BIOCHEMISTRY AND TAXONOMY 329 



The organs and blood of the Elasmobranchii contain considerable 

 quantities of urea. In the blood of marine Elasmobranchii, urea may attain 

 a concentration of 26g/l and is never less than 18g/I. Here we are dealing 

 with a case of selective retention of urea since the other nitrogenous com- 

 pounds in Elasmobranch blood are no more concentrated than in the blood 

 of other Vertebrates. 



As in the case of the Cyclostomes, the biochemical characters of the 

 Elasmobranchii are so clearly different from those of the teleost fishes that 

 they are strong arguments in the support of those zoologists who favour 

 the separation of the class of Elasmobranchii from the class of fishes. 



Like the Elasmobranchii and unlike teleost fishes, Amphibia have bile 

 salts of the C27 type. While the bile salts of teleost fishes are present in 

 combination with taurine, those of Amphibia are always combined as 

 sulphates as in the Elasmobranchii. Thyroxine has no action of the meta- 

 bolism of Amphibia, as in the fishes, but the thyroid secretion has a very 

 marked effect on their metamorphosis. 



The Amphibia are uniform in their amino-nitrogen and purine cata- 

 bolism which is always ureotelic. 



Biochemical support for the separation of the Amphibia into the orders 

 Anura and Urodela is found on comparison of the retinal pigments in the 

 two. While the retina of Urodela contains mostly porphyropsin (p. 291) 

 which is also present in Cyclostomes and is a derivative of vitamin Ag, the 

 retina of the adult Anura contain only rhodopsin, a derivative of vitamin 

 Aj. In this respect the Urodela are nearer to the Dipneusti which have 

 porphyropsin as their retinal pigment. It can be pointed out here, that in 

 the nature of their retinal pigment the class of fishes is very heterogeneous, 

 as is the case for nitrogen metabolism and a number of other characters. 



The nitrogen metabolism of the class Reptilia differs from that of fishes 

 and Amphibia. They always have a uricotelic amino-nitrogen metabolism, 

 a fact connected, as stated in dementi's law, with the lack of arginase in 

 the liver. Another interesting point is the fact that in terrestrial turtles, 

 such as Testudo graeca, a more or less active system of ureotelic metabolism 

 is present together with the enzyme system for uricotelic metabolism. With 

 regard to their biliar)' steroids, the Reptilia show a systematic variation : 

 among the sub-class Chelonia, the biliary steroids are derivatives of 

 sterocholanic acid, while most of the members of the order Serpentes con- 

 tain chloic acid. Among these, however, the family Boidea is characterized 

 by pythocholic acid. Turtles and snakes contain C24 biliary steroids, 

 while Haslewood has found in the sub -class CrocodiHni that the alligator 

 has a C27 steroid. 



In all Reptilia examined so far, the biliary steroids are combined with 

 taurine, as is the case with the teleost fishes, while in the Elasmobranchii 

 and Amphibia the steroids are always in the form of sulphates. Reptilia, 



