588 THE SPIRAL SHELLS [ch. 



the shell are of different and successive ages, that one part of the 

 shell is always relatively new, and the rest old in various grades 

 of seniority. 



The forms which we set together in the sister-group of Radio- 

 laria are very differently characterised. Here the cells or vesicles 

 of which each little composite organism is made up are but little 

 separated, and in no way walled off, from one another ; the hard 

 skeletal matter tends to be deposited in the form of isolated 

 spicules or of httle connected rods or plates, at the angles, the 

 edges or the interfaces of the vesicles ; the cells or vesicles form 

 a coordinated and cotemporaneous rather than a successive series. 



Fig. 308. Hastigerina sp. ; to shew the '"mouth." 



In a word, the whole quasi-fluid protoplasmic body may be 

 Ukened to a little mass of froth or foam : that is to say, to an 

 aggregation of simultaneously formed drops or bubbles, whose 

 physical properties and geometrical relations are very different 

 from those of a system of drops or bubbles which are formed one 

 after another, each soUdifying before the next is formed. 



With the actual origin or mode of development of the fora- 

 miniferal shell we are now but little concerned. The main factor 

 is the adsorption, and subsequent precipitation at the surface of 

 the organism, of calcium carbonate, — -the shell so formed being 

 interrupted by pores or by some larger interspace or "mouth" 

 (Fig. 308), which interruptions we may doubtless interpret as 

 being due to unequal distributions of surface energy. In many 



