AMEBOID MOVEMENT 133 



the special action of the cilia of a paramecium that causes it to 

 rotate and not the shape of the body. Again the shape of a Stentor 

 caeruleus is subject to very great variation due to varying amounts 

 of food eaten, and to surgical operation, but a spiral path is 

 nevertheless maintained while the body shape undergoes marked 

 changes. 



Although all free-swimming unicellular organisms revolve on 

 their long (antero-posterior) axis, an occasional one does not move 

 in spirals. This is observed in the large colonial flagellate Volvox 

 occasionally, but not always (Mast, '10). Since it is more fre- 

 quently seen in the larger individuals, it is probable that the forma- 

 tion of spirals is prevented because of the increased physical in- 

 ertia of the colony; for the older and larger colonies are much 

 more unsymmetrical than the younger and smaller, owing to the 

 unequal distribution of the reproductive elements. Spondylo- 

 morum and several other colonial forms describe smaller spirals 

 than smaller solitary organisms. These colonial organisms con- 

 sisting of from four to twenty thousand cells, each of which may 

 be possessed of cilia, are marvels of locomotory coordination, 

 but it is not at all clear how this coordination is brought about. 

 Since the colonies are symmetrical however, the spirality of the 

 path is clearly due to the special action of the cilia. 



Some organisms possess body shapes that seem to be due to the 

 habit of spiral swimming. Jennings ('01) describes a species 

 of rotifer whose body forms a segment of a spiral. When swim- 

 ming a spiral path is described, "of which its own twisted body 

 forms a part" (p. 376). Elsewhere he has pointed out that the 

 oral groove of, a paramecium likewise coincides with its own 

 spiral path. Indications of such correspondence between the 

 axis of a structure and the spiral path the organism possessing 

 it, describes, are numerous among free swimming animals. But 

 such correspondence (with an imaginary spiral path) is also 

 found in organisms that do not swim freely. One of the most 

 interesting of such cases is found in the Oscillatoriaceae. In a 

 previous chapter it was seen that many of these organisms are 

 capable of moving about by means of a film of what is probably 

 protoplasm, which moves spirally around the filament. A particle 

 attached to this film describes a spiral path like that of a flagellate 



