674 ON CONCRETIONS, SPICULES, ETC. [ch. 



characters connote) of the order of magnitude of which we are 

 speaking. 



Proceeding a Httle farther in our classification, we may conceive 

 each Httle skeletal element to be associated with, and developed by, 

 a single cell or vesicle, or alternatively a cluster or "system" of 

 consociat^d cells. In either case there are various possibilities. 

 For instance, the calcified or other skeletal material may tend to 

 overspread the entire outer surface of the cell or cluster of cells, 

 and so tend to assume a configuration comparable to the surface 

 of a fluid drop or aggregation of drops ; this, in brief, is the gist and 

 essence of our story of the foraminiferal shell. Another common 

 but very different condition will arise if, in the case of the cell- 

 aggregates, the skeletal material tends to accumulate in the inter- 

 stices between the cells, in the partition-walls which separate them, 

 or in the still more restricted edges, or junctions between these 

 partition-walls; conditions such as these will go a long way to 

 help us to understand many sponge-spicules and an immense 

 variety of radiolarian skeletons. And lastly (for the present), 

 there is a possible and very interesting case of a skeletal element 

 associated with the surface of a cell, not so as to cover it like 

 a shell, but only so as to pursue a course of its own within it, 

 and subject to the restraints imposed by such confinement to a 

 curved and limited surface. With this curious condition we shall 

 deal immediately. 



This preliminary and much simplified classification of the lesser 

 skeletal, or micro-skeletal, forms does not pretend (as is evident 

 enough) to completeness. It leaves out of account some conforma- 

 tions and configurations with which we shall attempt to deal, and 

 others which we must perforce omit. But nevertheless it may help 

 to clear or mark our way towards the subjects which this chapter 

 has to consider, and the con4itions by which they are at least 

 partially defined. 



Among the possible, or conceivable, types of microscopic skeletons 

 let us begin with the case of a spicule, more or less simply 

 linear as far as its intrinsic powers of growth are concerned, but 

 which owes its more complicated form to a restraint imposed by 

 the cell to which it is confined, and within whose bounds it is generated. 



