200 THE PROTOZOA 



of much investigation and discussion. The older view, which 

 ascribed them to contractility and assumed a complicated structure 

 in the protoplasm, has now been superseded generally bj r the theory 

 connected more especially with the names of Quincke, Berthold, 

 Butschli (37), and Rhumbler (34, 35, 40, etc.), according to which 

 differences of surface-tension are regarded as the efficient cause of 

 the streaming movements of the pseudopodia and the protoplasm. 

 The living substance is in a state of continual chemical change in 

 every part ; such changes are sufficient to account, in one way or 

 another, for the origin of local differences in the physical nature 

 (adhesion) of the surface of the body in contact with the surrounding 

 medium, or of internal protoplasmic surfaces in contact with 

 vacuoles or cavities filled with fluid ; and the resulting differences in 

 surface-tension cause flowing movements both in the protoplasm 

 and in the fluid with which it is in contact. The relation of such 

 currents to the movements of pseudopodia has been discussed above 

 (p. 47). Similar movements have been imitated artificially by 

 Butschli and Rhumbler in a manner which can leave no doubt that 

 the physical analogy is a reasonable interpretation of the mechanism 

 of amoeboid movement. 



The close structural similarity between flagella and cilia on the 

 one hand, and the axopodia on the other, makes it highly probable, 

 to say the least, that the same explanation of the movement applies 

 to both. The axis of the vibratile organ is commonly regarded as 

 a firm, elastic, form-determining structure ; the more fluid sheath 

 as the seat of the motile activity. Chemical differences set up in 

 the limiting membrane, causing differences in the surface-tension of 

 the sheath along certain lines, have been supposed to be responsible 

 for a deformation of the sheath, bending the axis and the whole 

 organ with it ; with equalization and disappearance of such differ- 

 ences, the elastic axis straightens itself again. How such chemical 

 differences are set up remains to be explained ; possibly they origi- 

 nate in chemical changes taking place explosively in the basal 

 apparatus of the vibratile organs ; in any case it is clear that, as com- 

 pared with pseudopodia, they act with extreme rapidity, and, further, 

 that they are localized on the surface of the flagellum or cilium. 

 From the movements of these organs, the contraction appears to run 

 a spiral course as a general rule at least in cilia (p. 54) ; flagella, 

 however, appear to be capable of various kinds of movements (p. 52). 



According to Prowazek (192), the flagellum of a trypanosome only retains 

 its motility so long as it remains in connection with the kinetonucleus. Wer- 

 bitzki (526), however, has succeeded in producing strains of trypanosomes 

 without kinetonuclei, and with apparently no resulting loss of motility. It 

 has been observed frequently that detached cilia or flagella continue to 

 contract, for a time at least ; and Schuberg (44) denies that the basal granules 

 of the cilia function as centres of kinetic activity. 



