CH. XII] THE FORAMINIFERA 851 



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 little 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. In a word, 

 the whole quasi-fluid protoplasmic body may be likened to a little 

 mass of froth or foam : that is to say, to an aggregation of simul- 

 taneously formed drops or bubbles, whose-physical properties and 

 geometrical relations are very different from those of a system of 



Fig. 422. Hastigerina sp.; to shew the "mouth." 



drops or bubbles which are formed one after another, each solidifying 

 before the next is formed. 



With the actual origin or mode of development of the foraminiferal 

 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. 422), which 

 interruptions we may doubtless interpret as being due to unequal 

 distributions of surface energy. In many cases the fluid protoplasm 

 "picks up" sand-grains and other foreign particles, after a fashion 

 which we have already described (p. 702); and it cements these 

 together with more or less of calcareous material. The calcareous 

 shell is a crystalline structure, and the micro-crystals of calcium 

 carbonate are so set that their little prisms radiate outwards in each 

 chamber through the thickness of the wall — which symmetry is 



