PHYSIOLOGICAL 233 



But, besides the movements in the plasma membrane, and 

 apparently more fundamental, is a deeper protoplasmic streaming 

 which is correlated with intricate physical and chemical changes, 

 such as a rise in hydrogen ion concentration in an active outflowing. 

 There are indications of rapid changes from "gel" to "sol" states, 

 and back again. Yet when we think {a) of the relatively rapid move- 

 ments in the plasma membrane with its surface-tension effects, and 

 (b) of the deeper protoplasmic streaming, we are still far from having 

 a connected picture of what takes place when one Amoeba pursues 

 another. 



It is interesting to find that an Amoeba, when comparatively free 

 from external stimulation, moves in a sinuous path, in a very loose 

 spiral ; and Schaeffer has shown that many different kinds of animals, 

 and man himself in swimming aimlessly, and all motile plants, like 

 free-swimming Algae, move in spirals of some sort, when moving 

 idly, that is to say when the orienting senses are not functioning. 

 At other times, of course, as when in pursuit of some booty, the 

 Amoeba shows a more direct controlled movement; and the un- 

 prejudiced observer will find it difficult to accept the view that it 

 is entirely destitute of "purpose", ignorant though he may be of 

 what "purpose" means at this low level of life. 



Myonemes. — A clear and familiar instance of a myoneme or 

 contractile plasmic thread is in the stalk of the beautiful Bell 

 animalcule (Vorticella), which often grows in groups on the stems 

 of water plants. It may be compared to a hand-bell borne mouth 

 upwards on a long contractile stalk, and it passes frequently from 

 an expanded phase with the stalk at its maximum length to a 

 sharply contracted phase when it is drawn in close to its support. 

 The sheath of the stalk is non-contractile, while the axial myoneme 

 is like a thread of muscle; therefore the sheath has to coil into a 

 spiral when the central thread contracts to a fraction of its maximum 

 length. It is a fascinating sight, the sudden retraction, the momen- 

 tary quiescence, the rapid shooting out again; and no one quite 

 understands how it is done! Again there must be rapid alterna- 

 tions of "sol" and "gel" states. 



Cilia and Flagella. — A cilium is a microscopic lash of living 

 matter that is continually flexed and straightened again, as one 

 might bend one's arm at the elbow and then straighten it out again. 

 Cilia are especially characteristic of one of the orders of Infusorians, 

 the Ciliata,well illustrated by the Slipper animalcule (Paramoecium), 

 the trumpet-shaped Stentor, the bell-like Vorticella. Among multi- 

 cellular animals cilia are also very common, from the lowest to the 

 highest, except in the Threadworms and in the great phylum of 

 Arthropods, where the abundant presence of chitin probably pre- 

 cludes their development. It is interesting to notice that the very 

 simple Turbellarian or Planarian worms move mainly by means of 



