254 THE FORMS OF CELLS [ch. 



contract downwards towards its base, and become confluent with 

 the general surface of the cell ; for it has no 

 longitudinal supports and no strengthening ring 

 at its periphery. But in all these collar-cells, 

 there stands within the annulus of the collar 

 a large and powerful cilium or flagellum, in 

 constant movement; and by the action of this 

 flagellum, and doubtless in part also by the 

 intrinsic vibrations of the collar itself, there is 

 set up a constant steady current in the sur- 

 rounding water, whose direction would seem to 

 be such that it passes up the outside of the 

 collar, down its inner side, and out in the middle 

 in the direction of the flagellum ; and there is a 

 Fig. 83.^ distinct eddy, in which foreign particles tend to 



be caught, around the peripheral margin of the collar. When the 

 cell dies, that is to say when motion ceases, the collar immediately 

 shrivels away and disappears. It is notable, by the way, that 

 the edge of this little mobile cup is always smooth, never notched 

 or lobed as in the cases we have discussed on p. 236: this latter 

 condition being the outcome of a definite instability, marking the 

 close of a period of equilibrium; while in the vibratile collar of 

 Codosiga the equilibrium, such as it is, is being constantly 

 renewed and perpetuated like that of a juggler's pole, by the 

 motions of the system. I .take it that, somehow, its existence 

 (in a state of partial equilibrium) is due to the current motions, 

 and to the traction exerted upon it through the friction of 

 the stream which is constantly passing by. I think, in short, 

 that it is formed very much in the same way as the cup-like ring 

 of streaming ribbons, which we see fluttering and vibrating in the 

 air-current of a ventilating fan. 



It is likely enough, however, that a different and much better 

 explanation may yet be found; and if we turn once more to 

 Mr Worthington's Study of Splashes, we may find a curious 

 suggestion of analogy in the beautiful craters encircling a central 

 jet (as the collar of Codosiga encircles the flagellum), which we see 

 produced in the later stages of the splash of a pebble*. 



* Op. cit. p. 79. 



i 



