ON THE CILIATION OF MELICERTIDIUM OCTOCOSTATUM. 459 



32. The Ciliation of the Leptomedusan Melicertidium octo- 

 costatum (Sars). By James F. Gemmill, M.A., M.D., 

 D.Sc. 



[Received August 18, 1919 : Read November 18, 1919.] 



(Text-figure 6.) 



Few hydromedusan gonophores have radial and ring canals 

 wide enough to allow the action of the ciliated lining of these 

 channels to be adequately studied. Melicertidmm is an exception, 

 and now that the importance of primary functions in relation 

 to form is more and more recognised, tlie following details cannot 

 fail to be of interest. (The adult medusa has eight radial canals 

 along which the gonads are developed as eight somewhat promi- 

 nent and sinuous ridges projecting into the sub-umbrellar cavity. 

 See E. T. Browne, Proc. Roy. Soc. Edinb., 1905, p. 72, and 

 also a forthcoming j)aper by the author on the Life History of 

 Melicertidiwm.) 



Internal Surfaces. 



1. Stomach lining. — Currents .... outwards or centrifugal from 



middle point of roof, and inwards or centripetal {i. e., 

 towards manubrial junction) along floor. 



2. Lining of mcmmbrial canal. — Currents .... weakly upwards 



(from the mouth-opening) all round, even in the radial 

 grooves. This is different from what obtains in Aurelia 

 where the radial grooves serve as channels exhalant 

 from the stomach. 



3. Lining of radial canals. — Currents .... strong, centrifugal 



(i. e., from stomach towards ring canal along roof (exum- 

 brellar surface) of each canal), and centripetal (^'. e., 

 towards stomach) along the floor of each canal. 



4. Lining of ring canal. — Currents .... confused, causing mixing, 



but on the whole there appears to be a flow clockwise 

 (as viewed from the aboral side) along the floor of the 

 canal over the openii^^gs into the tentacle cavities, and a 

 converse flow along the opposite wall of the canal. An 

 inward and an outward current may be noted on 

 opposite sides of the opening of each tentacle cavity 

 into the ring canal. 



External Surfaces. 



1. Exumhrellar surface. — Ciliation absent, 



2. Sub-umhrellar surface. — (a) Along the projecting edges of the 



gonads there is strong ciliation upwards (i. e., towards 

 the manubrial region) ; (6) along the sides of the gonads 

 the currents are towards the projecting edges but 



