XII] OF THE FORAMINIFERA ' 589 



cases the fluid protoplasm "picks up" sand-grains and other 

 foreign particles, after a fashion which we have already described 

 (p. 463) ; 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 thick- 

 ness of the wall: — which symmetry is subject to corresponding 

 modification when the spherical chambers are more or less sym- 

 metrically deformed*. 



In various ways the rounded, drop-like shells of the Fora- 

 minifera, both simple and compound, have been artificially 

 imitated. Thus, if small globules of mercury be immersed in 

 water in which a little chromic acid is allowed to dissolve, as the 

 httle beads of quicksilver become slowly covered with a crystalline 

 coat of mercuric chromate they assume various forms reminiscent 

 of the monothalamic Foraminifera. The mercuric chromate has 

 a higher atomic volume than the mercury which it replaces, and 

 therefore the fluid contents of the drop are under pressure, which 

 increases with the thickness of the pelhcle; hence at some weak 

 spot in the latter the contents will presently burst forth, so forming 

 a mouth to the httle shell. Sometimes a long thread is formed, 

 just as in RJiabdamniina linearis; and sometimes unduloid 

 swelhngs make their appearance on such a thread, just as in 

 R. discreta. And again, by appropriate modifications of the 

 experimental conditions, it is possible (as Rhumbler has shewn) 

 to build up a chambered shell f. 



In a few forms, such as Globigerina and its close allies, the 

 shell is beset during life with excessively long and dehcate 

 calcareous spines or needles. It is only in oceanic forms that 

 these are present, because only when poised in water can such 



* In a few cases, according to Awerinzew and Rhumbler, where the chambers are 

 added on in concentric series, as in Orbitolites, we have the crystalline structure 

 arranged radially in the radial walls but tangentially in the concentric ones: 

 whereby we tend to obtain, on a miniite scale, a system of orthogonal trajectories, 

 comparable to that which we shall presently studj- in connection with the structure 

 of bone. Cf. S. Awerinzew, Kalkschale der Rhizopoden, Z. f. w. Z. lxxiv, 

 pp. 478^90. 1903. 



f Rhumbler, L., Die Doppelschalen von Orbitolites und anderer Foraminiferen, 

 etc., Arch. J. Protistenkuxde, i, pp. 193-296, 1902; and other papers. Also Die 

 Foraminiferen der Planktoiiex'pedition, i, 1911, pp. 50-56. 



