328 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Within the sub-phylum Vertebrates each class has its own biochemical 

 peculiarities, some of which are of diagnostic value. The class Cyclostoma 

 is so different from other Vertebrates in some of its biochemical characters 

 that it deserves special mention. The biochemical mechanism for ossifica- 

 tion is completely lacking from the Cyclostomes, and they show no sign of 

 calcification. Another character distinguishing them from Vertebrates in 

 general is the nature of the blood haemoglobin. Like the myoglobin of 

 Vertebrates, the haemoglobin of the lamprey has a molecular weight of 

 17,000, its arginine and lysine content is similar to that of vertebrate 

 haemoglobins whereas the histidine and cystine content as about the same 

 as that of an invertebrate haemoglobin. The dissociation curve of lamprey 

 haemoglobin is hyperbolic. The blood serum of Cyclostomes on electro- 

 phoresis gives a pattern which is very different from that of the other 

 Vertebrates. But, in spite of the above differences, the Cyclostomes have a 

 number of the biochemical characters of Vertebrates : a keratinized 

 epidermis, a liver secreting a characteristic bile and specialized for the 

 manufacture of glycogen, and their phosphagen is creatine phosphate as is 

 general for Vertebrates. 



Among Vertebrates, the class of fishes is distinguished by the complexity 

 of their depot fats, the fatty acids in them covering a wide range of homo- 

 logues in 14, 16, 18, 20 and 22 C atoms. The character of the depot fat 

 is not only influenced by the character of the ingested fat, but also by the 

 ability of the particular fish to hydrogenate or dehydrogenate the fatty acids 

 presented to it. 



In the fishes, as among most aquatic forms, xanthophylls are accumu- 

 lated in preference to carotenes. But among the accumulators of 

 xanthophylls, fishes form a special group since they accumulate only three : 

 lutein, taraxanthin, and astaxanthin. These carotenoids are accumulated 

 particularly in the chromatophores of the skin where they play a part in 

 photo-response. 



The role of the thryoid secretion in fishes is still unknown, but it can be 

 said that, contrary to the case in the higher Vertebrates, the thyroid secre- 

 tion of fishes has no action on metabolism and, contrary to its action in the 

 Amphibia, it does not appear to influence metamorphosis. 



Among the fishes, the sub-class Elasmobranchii, to which skates and rays 

 belong, possesses certain special biochemical characters. The skeleton is 

 essentially cartilaginous. It is calcified in some regions but properly speak- 

 ing it is never bone tissue which is found only in the scales and teeth. Like 

 all Vertebrates, the Elasmobranchii possess a liver secreting a bile which 

 contains steroid substances. This bile contains a special bile salt, scymnol 

 sulphate, a C27 steroid which has been detected in all Elasmobranchii 

 studied so far, while in the true fishes cholic acid, a C24 steroid, is present 

 in combination with taurine. 



