MICROSCOPIC AQUATIC ORGANISMS. 359 



pseudopodia goes on without cessation, there is a continuous move- 

 ment of the whole animal, though not usually in any definite 

 direction. Examples of amoeboid movements of various kinds 

 are very common among the class to which the Amoeba belongs, 

 namely, the Rhizopoda, including the well-known Foraminifera. 

 Owing to the observations and ingenious experiments of Biitschli, 

 Rhumbler, and others, it now seems very probable that in its 

 main features amoeboid motion is essentially a physical pheno- 

 menon depending chiefly upon changes in the surface-tension of 

 the naked protoplasm of these organisms. 



The points to be remembered in relation to amoeboid motion 

 are that it is brought about by an actual flowing out of the 

 semi-liquid protoplasm, that the pseudopodia so produced are 

 only temporary organs of locomotion, and that their movements 

 are too slow to be effective except when they are in contact with 

 solid bodies. But in connection with the latter remark it is to 

 be noted that there are a few forms (e.g. one of the varieties of 

 Amoeba radiosa) in which the pseudopodia are long and pretty 

 vigorous in their movements, sometimes swaying about so rapidly 

 that they can be imagined to form a transition from typical 

 pseudopodia to the flagella or whip- like organs which we have 

 next to consider. In other intermediate forms there exists a 

 combination of pseudopodia and flagellum in the same individual 

 (e.g. Mastig amoeba). 



The second type of locomotion to which we will devote atten- 

 tion is that due to the action of flagella. There are certain 

 difficulties in framing an exact definition of a flagellum, but I 

 look upon it as a permanent, motile, whip-like extension of the 

 living protoplasm, capable of moving the whole organism to 

 which it belongs, and capable of independent motion if, as is 

 sometimes the case, it is accompanied by other flagella. I 

 believe also that the mechanical action of a flagellum in pro- 

 ducing locomotion, a subject to be dealt with presently, is quite 

 characteristic. 



The source of movement of a flagellum appears to reside in the 

 flagellum itself. A close connection has, however, been made out 

 in many cases between the flagellum and the nucleus of the 

 cell : and it is probable that even though the motive power of the 

 flagellum cannot be referred to the nucleus, the latter exercises 

 some control over the former. Flagella usually originate from 



