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sisting of Naviculaeform frustules enclosed and freely swimming 

 about in tubes, after the manner of Schizonema. In fact 

 there is nothing to separate these genera, except that the first 

 inhabits fresh water, whilst the latter is an inhabitant of the 

 sea, where it is to be found generally in profusion, covering 

 larger algce and rocks. The extension of the tube takes 

 place after the following manner. As the frustules increase 

 by the process of subdivision common to all of the 

 Diatomacefe, of course the tw^o frustules thus formed occupy 

 double the space of one, and as the cell division is continually 

 going on, after a time the tube must become choked with 

 individuals. At this period in their existence they appear to 

 be extremely active, moving wdth increased rapidity up and 

 down the tube as freely as their crowded condition will 

 permit. Whether the end of the tube is never closed, or 

 opens at certain seasons, I have been unable to determine ; 

 at all events it is now found to be open, and the frustules 

 slip over each other until they reach this opening, and one 

 or two will project outside as if prospecting, and will occa- 

 sionally return within the general envelope. When a 

 frustule thus projects from the open end of the tube, it 

 never, as far as I have seen, rushes onward with the vigorous 

 motion with which it moves within the envelope, but this is 

 doubtless only so Avhen the tube is being lengthened. It can 

 be easily understood that if the species be disseminated by 

 the distribution of perfect frustules, as seems to be most 

 likely, that they must then escape from the tube after the 

 manner I have recorded above as taking place in the allied 

 genus, Schizonema. When one or two frustules have pro- 

 jected from the open end of the tube, they often immediately 

 come to a rest just beyond the tube, or do so after moving 

 over each slowly outside of, but in a line with, the tube. 

 While at rest there appears to form around them a trans- 

 parent mucous sheath, which, so that it may not fix them 

 in their position, is kept in a tube form by the frustules again 

 moving over each other, and thus, as it were, fashioning and 

 smoothing the inside of the tube. This shealh becomes 

 more and more dense, until it is plainly visible as forming an 

 elongation of the tube, when the frustules again project from 

 the end, and a new portion is added. I have in this way seen 

 a tube grow across the field of the microscope, and the closely 

 packed frustules extend themselves in single file, each just 

 overlapping those in front and behind it. The membrane 

 constituting the tube, although dense and strong, is somewhat 

 elastic, but not very much so, for I have seen three or four 

 frustules become wedged together by one attempting to pass 



