skin of fishes may keep internal osmotic pres- 

 sure down (Keyes & Willmer, 1932) , and adrenalin, probably by 

 constricting blood-vessels, may decrease or abolish such activity 

 (Keyes &C Bateman, 1932) . Adult salmon have salt-secreting cells 

 in their skins, but young salmon in fresh water do not. 



Protective integumentary structures may manifest themselves as 

 thickenings or special outgrowths such as scales, feathers, or hairs; 

 chitinous or horny cuticle; bony or horny plates; or glands which 

 form mucus, fat, or other secretions. Hatch (1946) points out the 

 advantages of exoskeletal structures to insects and birds. He states 

 that beetles make up half of all existing insects. They are perhaps 

 best protected from water loss. Pantin (1931) says "the acquisition 

 by an animal of independence from its environment has necessitated 

 increased impermeability of its surface membranes." Kennedy 

 (1927) has discussed the advantages and disadvantages of the exo- 

 skeletons of insects in a very illuminating way. He says, "The pos- 

 session of an exoskeleton makes an animal very sensitive to tern- 



