BIOCHEMISTRY AND TAXONOMY 327 



The biochemical characteristics of various classes of algae are listed in 

 Tables XXI and XXII. We can see that the biochemical diversity of these 

 classes is clearly indicated. Moreover certain of these classes show biochemi- 

 cal similarities confirming the grouping of the Xanthophyceae, Chr}^sophy- 

 ceae and Bacillariophyceae under the heading Chrysophyta while the 

 affinities between the Dinophyceae and the Crj'ptophyceae justify their 

 grouping in the Pyrrophyta. 



When we come to the higher groups in animal taxonomy, the idea of 

 biochemical character is by no means a new one. For a long time, the taxo- 

 nomists have considered the siliceous skeleton of Radiolaria as a character- 

 istic of that order, and within this order the presence of strontium sulphate 

 in the skeleton as characteristic of the sub-order Acantharia. In the phylum 

 Porifera, the class Calcispongiae have long been distinguished from other 

 sponges by the calcareous nature of their spicules. The phylum Brachio- 

 poda has been divided into the Ecardines, characterized by a calcareous 

 shell, and the Testicardines. Similarly, the presence of chitin in the tegu- 

 ment of Arthropods has been considered as one of the characteristics of 

 that phylum, and the possession of a calcareous shell as characteristic of the 

 Mollusca. 



Several biochemical characteristics of the sub-phylum Vertebrate can 

 be enumerated : 



(a) the biochemical system of bone, involving a number of functional 

 subtances such as parathyroid hormone, vitamin D, etc. ; 



(b) the biochemical system of blood clotting, involving a protein 

 characteristic of Vertebrates, fibrinogen, and a mechanism trans- 

 forming fibrinogen into fibrin by the action of thrombin, resulting 

 from the action on prothrombin of a number of substances present 

 in plasma and blood platelets; 



(c) the presence of keratin in the skin; 



(d) the system of digestive enzymes in the form of a series of hydrolases, 

 starting with pepsin and acting at diflFerent points ; 



(e) the presence in the blood of red cells containing a typical haemo- 

 globin, characterized by its molecular weight and by certain definite 

 proportions of arginine, cystine, histidine and lysine; 



(/) the presence of a liver secreting a bile containing special steroids, 



the bile salts; 

 (g) the protein system of blood plasma ; 

 [h) the presence of a carbohydrate metabolism involving insulin and 



systems sensitive to its action; 

 (/) the presence in the blood plasma of a system of inorganic bases 



and especially of a high level of sodium; 



