852 THE SPIRAL SHELLS [ch. 



subject to corresponding modification when the spherical chambers 

 are more or less symmetrically deformed*. 



In various ways the rounded, drop-Uke shells of the Foraminifera, 

 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 little beads of quicksilver 

 become slowly covered with a crystalline coat of mercuric chromate 

 they assume various forms reminiscent of the monothalamic Fora- 

 minifera. "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 pellicle ; hence at some weak spot in the latter the contents 

 will presently burst forth, so forming a mouth to the little shell. 

 Sometimes a long thread is formed, just as in Rhabdammina linearis; 

 and sometimes unduloid swelhngs make their appearance on such, 

 a thread, just as in R. discreta. And again, by appropriate modi- 

 fications 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 delicate structures 

 endure; in dead shells, such as we are much more familiar with, 

 every trace of them is broken and rubbed away. The growth of 

 these long needles may be partly explained (as we have already said 

 on p. 675) by the phenomenon which Lehmann calls orientirte 

 Adsorption — the tendency for a crystalline structure to grow by 

 accretion, not necessarily in the outward form of a "crystal," but 

 continuing in any direction or orientation which has once been 

 impressed upon it: in this case the spicular growth is in direct 

 continuation of the 'radial symmetry of the micro-crystalhne 



* 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 minute scale, a system of orthogonal trajectories, 

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

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

 pp. 478-490, 1903. 



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

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

 Foraminiferen der Planktonexpedition, i, pp. 50-56, 1911. 



