ELEMENTARY VITAL PHENOMENA 



173 



are secreted at once upon the surface of the cell in the form of 

 membranes, shells, and coatings, such as cell-membranes, the 

 c^te-membrane of plant-cells, the chitinous coats of insects, 

 the silicious cases of diatoms, the delicate latticed skeletons of 

 Radiolaria (Fig. 64), and the calcareous shells of Foraminifera ; 



A B 



FIG. 64. Silicious skeletons of Radiolaria. (After Haeckel.) A, Dorataspis, B. Thcoconus. 



and sometimes they are stored in the tissues between the in- 

 dividual cells as the so-called connective substances, such as chondrin 

 in cartilage, glutin in bone, calciiim phosphate in bone, and the 

 great number of supporting or skeletal substances which belong 

 to the albuminoids and in the different groups of animals have 

 compositions very different and as yet little known. 



b. Excretions 



The excretions are much fewer in number than the secretions. 

 Chief among them are the products of retrogressive proteid- 

 metamorphosis which are excreted by all living substance. 



Among gaseous excretions the most important one, whose pro- 

 duction is associated with the life of every cell without ex- 

 ception, is carbonic acid, the end-product of respiration ; it is 

 produced chiefly by the oxidation and the decomposition of 

 proteid, but under certain circumstances by the fermentation of 

 carbohydrates. As has already been seen, in addition to carbonic 

 acid, plants excrete oxygen, which is derived from the splitting-up 

 of the carbonic acid received from their green parts. It has, 

 therefore, been thought that the supposed contrast in the meta- 

 bolism of plants and of animals, already spoken of, is to be found 



