234 Dr. G. C. Wallich on the Extent and Causes of 



already stated, the appearances just indicated are to be found not 

 only in the lageniform and mitriform, but also in the globular 

 series, and in the exceptional variety Difflugia spiralis. 



Lastly, I may mention that in certain individuals (as for ex- 

 ample in figs. 13, 23, 24, & 24 a) the whole of the test is covered 

 with minute cylindrical rods evidently of similar origin. Some- 

 times these httle cylinders are straight, sometimes irregularly 

 curved ; but, as in the former examples, they are arranged side 

 by side, and without any very distinct regularity *. 



It has been already stated that no effect is produced by the 

 rectangular plates when seen by polarized light. When, how- 

 ever, the discs (which in common with the rectangular plates and 

 cylindrical bodies are themselves perfectly colourless) are seen by 

 polarized light, the alternating changes of tint are not only 

 distinctly visible over the test generally, but to the same extent 

 in each particular disc, — thus indicating that the effect is not due 

 to the presence of micaceous or other mineral films such as 

 usually underhe the external coarse layer of sandy particles — a 

 fact which is verified when such forms as those now under notice 

 are broken up under the microscope. 



The inference which I draw from this singularly complete 

 series of transitionary forms is, that the chitinoid basal substance 

 of the test, or (as is quite possible when we take into considera- 

 tion the facts I shall presently adduce with regard to the mode 

 in which mineral particles are arranged by an external sarcode- 

 layer such as we see in Gromia) a portion of the viscid sarcode 

 mass, combines, under the law of "molecular coalescence '^f, 

 with the siliceous or other mineral elements, and thus serves to 

 produce all the transitionary colloid bodies which occur, from 

 the first alteration in shape of the mineral particles themselves, 

 to the development of the crystalline tablets which were first 

 described. One thing is quite manifest, namely, that the whole 

 series of bodies now under notice are derived in some way from 

 the animal, and not directly from the medium in which it lives ; 

 for none are traceable in a free condition in the material in 

 which the specimens occur. On the other hand, notwithstanding 

 the selective and adaptive faculty already shown to belong to the 

 Rhizopod, we are not warranted in assigning to it a special 

 formative power. The origin of the minute crystalloids, so 

 abundantly present in the Amoeban and Difflugian forms gene- 

 rally, is thus also accounted for ; and step by step we are 

 arriving at a knowledge of the mode in which the vital and 



* These bodies were accurately described by me in the * Annals ' for 

 December 1863, p. 456. 



t See Mr, Rainey's valuable papers on " Molecular Coalescence " in 

 vols. vi. and vii. of the Journal of Microscopical Science. 



