PROTOZOA. RADIOLARIA. 25 



The genera and species of the Foraminifera have as a 

 general rule a long range in time, as might be expected 

 from their low organization ; some of the species which 

 occur in the Palaeozoic are still living. 



ORDER. RADIOLARIA. 



In the Radiolaria the body consists of a central mass 

 of protoplasm enclosed in a definite membrane, known as 

 the central capsule. The intracapsular protoplasm contains 

 one or more nuclei, and is continuous, through pores in 

 the capsule, with a layer of protoplasm outside the 

 capsule ; this layer gives off filamentous pseudopodia, 

 which occasionally anastomose. A skeleton is generally 

 developed, which is composed either of silica, a silicate of 

 carbon, or a peculiar horny substance known as acanthin. 

 The skeleton may be entirely outside the central capsule or 

 partly within, and it consists either of isolated spicules, 

 or of a lattice-like structure frequently with projecting 

 spines. The earliest British examples occur in the 

 Ordovician rocks of Scotland, others have been found 

 in the Carboniferous, and they become abundant in 

 the Mesozoic and Cainozoic formations. Those forms 

 in which the skeleton is composed of acanthin do 

 not occur fossil. 



