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CHAPTER VIII. 



SUB-KINGDOM I. PROTOZOA (Continued}. 

 RADIOLARIA AND SPONGIDA. 



II. EADIOLARIA. 



UNDER the head of Radiolaria are grouped together at the 

 present day various, mostly microscopic, Protozoans which 

 typically possess a siliceous skeleton, the parts of which are often 

 more or less radiate, the sarcode of the body being differentiated 

 into a central mass, surrounded ly a membranous capsule, and 

 an outer layer usually containing cell -like bodies, while the 

 pseudopodia are long, filamentous, and ray - like (fig. 30). 

 Though the typical Eadiolarians are distinguished by the 

 above-mentioned characters, some of the forms which must 

 be included here are devoid of certain of these features. 

 Thus the so-called Heliozoa have no central capsule, and 

 only occasionally possess skeletal structures. In other cases, 

 though the general type of the group is retained, the skeleton 

 is wholly wanting, while the nature of the skeleton when 

 present varies greatly. From the last-mentioned point of 

 view, the Eadiolarians are divided into four groups. In the 

 first of these there is no skeleton at all ; we have therefore 

 nothing to do with these as fossils. In the second group 

 are forms in which there is a skeleton, but this consists 

 merely of scattered spicules, which lie wholly outside the 

 central capsule of the body ; and these also are unknown as 

 fossils. In the third group are forms with a skeleton of 

 radial spicules, but these are now disposed as a symmetrical 

 VOL. I. I 



