702 ON CONCRETIONS, SPICULES, ETC. [ch. 



are), we can explain their situation on the surface of the protoplasm, 

 and their arrangement around the radial spicules, all by the prin- 

 ciples of capillarity. 



This last case is not of the simplest; and I do not forget that 

 my explanation of it, which is wholly theoretical, impHes a doubt 

 of Schewiakoif 's statements, founded on his personal observation. 

 This I am none too wiUing to do; but whether it be justly done 

 in this case or not, I hold that it is in principle justifiable to look 

 with suspicion upon all such statements where the observer has 

 obviously left out of account the physical aspect of the pheno- 

 menon, and all the opportunities of simple explanation which the 

 consideration of that aspect might aiford. 



Whether it be applicable to this particular and complex case or 

 no, our general theorem of the localisation and arrestment of sohd 

 particles in a surface-film is of great biological significance ; for on 

 it depends the power displayed by many httle naked protoplasmic 

 organisms of covering themselves with an "agglutinated" shell. 

 Sometimes, as in Difflugia, Astrorhiza (Fig. 320) and others, this 

 covering consists of sand-grains picked up from the surrounding 

 medium, and sometimes, on the other hand, as in Quadrula, it 

 consists of sohd particles said to arise as inorganic deposits or 

 concretions within the protoplasm itself, and to find their way 

 outwards to a position of equihbrium in the surface-layer; and in 

 both cases, the mutual capillary attractions between the particles, 

 confined to the boundary-layer but enjoying a certain measure of 

 freedom therein, tends to the orderly arrangement of the particles 

 one with another, and even to the appearance of a regular "pattern" 

 as the result of this arrangement. 



The "picking up" by the protoplasmic organism of a solid 

 particle with which "to build its house" (for it is hard to avoid 

 this customary use of figures of speech, misleading though it be) 

 is a physical phenomenon akin to that by which an amoeba 

 "swallows" a particle of food. This latter process has been repro- 

 duced or imitated in various pretty experimental ways. For 

 instance, Rhumbler hsts shewn that if a splinter of glass be covered 

 with shellac and brought near a drop of chloroform suspended in 

 water, the drop takes in the spicule, robs it of its shellac covering, 



