Alcyonaria. These also must be looked upon as the simple result of chemical deposition, the form of the deposit being somewhat modified in conformity with the surrounding tissues, just as in the simple experiment the form of the concretionary precipitate is affected by the heterogeneity, visible or invisible, of the matrix. Lastly, the simple needles of amorphous 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 cryôÙstalôÙliôÙsaôÙtion, and as ordinary cryôÙstalôÙliôÙsaôÙtion is often started by a particle of atmospheric dust; in some cases they also, like the others, are apt to be roughened by more irregular secondary deposits, which probably, as in Hartingãs experiments, appear in this irregular form when the supply of material has become relatively scanty.
Our few foregoing examples, diverse as they are in look and kind and ranging 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 Hartingãs calcospheres