FORMS OF SPICULES. 67 



simply implies that the variations of spicule forms are nou- 

 controlled^ and that they persist without reference to the 

 economy of the organism. My objections to this hypothesis 

 are, in short, that the implied capacity of the protoplasm to 

 form the requisite spicule moulds is shown, in all probability, 

 to be non-existent by the facts both of experimental embryo- 

 logy and cell physiology, and that the resources of physical 

 science can provide examples of structures much more closely 

 allied to spicules than any class of bodies known to be 

 organically produced, i. e. by inheritance. 



Factors possibly concerned in the Production of 

 Spicule Forms. 



Assuming the above conclusion to be valid, we are thus at 

 liberty to consider purely physical factors as being fully 

 competent to account for the varied forms which spicules 

 assume. Before enumerating possible factors it will be as 

 well to state definitely that we do not consider crystallisation, 

 in the strict sense of the term,' to be one of these. To put 

 the argument in brief, we may reiterate that no crystal 

 possesses curved surfaces, and, since no spicule exists without 

 them, therefore no spicule is a crystal. But, apart from this, 

 we may point out that all spicules composed of crystalline 

 matter consist of calcite, and that the various crystalline 

 forms of calcite being strictly limited in number (none of 

 Avhich bear the slightest resemblance to any form of cal- 

 careous spicule) and all modifications of one type, it is 

 evidently impossible to refer the multitudinous and Avidely- 

 different forms of calcareous spicules to any such factor as 

 the crystalline properties of their substance. In fact, the 



1 It is important, iu view of the loose application of the term, that I should 

 explain that by a crystal I mean solely a mass of matter which has assumed, 

 on soldification from a dissolved or fused condition, a form bounded by plane 

 surfaces referable to one of the six systems recognised by crystallographers. 

 Crystals, as thus defined, may be simple (most compact crystals) or aggregate 

 (snow-crystals, e. g.). 



