io6 AMEBOID MOVEMENT 



ment in Oscillatoria. The spiral feature of the movement has no 

 explanation that is based on observational data. It may be added 

 here that the surface film m amebas is powerful enough to enable 

 them to move by means of it. One sometimes sees sphaeronucle- 

 osus or small individuals of verrucosa, that are lying loose on the 

 substratum, actively- streaming, but moving slowly and more or 

 less irregularly backwards. This movement is due to the activity 

 of the surface film. 



The suggestion that no extra-cellular protoplasmic layer has 

 been demonstrated in Oscillatoria is not a cogent argument 

 against the surface tension hypothesis, since the surface film 

 would need to be but a small fraction of a micron thick, too thin 

 to be demonstrated by histological methods now in vogue. It is 

 also to be remembered that the surface film in ameba can be 

 demonstrated in no other way at present than by its particle-car- 

 rying capacity. 



The main features of the movements of diatoms are very 

 similar to those of Oscillatoria. Miiller ('89, '97, '99) has shown 

 that the gliding movements of diatoms are not due to the ejection 

 of water, but to the streaming of protoplasm on the outside of the 

 shells. Foreign particles are carried by these shallow streams of 

 protoplasm in quite the same manner as by the surface film of 

 the ameba. And there seems to be no evidence against the as- 

 sumption that these shallow streams, at least the surface films 

 over them, owe their movement to changes in surface tension. 



Desmids also glide about slowly, leaving a track of mucus be- 

 hind. Only one explanation for locomotion has been advanced, 

 and that is that it is due to the secretion of mucus (Klebs, '85). 

 This explanation is likely to be as wide of the mark as the similar 

 explanation in the case of Oscillatoria. There is no question con- 

 cerning the excretion of mucus, but the source of the locomotive 

 energy is probably here also surface energy, though the observa- 

 tional data are too few to try to locate the regions where the 

 . changes in tension occur. 



It has been a matter of considerable surprise to me to find 

 that the so-called "crawling" euglenas, in addition to the diatoms, 

 desmids, Oscillatoria, Beggiatoa and perhaps other forms of life 

 such as the Gregarinidas, also possess extra-cellular films which 



