DIATOM LOCOMOTION. I59 



under side of the cover marks at once the great gulf lying 

 between these lowly creatures and even the most subtle 

 microscopic foam of the wise Bvitschli. And in the rhizopod, 

 whether naked or hampered by a carapace, anyone may 

 easily see the thing done before his own eyes by actual con- 

 tact and movement of the living substance. Actually to see 

 the propelling protoplasm in the living diatom is not so easy, 

 and almost always it is quite impossible.* Heavily housed 

 in silica, it streams mostly within ingenious and complicated 

 tubes and minute clefts, protruding when necessary only 

 slightly, and m thinnest threads or sheets that are so hyaline 

 as to be invisible in water with the ordinary arrangements 

 of lenses and illuminators. 



The analogy with the rhizopods need not be pushed too 

 far. In one respect there may seem to be a contrast so com- 

 plete as to render the comparison of no value. The amoeba 

 moves in the direction of its general flow, while the diatom, 

 b\' hypothesis, goes in the opposite direction to the flow of its 

 protoplasmic streams. But even so, the difference is not so 

 great as might at first appear, since while the interior currents 

 of the amoeba move forward, the peripheral currents next the 

 substratum on which it rests, move backward just as does the 

 stream in the raphe of the Naviaila or the Siirirella. The 

 contrast is not in this respect so violent after all. But cer- 

 tainly the methods of motion in the two cases are unlike in 

 important respects, for the amoeba rolls about vaguely by 

 means of a continuous succession of bodily deformations, 

 impossible to the rigidly confined diatom, which must push 

 all its heterogenous apparatus by specialized surface-layers. 

 And these layers and threads, uulike the surfaces of the 

 amoeba, continually bathe themselves in a thin secretion of 

 "gallerte," sufficiently adhesive to afford a basis for a real 



* In the "Journal of the Royal Microscopical Society" for June, 

 1912, an account is given of a Coseinodiscus exhibited by Mr. Siddall, 

 showing pseudopodial protnisions with dark-ground illuniination. 



