246 ON THE STRUCTURE OF OSCILLATORI^. 



and forwards, or leaving' them altogether ; in the latter 

 instance, the filament, on sliding out, receives an impetus, as 

 it seems, from the sudden cessation of the impediment pre- 

 sented by the sheath to its forward motion. While sliding 

 within its sheath, I have repeatedly observed that the tapering 

 and bent extremity of the filament in its progress altered its 

 position with regard to the sides of the sheath, now pointing 

 upwards, now downwards ; performing, therefore, a kind of 

 rotation around its own axis in its progress. 



The filaments themselves have been supposed to consist 

 wholly of protoplasm ; this view is not correct, since the 

 protoplasm is enclosed in a proper cell-membrane, which has 

 not, to my knowledge, been noticed before. This cellulose 

 coat ahcays shows the cross-markings corresponding to the 

 striae when such were observable in the filament, and which 

 divide it into distinct joints or cells ; these cells, however, 

 seem to be what Kiitzing calls " cellulse hologonimicae " 

 cells, completely filled out by the gonimic substance, or 

 endochrome, which circumstance causes the cells to resume 

 their former shape, after desiccation, on the addition of water, 

 and accounts for the difficulty of demonstrating their struc- 

 ture. They form, with the protoplasm deposited in them, 

 annular bands or concentric rings, around the solid axis of the 

 filament (formatio perigenata). A reference to PI. XIV., fig 7, 

 will render its structure more intelligible. The presence of 

 this cell-membrane may be best demonstrated by breaking up 

 the filaments, either by moving the thin glass cover, or by 

 cutting through a mass of them in all directions with a pair 

 of fine dissecting knives. On now examining the slide, in 

 most instances many detached empty pieces of this cell- 

 membrane, with its striae, will be found, as well as filaments 

 partly deprived of the protoplasm, showing in those places the 

 empty, striated cellulose coat, figs. 1, 2. On the subsequent 

 addition of iodine all these appearances will become unmis- 

 takeably evident ; the entire portions of the filament turning 

 brown or red, while the empty, with its striae, remain either 

 unaffected, or at most present a slight yellowish tint, as is 

 frequently the case with cellulose when old, for instance. (PI. 

 XIV., figs. 3, 8, 11.) Many specimens, however, do not readily 

 show the above appearances, but require some trouble and 

 management, while others do so readily enough ; this arises 

 from the peculiar state they are in ; as a general rule, I found 

 that those which admit of being easily broken up are the most 

 fit for demonstrating this cell-membrane. In case it might be 

 imagined I had mistaken the external enclosing sheath for the 

 (ell-membrane, I will observe that I have repeatedly isolated 



