130 



BIOLOGY OF THE PROTOZOA 



of silica are deposited in the inner protoplasm and passed out during 

 reproduction to be cemented on the chitinous membrane in regular 

 patterns (Euglypha alreolata. Fig. S, p. 30). Foreign bodies caught 

 up in the wrinkles of withdrawing pseudopodia are similarly stored 

 in the protoplasm to be used for shell-building purposes, Verworn, 

 for example, compelling Difflugia to build its shell of difl'erent kinds 

 of powdered glass. 



The lime shells of Foraminifera are formed in quite a different 

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

 ella? 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 



f Fig. 66. — A complex polythalamous shell of Opcrcidina (achematic). 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.) 



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 limestone 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. 60). 



Skeletons of Heliozoaand 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 indicate that 



