138 



BIOLOGY OF THE PROTOZOA 



The lime shells of Foraminifera are formed in quite a different 

 manner. Here, calcium carbonate is precipitated between two lam- 

 ellae of chitin very much as a cement wall is made between board 

 surfaces. Except for a single mouth opening such limestone shells 

 may form an unbroken wall about the organism (imperforata) or 

 they may be perforated by myriads of minute pores (foramina) 

 through which the pseudopodia pass to the outside, a condition 

 which gave rise to the name Foraminifera. In the more compli- 

 cated types of these lime-stone shells, which may reach a diameter of 

 2 or 3 inches, the calcium carbonate may be deposited at successive 

 intervals of growth, thus giving rise to chambered structure of the 

 cells. Such polythalamous shells are complicated by the presence 

 of an intricate system of canals which, in life, are filled by moving 

 protoplasm (Fig. 74). 



Fig. 74. — A complex polythalamous shell of Operculina (schematic). The shell is 

 represented as cut in different planes to show the distribution of the canals and the 

 arrangement of septa and chambers. (After Carpenter.) 



Skeletons of Ileliozoa and Radiolaria, unlike the more clumsy 

 shells of the Foraminifera, are usually delicate in structure and 

 graceful in design. They are formed for. the most part by a deposit 

 of silica upon a chitinous base. Dreyer has given evidence to indi- 

 cate that such skeletons have their beginnings in spicules which 

 conform in shape and size with the nodal points in the alveolar walls 

 of the cytoplasmic reticulum (Fig. 12, p. 33). Isolated spicules are 

 characteristic of several Heliozoa and Radiolaria where they form a 

 loose or felted covering in the outer protoplasm. Such spicules 

 invariably grow by accretion, that is, by the addition of new sub- 

 stance to the outside of that already formed. If such added material 

 is formed in a limited region of the protoplasm, the result is a con- 



