424 ON CONCRETIONS, SPICULES, ETC. [ch. 



added to the mixture, the spheroidal globules tended to become 

 rough, by an outgrowth of spinous or digitiform projections; and 

 in some cases, but not without the presence of the phosphate, the 

 result was an irregularly shaped knobby spicule, precisely similar 

 to those which are characteristic of the Alcyonaria*. 



The rough spicules of the Alcyonaria are extraordinarily variable in shape 

 and size, as, looking at them from the chemist's or the physicist's point of 

 view, we should expect them to be. Partly upon the form of these spicules, 

 and partly on the general form or mode of branching of the entire colony of 



Fig. 200. Additional illustrations of Alcyonarian spicules : Eunicea. (After 



Studer.) 



polypes, a vast number of separate "species" have been based by systematic 

 zoologists. But it is now admitted that even in specimens of a single species, 

 from one and the same locahty, the spicules may vary immensely in shape 

 and size: and Professor Hickson declares (in a paper published while these 

 sheets are passing through the press) that after many years of laborious work 

 in striving to determine species of these animal colonies, he feels "quite con- 

 vinced that we have been engaged in a more or less fruitless taskf". 



The formation of a tooth has very lately been shown to be a phenomenon 

 of the same order. That is to say, " calcification in both dentine and enamel 



* .Spicules extremely like those of the Alcyonaria occur also in a few sponges ; 

 of. (e.g.), Vaughan Jennings, Journ. Linn.. Soc. xxni, p. 531, pi. 13, fig. 8, 1891. 

 t Mem. Manchester Lit. and Phil. Soc. lx, p. 11, 1916. 



