414 ON CONCRETIONS, SPICULES, [ch. 



rounded body, sometimes spherical, sometimes flattened, in which 

 the calcareous matter is laid down in concentric zones, denser 

 and clearer layers alternating with one another. In the develop- 

 ment of the molluscan shell and in the calcification of a bird's 

 egg or the shell of a crab, for instance, spheroidal bodies with 

 similar concentric striation make their appearance ; but instead of 

 remaining separate they become crowded together, and as they 

 coalesce they combine to form a pattern of hexagons. In some 

 cases, the carbonate of lime on being dissolved away by acid 

 leaves behind it a certain small amount of organic residue ; in 

 most cases other salts, such as phosphates of lime, ammonia or 

 magnesia are present in small quantities ; and in most cases if 

 not all the developing spicule or concretion is somehow or other 

 so associated with Uving cells that we are apt to take it for granted 

 that it owes its peculiarities of form to the constructive or plastic 

 agency of these. 



The appearance of direct association with living cells, however, 

 is apt to be fallacious ; for the actual precipitation takes place, 

 as a rule, not in actively living, but in dead or at least inactive 

 tissue* : that is to say in the "formed material" or matrix which 

 (as for instance in cartilage) accumulates round the living cells, 

 in the interspaces between these latter, or at least, as often happens, 

 in connection with the cell-wall or cell-membrane rather than 

 within the substance of the protoplasm itself. We need not go 

 the length of asserting that this is a rule without exception ; but, 

 so far as it goes, it is of great importance and to its consideration 

 we shall presently return f. 



Cognate with this is the fact that it is known, at least in some 

 cases, that the organism can go on living and multiplying with 

 apparently unimpaired health, when stinted or even wholly 

 deprived of the material of w^hich it is wont to make its spicules 



* In an interesting paper by Irvine and 8ims Woodhead on the "Secretion of 

 Carbonate of Lime by Animals" (Proc. E. S. E. xvi, 1889, p. 351) it is asserted 

 that "lime salts, of whatever form, are deposited only in vitally inactive tissue." 



t The tube of Teredo shews no trace of organic matter, but consists of irregular 

 prismatic crystals : the whole structure •' being identical with that of small veins 

 of calcite, such as are seen in thin sections of rocks" (Sorby, Proc. Geol. Soc. 1879, 

 p. 58). This, then, would seem to be a somewhat exceptional case of a shell laid 

 down completely outside of the animal's external layer of organic or colloid sub- 

 stance. 



