CHAPTER IX 



ON CONCRETIONS, SPICULES, AND SPICULAR SKELETONS 



The deposition of inorganic material in the Hving body, usually 

 in the form of calcium salts or of sihca, is a very common and 

 wide-spread phenomenon. It begins in simple ways, by the 

 appearance of small isolated particles, crystalline or non- 

 crystalline, whose form has little relation or sometimes 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 many very simple organ- 

 isms, such as the Diatoms, the Radiolarians, the Foraminifera, 

 or the Sponges, the skeleton displays extraordinary variety and 

 beauty, whether by reason of the intrinsic form of its elementary 

 constituents or the geometric symmetry with which these are 

 arranged and interconnected. 



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

 is all that immediately concerns us here), it is plain that we have 

 to do with two distinct problems, which however, though 

 theoretically distinct, may merge with one another. For the 

 form of the spicule or other skeletal element may depend simply 

 upon its chemical nature, as for instance, to take a simple but 

 not the only case, when the form is purely crystalline; or the 

 inorganic solid material may be laid down in conformity with the 

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

 were, moulded to the shape of the living organism ; and again, 

 there may well be intermediate stages in which both phenomena 

 may be simultaneously recognised, the molecular forces playing 

 their part in conjunction with, and under the restraint of, the 

 other forces inherent in the system. 



