Dr. G. C.TVallich on the Diatomacea. 15 



granules, the movements become either wholly or partially 

 arrested. 



The power of turning on the long axis also exists, and, 

 further, of wheeling round on the centre abruptly. It is proba- 

 ble, however, that this last kind of motion is effected only when 

 the frustule happens to l)e obstructed iu its course by foreign 

 particles, or between the glass slide and cover of the observer. 

 When thus impeded, the smooth gliding character of the motion 

 becomes destroyed, and in its place may be seen a somewhat 

 ''drunken^' or, at times, jerky progress. This., although evi- 

 dently due to the abnormal position in which the Diatom hap- 

 pens to be placed, proves of the highest value, as will be pre- 

 sently understood, in arriv^ng at a proper view of the question 

 before us. 



Allien a frustule comes into contact with particles of matter in 

 its vicinity (as constantly occurs whilst it is under the eye of 

 the observer), it either cleaves its way steadily and slowly through 

 them, or, by a series of abrupt jerks, becomes freed from the 

 obstacles, and continues its progress, with perhaps some slight 

 change of its original direction. Should the particles prove too 

 heavy for its powers, or too firmly fixed, the jerks nevertheless 

 are continued, until, on the recurrence of the retrograde in- 

 terval, the frustule reverses its direction and retires from the 

 obstructing particles. 



But, instead of merely thrusting aside a ])article in its way, 

 the Diatom may frequently be noticed to seize upon it, and 

 carry it along with it for an indefinite period on one or other of 

 its surfaces — often, moreover, in an opposite direction to that 

 pursued by the frustule for the time being. It is not by any 

 means essential that the particle laid hold of should be placed 

 directly in its path, or even very close to it ; for, at times, 

 without any connecting bond of union being detected, the 

 particle is forcibly drawn towards the moving frustule, and is 

 either released after a while, or subjected to the handling above 

 referred to. 



The Diatom may, again, pass over, or under, or through a 

 mass of impeding objects, and may appear for a time as if it 

 had got clear of these. Such is not the case, however. When 

 it has advanced fo some distance, the particles are suddenly ob- 

 served to be bound together, as it were, and to follow accurately in 

 the wake of the frustule, the relative positions and distance being 

 accurately maintained. Should the frustule, with its particle 

 or particles "in totv," now meet with any sudden impediment, 

 the instant it is checked in its course, so is the ])article, — 

 every jerk, turn, and movement of tlie body dragging being 

 synchronously and faithfully repeated by the particle dragged. 



