426 THE FORMS OF CELLS [ch. 



fabric is in a flux, and currents flow and granules are carried 

 hither and thither. Here again there is no statical equilibrium; 

 but surface tension varies, as does the chemistry of the protoplasm, 

 from oiie spot to another. 



The great oceanic group of the Radiolaria, and the highly com- 

 plicated skeletons which they construct, give us many beautiful 

 illustrations of physical phenomena, among which the efl'ects of 

 surface-tension are as usual prominent. But we shall deal later on 

 with these little skeletons under the head of spicular concretions. 



In a simple and typical Heliozoan, such as the sun-animalcule, 

 Actinophrys sol, we have a "drop" of protoplasm, contracted by its 

 surface tension into a spherical form. Within this heterogeneous 

 protoplasm are more fluid portions, and a similar surface-tension 

 causes these also to assume the form of spherical "vacuoles," or of 

 little clear drops within the big one; unless indeed they become 

 numerous and. closely packed, in which case they run together and 

 constitute a "froth," such as we shall study in the next chapter. 

 One or more of such clear spaces may be what is called a "con- 

 tractile vacuole " : that is to say, a droplet whose surface-tension 

 is in unstable equilibrium and is apt to vanish altogether, so that 

 the definite outHne of the vacuole suddenly disappears*. Again, 

 within the protoplasm are one or more nuclei, whose own surface- 

 tension draws them in turn into the shape of spheres. Outwards 

 through the protoplasm, and stretching far beyond the spherical 

 surface of the cell, run stiff linear threads of modified protoplasm, 

 reinforced in some cases by delicate siliceous needles. In either 

 case we know little or nothing about the forces which lead to their 

 production, and we do not hide our ignorance when we ascribe 

 their development to a "radial polarisation" of the cell. In the 

 case of the protoplasmic filament, we may (if we seek for a hypo- 

 thesis) suppose that it is somehow comparable to a viscid stream, 

 or "liquid vein," thrust or spirted out from the body of the cell. 

 But when it is once formed, this long and comparatively rigid 

 filament is separated by a distinct surface from the neighbouring 



* The presence or absence of the contractile vacuole or vacuoles is one of the 

 chief distinctions, in systematic zoology, between the Heliozoa and the Radiolaria. 

 As we have seen on p. 295 (footnote), it is probably no more than a physical con- 

 sequence of the different conditions of existence in fresh and in salt water. 



