V] OF VARIOUS UNDULOIDS 247 



usually see signs of an unequal tension of the membrane in the 

 neighbourhood of the base of the fiagellum. Here the tension is 

 usually less than elsewhere, and the radius of curvature is accord- 

 ingly less : in other words that end of the cell is drawn out to a 

 tapering point (Fig. 73). But sometimes it is the other way, as 

 in Noctiluca, where the large fiagellum springs from a depression 

 in the otherwise uniformly rounded cell. In this case the explan- 

 ation seems to lie in the many strands of radiating protoplasm 

 which converge upon this point, and may be supposed to keep it 

 relatively fixed by their viscosity, while the rest of the cell-surface 

 is free to expand (Fig, 74). 



A very large number of Infusoria represent unduloids, or 

 portions of unduloids, and this type of surface appears and 

 reappears in a great variety of forms. The cups of the various 

 species of Vorticella (Fig. 75) are nothing in the world but a 



Fig. 75. Various species of Vorticella. (Mostly after Saville Kent.) 



beautiful series of unduloids, or partial unduloids, in every grada- 

 tion from a form that is all but cylindrical to one that is all but 

 a perfect sphere. These unduloids are not completely symmetrical, 

 but they are such unduloids as develop themselves when w^e 

 suspend an oil-globule between two unequal rings, or blow a 

 soap-bubble betw^een two unequal pipes; for, just as in these 

 cases, the surface of our Vorticella bell finds its terminal supports, 

 on the one hand in its attachment to its narrow stalk, and on the 

 other in the thickened ring from which spring its circumoral cilia. 

 And here let me say, that a point or zone from which cilia arise 

 would seem always to have a pecuKar relation to the surrounding 

 tensions. It usually forms a sharp salient, a prominent point 

 or ridge, as in our little monads of Fig. 73; shewing that, 

 in its formation, the surface tension had here locally diminished. 

 But if such a ridge or fillet consolidate in the least degree, it 

 becomes a source of strength, and a 'jwint (Vapfui for the adjacent 

 film. We shall deal with this point again in the next chapter. 



