CHAPTER IX 



ON CONCRETIONS, SPICULES, AND 

 SPICULAR SKELETONS 



The deposition of inorganic material in the living body, usually 

 in the form of calcium salts or of silica, is a common phenomenon. 

 It begins by the appearance of small isolated particles, crystalline 

 or non-crystalline, whose form has little relation or none to the 

 structure of the organism; it culminates in the complex skeletons 

 of the vertebrate animals, in the massive skeletons of the corals, or 

 in the polished, sculptured and mathematically regular molluscan 

 shells. Even among very simple organisms, such as diatoms, 

 radiolarians, foraminifera or sponges, the skeleton displays extra- 

 ordinary variety and beauty, whether by reason of the intrinsic 

 form of its elementary constituents or the geometric symmetry with 

 which these are interconnected and arranged. 



With regard to the form of these various structures (and this is 

 all that immediately concerns us here), we have to do with two 

 distinct problems, which merge with one another though they are 

 theoretically distinct. For the form of the spicule or other skeletal 

 element may depend solely on its chemical nature, as for instance, 

 to take a simple but not the only case, when it is purely crystalline ; 

 or the inorganic material may be laid down in conformity with the 

 shapes assumed by cells, tissues or organs, and so be, as it were, 

 moulded to the living organism ; and there may well be intermediate 

 stages in which both phenomena are simultaneously at work, the 

 molecular forces playing their part in conjunction with the other 

 forces inherent in the system. 



So far as the problem is a purely chemical one we must deal 

 with it very briefly indeed: all the more because special investiga- 

 tions regarding it have as yet been few, and even the main facts 

 of the case are very imperfectly known. This at least is clear, that 

 the phenomena with which we are about to deal go deep into the 

 subject of colloid chemistry, and especially that part of the science 



