270 A. EARLAND ON RADIOLARIA. 



The acanthin, of which the Acantharia build their skeletons, is 

 very easily destroyed, being soluble in both acids and alkalis, and 

 also in sea- water. The skeletons of this legion are therefore 

 practically known only from such specimens as have been 

 observed in the living state, being of very rare occurrence in the 

 Radiolarian oozes now forming in the deep sea, and absolutely 

 unknown in the fossil state. 



The carbonic silicate, of which the Phfeodaria build their 

 skeletons, is less easily destroyed than acanthin, but it is strongly 

 acted upon by heat, and also by alkalis, though proof against 

 acids. It offers greater resistance to the solvent power of sea- 

 water than acanthin, and hence skeletons of the Phseodaria may 

 be found sparingly in deep-sea deposits. In the fossil state the 

 legion is represented by one family only, the Dictyochida, the 

 members of which are characterised by the possession of a purely 

 siliceous skeleton. 



The pure siliceous skeletons of the Spumellaria and Nassellaria, 

 which were first recognised and described by Ehrenberg in 1838, 

 are practically indestructible, and hence occur in fossil deposits of 

 all ages, and in enormous numbers in the deep-sea Radiolarian 

 oozes. It should be borne in mind that Ehrenberg's name for 

 the class, " Polycystina," is only applicable to these two legions, 

 as, with the exception of the Dictyochida, which he regarded 

 as diatoms, Ehrenberg had no know^ledge of the Acantharia or 

 Phseodaria. 



The skeletons of the Radiolaria are characterised by great 

 strength and firmness, the acanthin skeleton of the Acantharia 

 being only slightly inferior in stiffness to the siliceous skeletons of 

 the Polycistina. The silicate shells of the Phseodaria, which in 

 many cases have skeleton tubes filled with a gelatinous material, 

 are much weaker, and sometimes are extremely brittle. 



In the second legion, the Acantharia, the skeleton is 

 centrogenous — that is to say, the formation of the skeleton begins 

 in the middle of the central capsule, from which point twenty 

 (the normal number) radial spines of acanthin are centrifugally 

 developed. The three other legions, on the other hand, possess 

 perige7ious skeletons — that is to say, the skeleton originally 

 develops outside the central capsule. In the Nassellaria and 

 Phseodaria the skeleton remains outside the central capsule, as 

 also in a part of the Spumellaria ; but in the greater part of this 



