94 TOILERS IN THE SEA. 



shapes, and exhibits at the same time such wonderful 

 regularity and delicacy in its adjustments, that in 

 both these respects this group excels all other classes 

 of the organic world. For, in spite of the fact that 

 the Radiolarian organism always remains merely a 

 single cell, it shows the potentiality of the highest 

 complexity to which the process of a skeleton forma- 

 tion can be brought by a single cell." The chemical 

 composition of the skeleton, in many cases, is pure 

 silica, or flint ; in some it is a silicate of carbon, and, 

 in a few, of a peculiar organic substance, called 

 acanthin. Calcareous skeletons, or skeletons com- 

 posed of lime, do not occur. In the great majority 

 this skeleton has the form of a delicate lattice shell, 

 or a receptacle, in which the central capsule is 

 enclosed. In a small minority, however, this is not 

 the case. The skeleton then consists only of isolated 

 rigid pieces, or of a simple ring, or of a basal tripod, 

 with or without a loose tissue, and the central capsule 

 is not then surrounded by a special latticed recep- 

 tacle, but only rests upon the skeleton. ;$#,:<. 



We pass on, from this brief description, to what is 

 known, up to the present, of the development of the 

 Radiolaria, as set forth by Haeckel. 1 



It may be assumed that in all Radiolaria, on 



1 "Report on the Radiolaria of the Challenger Expedition," 

 by Professor Ernst Haeckel, vol. xviii. (1887), p. xciii. 



