1877J 



On the Structure o/Magelona, 



559 



The rest of the diagram represents the forces which we found when 

 the disks were at distances of 10 and 5 millims. asunder. The forces 

 which presented themselves at these distances are to be attributed 

 mainly to a true Crookes's reaction between the disks ; and they seem to 

 warrant the conclusion that Crookes's reaction was manifested at a dis- 

 tance of at least 10 millims. in a hydrogen vacuum, when the outstand- 

 ing tension was as much as 5 milHms. of mercury. 



At distances of from 20 to 80 millims. the very feeble force acting on 

 the glass disk in our apparatus seemed to vary about inversely as the 

 tension. As already mentioned, it appeared to be nearly independent of 

 the distance when the distance exceeded 20 millims. 



At distances of 5, 10, and 20 miUims. the force on the swinging disk 

 made some approach to varying at each tension inversely as the distance. 

 But, so far as may be judged from our measures of such exceedingly 

 feeble forces, there is a sensible deviation from this law at most of the 

 tensions. 



Moreover the diagram, taken as a whole, seems to suggest, in con- 

 formity with the dynamical theory, that the law changes with variations 

 of density. For if the law were the same at all the observed tensions, 

 the converging lines in the diagram should converge to points in the 

 axis of abscissas, whereas they converge towards points lower down. 



We will postpone the further discussion of the observations already 

 made with this apparatus until we can supplement them by others. 



II. ^'On the Structure of Magelona." By W. C. M^ntosh, 

 M.D., F.R.S.E., F.L.S. Communicated by G. Busk, F.R.S., 

 V.P.L.S. and Z.S. Received January 23, 1877. 



(Abstract.) 



This annelid was first discriminated * by Dr. Greorge Johnston, of 

 Berwick ; but as his description (under the name Mcea mimhilis) was not 

 published till 1865, the above-mentioned title, given to the same type, 

 from the Island of St. Catherine, off the coast of Brazil, by Dr. Fritz 

 MuUer, has the priority. It is a comparatively small form, its slender 

 body being divided into two well-marked regions; w^hile anteriorly 

 two long papillose tentacles are attached to the base of a remarkable 

 spathulate, eyeless snout, which it dextrously uses to perforate sand near 

 low-water mark at St. Andrews and other sandy shores on both east and 

 west coasts. 



The structure may be examined under the following heads : — 

 Cuticle. — This chitinous transparent layer is densest on the snout and 

 anterior region of the body, both being much exposed in the boring- 

 operations. Throughout the rest of the body it attains its maximum 

 thickness over the nerve-cords in the ventral median line. Ko cilia occur 

 * From specimens procured by the distinguished botanist, Dr, Greyille. 



