PHYLUM PROTOZOA 



31 



subsequently become lost, pseudopodia appearing in their 

 place. 



The Radiolaria are marine Rhizopoda which have exceed- 

 ingly delicate, thread-like pseudopodia (Fig. 9, psd) and a 

 skeleton usually composed of silica. This skeleton may be 

 composed of loosely woven needle-like bodies or spicules ; 

 more usually it is in the form of a globular, conical, star- 

 shaped, or disc-shaped shell, perforated by numerous open- 

 ings, and often supported by spines which radiate out from 



Int. caps. pr 

 cent caps 



SKcl. 



•Mxt.caps.jm 



Fig. 9. — Liteocircus annularis, 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 Biltschli, 

 from Parker's Biology.') 



the centre ; sometimes (Fig. 10) there are several such 

 shells one within the other. In some Radiolaria the skele- 

 ton is composed not of silica, but of a chitinoid substance 

 called acanthin. Embedded in the protoplasm is a perfor- 

 ated 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. 



