Integument 99 



outgrowths (Anabas, Birgus, Ocypode) or surround cavities within 

 the animal (vertebrate lungs) . When animals live where oxygen is 

 deficient or difficult to obtain, gills or dermal breathing structures 

 are most often present. Harms (1929) in speaking of beach-skip- 

 ping gobies makes the generalization that the more a fish lives in 

 mud the more vascular is its skin. 



In leaving the ocean for fresh water, animals encounter two diffi- 

 culties which require special abilities or adjustments: (1) respiration 

 is more difficult, and (2) there is danger of loss of essential sub- 

 stances from body fluids to the less dense surrounding water by 

 osmosis. The changes by which the external membranes of animals 

 have been able to meet these will be considered in the next two sec- 

 tions. It is worth while, however, to mention here a few points which 

 bear on the adjustments of the skin in passing from sea water to 

 fresh water. Such changes have been studied extensively by Adolph 

 (1925, 1927a, d, 1943) . No fresh-water organism is ever in absolute 

 equilibrium with the environment. Bony fishes in the ocean have 

 body fluids which are hypotonic to sea water. Fishes in fresh water 

 are not very different from those in the ocean, and their body fluids 

 are of course hypotonic to sea water. Marine animals can usually 

 endure variations in the salinity of the medium in which they live 

 better than fresh-water animals. The integument may be nearly 

 perfectly permeable and exercise little or no control over the passage 

 of salts, as in a sea cucumber. Such an animal as an earthworm, a 

 fish, or a frog has some degree of control over the passage of solutes 

 through its skin, kidney, or gill membranes. In earthworms and 

 frogs control appears to be more through the skin than the kidneys 

 (Adolph, 1927a, b) . 



In migrating from the ocean, animals with an exoskeleton or 

 more or less impermeable external membranes have marked advan- 

 tages over those which can exercise little or no control over the pas- 

 sage of fluids into or out of their bodies. They are able to keep 

 their body fluids in more or less steady state, and their cells con- 



