20 JOURNAL OF THE [January, 



enveloping sack or skin, its movement is that of swimming, ac- 

 complished by the use of permanent hair-like filaments, called 

 cilia, or whip-like lashes known as flagella, which act against the 

 water and propel the creature with considerable velocity. 



But the motion of the free diatoms is like neither the quick and 

 nervous swimming, effected by cilia or flagella, nor the slow and 

 measured crawling, performed by pseudopodia. It is in fact a 

 somewhat jerky sort of glide. There are, nevertheless, advocates 

 of the existence both of cilia and of pseudopodia as the means of 

 the diatom's propulsion, although no certain glimpse has ever 

 been obtained of either kind of organ. 



In the genus Navicula, which includes most of the motile forms,. 

 a pretty good magnifying power discloses a central spot, or nodule, 

 in the siliceous valve, with a line, or, in some cases, a double 

 or bifurcate line, extending from it in either direction to the nar- 

 rowed ends, as the frustule lies with its boat-like outline towards 

 us. This median line is believed to be a lapped or rolled seam 

 between two contiguous plates of which the valve is composed. 

 Whether the edges are rolled so tightly as to form a closed joint 

 or the union is so loose as to leave an open slit, is still a de- 

 batable question ; but there seems to be no room for doubt 

 that the thickened welt, to which the name raphe has been given^ 

 has with it a narrow groove of more or less depth. Now, the 

 advocates of the pseudopod theory of locomotion commonly 

 adhere to the belief in an actual cleft, through which the feet are 

 supposed to be protruded from within the diatom; while the 

 believers in the existence of cilia are generally disposed to accept 

 either a closed or an open furrow as the seat of these oar-like 

 appendages. Since the diatom is really a bivalve, you may begin 

 to wonder why the workers over this problem have not located 

 the organs of locomotion along the suture, or line of junction, 

 between the two valves. But the objection to doing this is that 

 the edges of the two valves are not actually in contact, but rather 

 overlap, like the sections of a telescope tube, the diameter of one 

 valve being slightly greater than that of the other. This over- 

 lapping is quite extensive in the larger forms, and gives rise to 

 the appearance of a broad band or hoop encircling the frustule. 



Unless, therefore, the raphe is accompanied by an open slit,, 

 the diatom is a tightly closed box with its living substance shut 



