i PHYLUM PROTOZOA 31 



Radiolaria the skeleton is composed not of silica, but of a 

 chitinoid substance called acanthin. Embedded in the 

 protoplasm is a perforated membranous sac, the central 

 capsule (Figs. 9 and 10, cent, caps.}, in the protoplasm within 

 which is a single nucleus or a number of nuclei, and a 

 number of oil-drops. There is no contractile vacuole, but 

 in many Radiolaria the protoplasm outside the central 

 capsule contains numerous non-contractile vacuoles, the 

 presence of which gives it a frothy appearance. 



Inl.caps.fr 



SKel. 



FIG. 9. Lithocircus q,nnularis. cent. caps, central capsule ; ext. caps. pr. extra- 

 capsular protoplasm ; int. caps. pr. intra-capsular protoplasm ; nu. nucleus ; 

 psd. pseudopods ; skel. skeleton ; z. cells of Zooxanthella. (After Biitschli, from 

 Parker's Biology.) 



Radiolaria which give rise to colonies are exceptional, 

 but a few cases occur. In these (Fig. n) the central 

 capsule divides again and again, giving rise to a number of 

 central capsules which remain embedded in a firm gelatinous 

 substance the vacuolated protoplasm outside the central 

 capsules. Such a mass, which may attain considerable size, 

 floats about freely in the sea. 



In addition to reproduction by simple binary fission, 

 spore-formation also occurs in some of the Radiolaria. 

 The protoplasm contained in the central capsule breaks up 



