IX] AND SPICULAR SKELETONS 437 



'■calcospherites"' of Harting's experiments; and as these grow 

 they become closely aggregated together (Fig. 210), and assume an 

 angular, polyhedral form, once more in complete accordance with 

 the results of experiment*. Again, in many Monaxonid sponges, 

 we have irregularly shaped, or branched spicules, roughened or 

 tuberculated by secondary superficial deposits, and reminding one 

 of the spicules of some 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 surround- 

 ing tissues, just as in the simple experiment the form of the con- 

 cretionary precipitate is affected by the heterogeneity, visible or 

 invisible, of the matrix. Lastly, the simple needles of amorphous 

 silica, w^hich constitute one of the commonest types of spicule, 

 call for little in the w^ay 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 

 crystalhsation 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 

 Harting's experiments, appear irt 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 Harting's calcospheres made their appearance in 

 the albuminous content of a dish. But we now 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 crystallisation or molecular adhesion is mani- 

 festly necessary. As we enter on this subject, which is certainly 

 no small or easy one, it may conduce to simplicity, and to brevity. 



* The peculiar spicules of Astrosclera are now said to consist of spherules, or 

 calcospherites, of aragonite, spores of a certain red seaweed forming the nuclei, 

 or starting-points, of the concretions (R. Kirkpatrick, Proc. R. S. Lxxxiv (B), 

 p. -u9, 1911. 



