v] OF CILIA AND FLAGELLA 407 



microcosmic, properties of these minute, mobile, pointed, fluid or semi-fluid 

 threads.* 



Cilia, like flagella, tend to occupy positions, or cover surfaces, 

 which would otherwise be unstable; and often indeed (as in a 

 trochosphere larva or even in a Rotifer) a ring of cilia seems to 

 play the very part of one of Plateau's wire rings, supporting and 

 steadying the semi-fluid mass in its otherwise unstable configura- 

 tion. Let us note here (in passing) what seems to be an analogous 

 phenomenon. Chitinous hairs, spines or bristles are common and 

 characteristic structures among the smaller Crustacea, and more or 

 less generally among the Arthropods. We find them at every 

 exposed point or corner; they fringe the sharp edge or border of 

 a limb; as we draw the creature, we seem to know where to put 

 them in ! In short, they tend to occur, as the flagella do, just where 

 the surface-tension would be lowest, if or when the surface was in 

 a fluid condition. 



Of the other surfaces of Plateau, we find cylinders enough and 

 to spare in Spirogyra and a host of other filamentous algae and 

 fungi. But it is to the vegetable kingdom that we go to find them, 

 where a cellulose envelope enables the cyUnder to develop beyond 

 its ordinary limitations. 



The unduloid makes its appearance whenever sphere or cyhnder 

 begin to give way. We see the transitory figure of an unduloid in 

 the normal fission of a simple cell, or of the nucleus itself; and we 

 have already seen it to perfection in the incipient headings of a 

 spider's web, or of a pseudopodial thread of protoplasm. A large 

 number of infusoria have unduloid contours, in part at least; and 

 this figure appears and reappears in a great variety of forms. The 

 cups of various Vorticellae (Fig. 124), below the ciliated ring, look 

 like a beautiful series of unduloids, in every gradation of form, from 

 what is all but cylindrical to all but a perfect sphere; moreover 

 successive phases in their hfe-history appear as mere graded changes 



* It is highly characteristic of a cilium or a flagellum that neither is ever seen 

 motionless, unless the cell to which it belongs is moribund. "I believe the motion 

 to be ceaseless, unconscious and uncontrolled, a direct function of the chemical 

 and physical environment "; tieorge Bidder, in Presidential Address to Section D, 

 British Associqtion, 1927. Cf. also James Gray, Proc. R.S. (B), xcix, p. 398, 

 1926. 



