678 ON" CONCRETIONS, SPICULES, ETC. [ch. 



Furthermore, if these mechanical methods of conformation, and 

 others hke to these, be the true cause of the shapes which the 

 spicules assume, it is plain that the production of these spicular 

 shapes is not a specific function of the sponge, but that we should 

 expect the same or similar spicules to occur in other organisms, 

 wherever the conditions of inorganic secretion within closed cells 

 were very much the same. As a matter of fact, in the sea-cucumbers, 

 where the formation of intracellular spicules is a characteristic 

 feature of the group, all the principal types of conformation which 

 we have just described can be closely paralleled; indeed, in many 

 cases, the forms of the holothurian spicules are identical and indis- 

 tinguishable from those of the sponges*. But the holothurian 

 spicules are composed of calcium carbonate while those which we 

 have just described in the case of sponges are siUceous: this being 

 just another proof of the fact that in such cases as these the form 

 of the spicule is not due to its chemical nature or molecular 

 structure, but to the external forces to which it is subjected. 



The broad fact that the skeleton is calcareous in certain large 

 groups of animals and calcareous in others is as remarkable as its 

 causes are obscure. I for one have no idea why some sponge- 

 skeletons are of the one and some the other, with never the least 

 admixture of the two; or why the diatoms and radiolarians are all 

 the one, and the molluscs and corals and foraminifera are all the 

 other!. 



So much for that small class of sponge-spicules whose forms seem 

 due to the fact that they are developed within, or under the restraint 

 imposed by, the surface of a single cell or vesicle. Such spicules 

 are usually of small size as well as of simple form ; and they are 

 greatly outstripped in number, in size, and in supposed importance 

 as guides to zoological classification, by another class of spicules. 

 These are the many and various cases which we explain on the 



* See for instance the plates in' Theel's Monograph of the Challenger Holo- 

 thuroidea; also Sollas's Telractinellida, p. Ixi. Cf. also E. Merke, Studien am Skelet 

 der Echin^dernien, Zool. Jahrbiicher (Abth. f. allgem. Zoologie), 1916-19. 



t The particles of lime and silica tend to bear opposite charges; siliceous 

 organisms seem to flourish in the colder waters as the calcareous certainly do in 

 warmer seas. And such facts, or tendencies, as these may help some day to explain 

 the phenomenon. 



