330 UNITY AND DIVERSITY IN BIOCHEMISTRY 



like all Vertebrates, have keratin in their integument, but in addition 

 another keratin is found in their skin which is called "feather keratin" 

 because it is also found in bird's feathers. This is not the only biochemical 

 character that reptiles and birds have in common; both have a uricotelic 

 nitrogen catabolism. 



Although it is true that Reptilia and birds have many biochemical 

 characters in common, there are also some peculiar to birds alone. Among 

 them is the mechanism for the detoxification of benzoic acid by synthesis 

 of ornithuric acid, a conjugate of benzoic acid and ornithine. 



The biliary steroids of more than sixty species of mammal have been 

 studied. In general, the characteristic biliary steroid is cholic acid. It is 

 particularly interesting to find in groups of related species certain unique 

 bile salts, although not necessarily as the principal constituents. For 

 instance, in all members of the families Otaridae, Odobenidae and Phocidae 

 (the seals and walruses) so far examined, a special bile acid, ^-phocaecholic 

 acid, has been found in addition to cholic acid. Certain genera too have bile 

 salts of their own, for example the genus Sus which has no cholic acid but 

 hyodesoxycholic acid instead. As already stated, biliary steroids are con- 

 jugated in the form of sulphates in the Elasmobranchii and Amphibia, and 

 with taurine in the teleost fishes and reptiles. In mammals they are con- 

 jugated with taurine and glycine in varying proportions. The presence of 

 glyco-acids is characteristic of mammalian biles. 



The members of the sub-phylum Urochorda do not show the bio- 

 chemical features we have described as being characteristic of Vertebrates 

 but they have features of their own. They possess a cellulose coat and they 

 accumulate vanadium in their tissues and blood cells. Although there is no 

 thyroid secretion, they are sensitive to thyroxine which activates the 

 development of the larvae. Also, their neural glands contain principles 

 similar to the hypertensive, melanophore-expanding, oxytocic and gona- 

 dotrophic principles of the vertebrate hypophysis. 



All the facts so far collected pertain to the Chordates, but many facts of 

 a similar nature could be gathered through the study of each of the groups 

 of the Metazoa. In what follows we shall limit our enquiry to just a few 

 examples. 



One of the oldest identified features of the phylum Arthropoda is the 

 presence in their integument of a polyacetyglucosamine called chitin. In 

 the integument the chitin is combined with protein. Another general 

 character of Arthropods is the peculiar nature of their blood coagulation 

 mechanism. The physiological phenomenon of blood clotting is found in 

 Vertebrates and Arthropods and the two systems show certain similarities. 

 The blood clotting mechanism in Vertebrates is summarized in Fig. 90a. 



In Arthropods, the phenomenon of plasma coagulation depends upon 

 the action of tissue coagulins on a coagulable protein. In the lobster, which 



