IX] OF AGGLUTINATED SKELETONS 463 



of the protoplasm, and their arrangement around the radial 

 spicules, all on the principles of surface-tension*. 



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

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

 of Schewiakoff's statements, which are founded on direct personal 

 observation. This I am none too willing to do ; but whether it 

 be justly done in this case or not, I hold that it is in principle 

 justifiable to look with great suspicion upon a number of kindred 

 statements where it is obvious that the observer has left out of 

 account the purely physical aspect of the phenomenon, and all 

 the opportunities of simple explanation which the consideration 

 of that aspect might afford. 



Whether it be wholly applicable to this particular and complex 

 case or no, our general theorem of the localisation and arrestment 

 of solid particles in a surface-film is of very great biological 

 importance ; for on it depends the power displayed by many 

 little naked protoplasmic organisms of covering themselves with 

 an "agglutinated" shell. Sometimes, as in Difflugia, Astrorhiza 

 (Fig. 219) 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 solid particles which are said 

 to arise, as inorganic deposits or concretions, within the protoplasm 

 itself, and which find their way outwards to a position of equilibrium 

 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 anthropomorphic figures of speech, misleading 

 though they be), is a physical phenomenon kindred to that by which 

 an Amoeba "swallows" a particle of food. This latter process 

 has been reproduced or imitated in various pretty experimental 



* The manner in which the minute spicules of Raphidophrys arrange themselves 

 round the bases of the pseudopodial rays is a similar phenomenon. 



