82 



PROTOZOA 





CHAP. 



in the several divisions. 1 The Acantharia (Figs. 24, 25, A) have a 

 skeleton of radiating spines meeting in the centre of figure of the 

 endoplasm, and forcing the nucleus to one side. The spines are 

 typically 2 in number, and emerge from . the surface of the 



Fig. 26. A, Lithocercus annularis, 

 with sagittal ring (from Parker 

 and Haswell). B, Aulactinium 

 actinastrum. C, calymma ; cent, 

 caps., km, central capsule ; 

 Ext. caps, pr., Extracapsular, 

 and Int. caps.pr., intracapsular 

 protoplasm ; n, mi, nucleus ; 

 op, operculum ; ph, phaeodium ; 

 psd, pseudopodium ; Skel., 

 ''Jpo^^^jr, -** **./ skeleton; z, Zooxanthella. 



(From Lang's Comparative 

 Anatomy, after Haeckel.) 



. 



regular spherical forms (from which the others may be readily 

 derived) radially, in five sets of four in the regions corresponding 

 to the equator and the tropics and polar circles of our world. 



1 Dreyer has shown that in many cases it may be explained by geometrical 

 considerations. V. Hacker has written a most valuable account of the Biological 

 relations of the skeleton of Radiolaria in Jen. Zeitschr. xxxix. 1904, p. 297. 



