718 ON CONCRETIONS, SPICULES, ETC. [ch. 



common occurrence in diverse groups of organisms. And the simple 

 fact is, that we seem to know less and less of these things on the 

 biological side, the more we come to understand their physical and 



Fig. 335. Various species of Distephanus (SilicojEIagellata). 

 From Deflandre, after Ehrenberg. 



mathematical characters. I have lost faith in Haeckel's four 

 thousand "species" of Radiolaria. 



In Callimitra itself, and elsewhere where the boundary- walls (and 

 not merely their edges) are sihcified, the skeletal matter is not 



Fig. 336. Holothurian spicules. A, of Thyone (Mortensen) ; B, Holothuria ladea 

 (Perrier); C, D, Holothuria and Phyllophorus (Deichman). 



deposited in an even layer, like the waxen walls of a bee's cell, but 

 in a close mesh work of fine curvilinear threads; and the curves 

 seem to form three main series more or less closely related to the 

 three edges of the partition. Sometimes (as may also be seen in 

 our figure) the system is further comphcated by a radial series 

 running from the centre towards the free edge of each partition. 



