160 THE SEA. 



all other cases the atoms possess the power of spon- 

 taneous movement to some extent. Often this takes 

 place by a series of intermittent jerks, which carry the 

 Diatom onward in a given direction for a while, and 

 then suddenly ceasing, yield to similar motions in an 

 opposite direction, by which the progress made is re- 

 versed. In some cases, as in the genus JBacillaria, 

 which we have just compared to a column of bricks 

 sliding one over the other, this movement of sliding 

 goes on till the frustules are on the point of separat- 

 ing, which then retrace their course till such a catas- 

 trophe seems equally imminent in the opposite direc- 

 tion. 



It is generally considered that no power of choice, 

 no real volition is manifested by these motions, which 

 are asserted to be merely mechanical, and not pro- 

 duced by any motile organs, properly so called. But 

 Dr G. C. Wallich has recently published some elabo- 

 rate researches made upon the free-swimming Dia- 

 tomacece of the South Atlantic, which lead to a differ- 

 ent conclusion. He has shewn that particles of ex- 

 traneous matter lying in the path of a moving Diatom 

 are occasionally pushed forward by it, or, if behind, 

 are taken in tow, and dragged after it ; the object in 

 neither case being in contact with the frustule, but 

 considerably distant from it. The object in tow 

 accurately exhibits and repeats every jerk, progression, 

 or pause of the ' tug ;' and at tunes is even drawn up 

 to it, it may be, from an oblique position, and is then 

 either released or carried along with it, adhering to 



