"j^nla^ Ma^Tim '] the Econoimj of Stephanoceros. 241 



point, and with the view to form a true comparison with that organ 

 of the normal forms, the nature and effects of the currents produced 

 by their marginal cilia first claim our attention ; and the diagrams 

 (Plate LI., Figs. 1, 2, 3, 4), which were constructed in the summer 

 of 1867 to illustrate their courses in the disc of Melicerta are now 

 submitted to facilitate such a comparison ; and to prevent any con- 

 fusion of terms requiring to be frequently repeated throughout these 

 remarks, it is to be understood that the word cilia is used to express 

 the active vibratile hairs in contradistinction to the more quiescent 

 setm, and hristles as distinct from both. 



In the disc of Melicerta, let us first consider the action of a 

 single marginal cilium (Figs. 1 and 2, a, a). Its point describes 

 a circle from a centre (h, h), in the direction indicated by the small 

 arrows, which at first sight would appear to drive particles away 

 from, and not to the ventral surface of the disc, and in vacuo such 

 would be the inevitable result ; but here the motion of the particle is 

 subservient to that of the medium supporting it, in fact to the water. 

 Consider therefore, the point of this cilium to represent one of the 

 teeth of a small pinion working into a large wheel whose diameter 

 is proportionate to that of the pinion inversely as the resisting power 

 of the water is to that exerted by the cilium, and the course of this 

 wheel will be in the direction indicated by the arrow (c), precisely 

 that of the current actually produced by the motion of that cilium, 

 by which means particles are conveyed to, and impinge on, the ven- 

 tral surface of the disc at a tangent perpendicular thereto (e, f) ; the 

 same thing occurs mth each individual cilium all round the margins 

 of the four frontal lobes (c' c c), and the lobes being deflected from 

 the plane of the disc, form an irregular funnel, and so produce by a 

 conflaence of numerous currents a resultant volume which escapes 

 in part through the upper sinus, and then, meeting a current from 

 the dorsal lobe (g) (the chin of Gosse), is thereby deflected more or 

 less from its primal course, according to the varying angles assumed 

 by the frontal lobes, as indicated by arrow (h), passing off in a dense 

 stream to be dispersed around or to be brought again and again 

 unappropriated over the same course ; clearly showing that the 

 rotary organ by the currents it creates does not convey particles 

 within its range to the mouth. 



In considering the process by which food is actually carried to the 

 mouth, let us see how far these marginal cilia are concerned. They 

 possess a certain power of prehension, and are seen to wliij:) those par- 

 ticles that, impinging on the disc, are so brought within their hmited 

 range, over the margin into the channel formed by the second range of 

 cilia ; and here their function ends. If fm'ther proof be needed, it is 

 found in the fact that while their onward procession, or the appearance 

 of such, is in one and the same continuous direction (Fig. 3, n, n), the 

 particles that do find their way to the mouth course along from the 



VOL. III. E 



