24 



GENERAL ORGANIZATION OF THE PROTOZOA 



the protozoon has the power of manufacturing by chemical processes, 

 over and above those which are devoted to nutrition, various products 

 which are secreted just within or outside the peripheral protoplasm, 

 where they may form a protective armor in the shape of shells, or tests. 

 The materials thus formed within the cell body may be chitin (as in 

 the case of Arcella vulgaris or in any other shelled rhizopod where the 

 shell material is always laid down upon a chitin base); cellulose (as 



Fig. 6 



Ceratium tri|.ios, a dinuHagellale. (Alter Slein.) 



in the dinoflagellates) ; calcium carbonate (as in the foraminifera); 

 or silica (as in the radiolaria). The secretions may take the form 

 of definite plates, as in dinoflagellates, of continuous deposits, or of 

 symmetrical skeletons which are often very complex. When the 

 deposit is regular and continuous the shell material is added to the 

 chitin membrane, the walls growing thicker ^A'ith age of the organism; 

 but when the material is deposited at one time (dictyotic moment), 



