ON CONCRETIONS, SPICULES, 



[CH. 



672 



silica which constitute one of the commonest types of spicule call 

 for little in the way of explanation ; they are accretions or deposits 

 about a linear axis, or fine thread of organic material, just as the 

 ordinary rounded calcospherite is deposited about some minute 

 point or centre of crystallisation, and as ordinary crystallisation 

 may be started by a particle of dust; in some cases they also, like 

 the others, are apt to be roughened by more irregular secondary 



NMNf 



Fig. .*i09. Close-packed caleospherites, or so-called '"spicules," 

 oi Astrosclera. After Lister. 



deposits, which probably, as in Harting's experiments, assume this 

 irregular form when material runs short. 



Our few foregoing examples, diverse as they are in look and kind, 

 from the spicules of Astrosclera or Alcyonium to the otoliths of a 

 fish, seem all to have their free origin in some larger or smaller 

 fluid-containing space or cavity of the body: pretty much as 

 Harting's calcospheres made their appearance in the albuminous 

 content of a dish. But we come at last to a much larger class of 

 spicular and skeletal structures, for whose regular and often complex 

 forms some other explanation than the intrinsic forces of crystal- 

 lisation or molecular adhesion is required. As we enter on this 



