30 



THE PROTOZOA 



homogeneous that is to say, of similar nature in all parts ; if, 

 however, its surface be heterogeneous, and differs in different parts, 

 local inequalities of surface-tension may be the result, and then a 

 perfectly spherical form cannot be maintained so long as the surface 

 remains heterogeneous. Thus an organism, such as an amoeba, in 

 which the protoplasm is quite naked and exposed at the surface of 

 the body, tends always to have a spherical form in the resting state ; 

 but when it enters upon a phase of vital activity, it may assume 

 various forms which can be explained by supposing that the surface- 

 tension is altered at one or more regions of the surface as the result 



EP 



FIG. 13. Thalassicdlla (Thalassophysa) pelagica, Haeckel, an example of a species 

 of floating habit combined with radiate symmetry and spherical body-form. 

 GK, Central capsule ; EP, extracapsular protoplasm ; al, vacuoles in the 

 calymma (see p. 251) ; ps., pseudopodia. The small dots in the calymma 

 represent " yellow cells " (p. 252). After Lankester, magnified 25 diameters. 



of local changes in chemical constitution, brought about by the 

 vital activity of the protoplasm (Rhumbler, 34, and p. 200 infra). 

 In consequence, the spherical form characteristic of the resting 

 state undergoes modification in various ways when the organism 

 becomes active. In floating forms the sphere throws out radiating 

 processes, so-called "pseudopodia," in all directions (Figs. 3, 13). 

 In creeping species the body assumes the indefinite and constantly 

 changing form, with pseudopodia extruded in every direction, which 

 is characteristic of the amoeba (Fig. 2), and hence commonly termed 

 ' amoeboid." In all such cases, when the animal passes into a 



