648 ON CONCRETIONS, SPICULES [ch. 



crowded together, and in doing so are apt 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 many 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 so 

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

 that it' owes its form to the constructive or plastic agency of these. 



The appearance of direct association with hving cells, however, 

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

 a rule, not in actively hving, but in dead or at least inactive tissue * ; 

 that is to say in the "formed material" or matrix which accumulates 

 round the living cells, or in the interspaces between these latter, 

 or, as often happens, in 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, at least in some cases, the 

 organism can go on, in apparently unimpaired health, when stinted 

 or even wholly deprived of the material of which it is wont to make 

 its spicules or its shell. Thus the eggs of sea-urchins reared in lime- 

 free water develop, in apparent health and comfort, into larvae 

 which lack the usual skeleton of calcareous rods: and in which, 

 accordingly, the long arms of the Pluteus larva, which the rods 

 should support and extend, are entirely absent {. Again, when 

 foraminifera are kept for generations in water from which they 

 gradually exhaust the lime, their shells grow hyaline and trans- 

 parent, and dwindle to a mere chitinous peUicle; on the other hand, 



♦ In an interesting paper by Robert Irvine and Sims Woodhead on the Secretion of 

 carbonate of lime by animals (Proc. R.S.E. xv, pp. 308-316; xvi, pp. 324-351, 

 1889-90) it is asserted (p. 351) that "Hme 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 substance. 



t Cf. Pouchet and Chabry, C.R. Soc. Biol. Paris (9), i, pp. 17-20, 1889; C.R. 

 Acad. Sci. cvm, pp. 196-198, 1889. 



