OF THE DIATOME.E. OO 



length^\dse over its contiguous fmstule ; the second acts simultaneously in a 

 similar manner mth regard to the third, and so on throughout the whole 

 filament, — the same action having been going on at the same time at both 

 ends of the filament, but in opposite directions. The central frustule thus 

 appears to remain stationary, or nearly so, — while each of the others has 

 moved with a rapidity increasing with its distance from the centre, its OAvn 

 rate of movement ha\ing been increased by the addition of that of the inde- 

 pendent movement of each fmstule between it and the central one. This 

 lateral elongation of the filament continues imtil the point of contact between 

 the contiguous frustules is reduced to a veiy small portion of theii' length, 

 when the filament is again contracted by the fnistules shding back again as 

 it were over each other ; and this changed direction of movement proceeding, 

 the filament is again di'awn out until the frustules are again only shghtly in 

 contact. The direction of the movement is then again reversed, and con- 

 tinues to alternate in opposite dii'ections, the time occupied in passing from 

 the elongation in one direction to the o^^posite being generally about 45 

 seconds. If a filament while in motion be forcibly divided, the iminjured 

 frustules of each portion continue to move as before, proving that the filament 

 is a compoimd structure, notwithstanding that its frustules move in unison. 

 When the filament is elongated to its utmost extent, it is extremely rigid, and 

 requires some comparatively considerable force to bend it, the whole filament 

 moving out of the way of any obstacle rather than bending or separating at the 

 joints. A higher temperatiu'e increases the rapidity of the movement." 



To this account Prof. Smith appends these observations : — " The motion 

 here so accurately described is not essentially diff'erent from that noticeable 

 in many of the free species of Diatomeae, the pecuHarity being that it is 

 here exhibited in numerous united frustules ; when observed in a band of 

 one hundred or more frustules, the singular appearances assumed by the 

 filament under the action of so many individuals moving at one time in 

 apparent concert, and another in opposition, never fail to excite astonish- 

 ment." 



Mr. Thwaites's account conveys the impression that the movements are 

 always regular : but this is not the case ; for Mr. Ealfs tells us, by letter, 

 that both Dr. Bailey and himself have convinced themselves that they are 

 at many times irregular. 



Dr. Donkin, in his description of a new species of Bacillaria he names B. 

 cursoria (J. M. S. 1858, p. 27), has the following account of its singular move- 

 ments : — " When the filament is in a quiescent state, the frustules are all dra^Ti 

 up side by side, their extremities being all in a line, thus forming a group. 

 When a filament previously at rest resumes its activity, the movement] is 

 commenced by the second or inner fmstule at one end of the filament gliding 

 forward along the contiguous surface of t\iQ first or outer frustule until their 

 opposite extremities overlap each other. This is soon followed by a similar 

 movement of the third, fom^h, and fifth, &c., all moving forward in the same 

 direction, and each fmstule gliding along the suiface of the one preceding it, 

 imtil they have extended themselves into a lengthened filament or chain. In 

 the course of two or thi'ee seconds after this has been accomplished, a retrograde 

 movement, exactly of the same character, begins to take jDlace, and continues 

 until the filament has retraced its course, and stretched itself out in a direction 

 exactly opposite to the position it had previously occupied. This phenomenon 

 is repeated again and again ; and in this manner the whole group is kept in 

 a state of activity for an indefinite period of time ; and all the while, if no 

 impediment produces irregularity, the outer or terminal frustide, next to 

 which the movement commenced, maintains a stationary atul fixed position. 



