MOVEMENTS 1 7 



growths, aud " contraction," leading to their diminution and dis- 

 appearance within the general surface.^ Expansion is probably 

 due to the lessening of the surface-tension at the point of out- 

 growth, contraction to the increase of surface-tension. Verworn 

 regards these as due respectively to the combination of the 

 oxygen in the medium with the protoplasm in diminishing sur- 

 face-tension, and the ejBFect of combination with substances from 

 within, especially from the nucleus in increasing it. Besides 

 these external movements, there are internal movements revealed 

 by the contained granules, which stream freely in the more fluid 

 interior. Those Protista that, while exhibiting amoeboid move- 

 ments, have no clear external layer, such as the Eadiolaria, Fora- 

 minifera, Heliozoa, etc., present this streaming even at the 

 surface, the granules travelling up and down the pseudopodia at 

 a rate much greater than the movements of these organs them- 

 selves. In this case the protoplasm is wetted by the medium, 

 which it is not where there is a clear outer layer : for that 

 behaves like a greasy film. 



Motile organs. — Protoplasm often exhibits movements much 

 more highly specialised than the simple- expansion or retraction 

 of processes, or the general change of form seen in Amoeba. If 

 we imagine the activities of a cell concentrated on particular 

 parts, we may well suppose that they would be at once more 

 precise and more energetic than we see them in Amoeba or the 

 leucocyte. In some free-swimming cells, such as the individual 

 cells known as " Flagellata," the reproductive cells of the lower 

 Plants, or the male cells (" spermatozoa ") of Plants as high as 

 Ferns, and even of the Highest Animals, there is an extension of 

 the cell into one or more elongated lash-like processes, termed 

 " flagella," which, by beating the water in a reciprocating or a 

 spiral rhythm, cause the cell to travel through it ; or, if the cell 

 be attached, they produce currents in the water that bring food 

 particles to the surface of the cell for ingestion. Such flagella 

 may, indeed, be seen in some cases to be modified pseudopodia. 

 In other cases part, or the whole, of the surface of the cell may 

 be covered with regularly arranged short filaments of similar 

 activity (termed " cilia," from their resemblance to a diminutive 

 eyelash), which, however, instead of whirling round, bend sharply 



1 The terms " expansion " and " contraction " refer only to tlie superficial area : 

 it is very doubtful whether the vohtme alters during these changes. 



VOL. I ° 



