52 



Comparative Animal Physiology 



a weak condition. It is certain that marine cyclostomes do not maintain hyper- 

 tonicity by urea retention, nor are they as dilute as teleosts. 



Fig. 21. Concentration of blood (Ai) as a function of external concentration (Ao): 

 O the elasmobranch Scyllium; • in a fresh-water eel; in a carp X when transferred 

 directly, and ▲ when adapted gradually to dilute sea water; and ® in a frog (Rana). 

 Data from Duval.'^ 



A PREDOMINANTLY FRESH- WATER GROUP; AMPHIBIA 



The amphibia are a group of fresh-water animals. Some are permanently 

 aquatic; others spend at least a part of their life on land. Eggs are laid either 

 in water or in a very moist environment. 



The blood of a frog is hypertonic to pond water (Rnwn blood Ai^O.563, 

 Bufo serum Ai=0.761). ^'^ The urine is hypotonic to the blood and is more 

 copious in a dilute than in a concentrated medium. Bottazzi adapted frogs to 

 salt solutions, resulting in the following values: 



The osmotic behavior of frogs has been extensively studied. Krogh^"*'^ 

 summarizes Overton's observations, and Adolph^- '^ reviews the work by 

 himself and his students. If the cloaca of a frog is tied shut and the frog is 

 left in pond or tap water, the body weight increases; when the cloaca is later 



