250 SPH.ERELLA CHAP. 



Each flagellum (Fig. 68, K,fl] is a thread of protoplasm 

 of uniform diameter except at its distal or free end, where 

 it tapers to a point. The lashing movements are brought 

 about by the flagellum bending successively in different 

 directions (F). Thus the ciliary movement of Sphaerella, 

 like the amoeboid movement of Amoeba, is a phenomenon 

 of contractility. Imagine an Amoeba to draw in all its 

 pseudopods but two, and to protrude these two until they 

 became mere threads ; imagine further these threads to 

 contract rapidly and more or less regularly instead of slowly 

 and irregularly ; the result would be the substitution of 

 pseudopods by flagella, i.e., of temporary slow-moving pro- 

 toplasmic processes by permanent rapidly-moving ones. 



To put the matter in another way : in Amoeba the func- 

 tion of contractility is performed by the whole organism ; 

 in Sphserella it is discharged by a small part only, viz., 

 the flagella, the rest of the protoplasm being incapable of 

 movement. 



Sphaerella multiplies after becoming quiescent in the 

 encysted condition (Fig. 68, c, D) ; as in the active Amoeba 

 (p. 236) its protoplasm undergoes simple or binary fission, 

 but with the peculiarity that the process is immediately 

 repeated, so that four daughter-cells are produced within the 

 single mother-cell-wall. By the rupture of the latter the 

 daughter-cells are set free as the ordinary motile form, 

 acquiring their fkgella and detached cell- wall before making 

 their escape (D). 



Under certain circumstances the resting form divides into 

 numerous smaller daughter-cells, and these when liberated 

 are found to have no cell-wall. Sphaerella therefore occurs, 

 in the motile condition, under two distinct forms, i.e., is 

 dimorphic : the larger or ordinary form with detached cell- 

 wall is called a mcgazpoid, the smaller form without a cell- 

 wall a microzooid. The microzooids, which are all similar 



