IX] AND SPICULAR SKELETONS 439 



may be comprised within the bounds of the organism, and which 

 are (as their names and characters connote) of the order of 

 magnitude of which we are speaking. 



Proceeding a little farther in our classification, we may conceive 

 each little skeletal element to be associated, in one case, with 

 a single cell or vesicle, and in another with a cluster or "system" 

 of consociated 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 accordingly to assume some configuration comparable 

 to that of a fluid drop or of an 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 interstices between the cells, in the partition-walls which 

 separate them, or in the still more restricted distribution indicated 

 by the lines of junction between these partition- walls. Conditions 

 such as these will go a very long way to help us in our under- 

 standing of many sponge-spicules and of 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 skeletal 

 forms (as is evident enough) does not pretend to completeness. 

 It leaves out of account some kinds of conformation and con- 

 figuration with which we shall attempt to deal, and others which 

 we must perforce omit. But nevertheless it may help to clear 

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

 to consider, and the conditions by which they are at least partially 

 defined. 



Among the several possible, or conceivable, types of microscopic 

 skeletons let us choose, to begin with, the case of a spicule, more 

 or less simply linear as far as its intrinsic powers of growth are 



