507 



viz., that a cilium is composed of contractile substance along one side 

 and elastic material on the other and that the intermittent action of 

 the contractile substance efl'ects the bending over. But to obtain such 

 a curve as is presented by the acting cilium the supposed elastic 

 material must show no "give" in the longitudinal direction: it there- 

 fore must be as rigid in that direction as a steel band: no soft or 

 yielding material would serve, for the shortening of the contractile side 

 of the cilium would merely throw such into wrinkles without bending it 

 bodily over. Models with steel to represent the elastic substance and 

 a stretched india-rubber band or a string to represent the contractile 

 substance are to be found in most lecture-rooms, but a model of this 

 nature which attempts also to imitate the physical properties of a cilium 

 has yet to be constructed and it may safely be said it will never be made. 

 And with it must go to the ground all theories which involve the 

 existence of contractile filaments within cilia — whether we denominate 

 the filaments lines of inotagmata or merely lines of contraction — for it 

 is impossible to conceive how such filaments can produce ciliary move- 

 ment in the absence of a longitudinally rigid but laterally flexible 

 skeleton. Which remark leads me to the last point which I propose 

 to discuss, viz., the new theory which Dr. Pütter himself brings for- 

 ward. 



Putter's Theory of Ciliary Action. Dr. Pütter strikes 

 the keynote of this subject in remarking upon the importance of the 

 tentacles of Suctoria for the theory of ciliary action. In this remark 

 I agree with him ; naturally so, because these objects afford the stron- 

 gest possible argument in favour of the probability of the theory of 

 tubular structure and hydraulic action which I have advanced. This 

 is clear from the accounts which are given of their expansion and re- 

 traction; moreover their structure — tubes, with fluid contents con- 

 tinuous with the cell-fluid — is obviously adapted to this mode of 

 movement; and, as we have seen, Plate has observed a particular 

 species (Ascellicola digitata) in which the end of the tentacle makes 

 rapid movements of invagination and extrusion which can only be pro- 

 duced by such a hydraulic process. And not only is this kind of 

 movement found in Suctoria but the tentacles in some species are ob- 

 served (BÜTSCHLI, "Protozoen" in Bronn's Klassen und Ordnungen 

 des Tierreiches, Bd. 1, 1887) to make frequent to-and-fro movements 

 from their base and in one genus (Ophryodendron) to possess the pro- 

 perty of steady rhythmic movement, so that in them we have every 

 transition between true cilia and the fully developed Suctorian tentacle 

 which has obviously been evolved out of a process of the cell-body 



