BRITISH DIATOMACE.E. V 



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 own rate of move- 

 ment having been increased by the addition of that of the independ- 

 ent movement of each frustule between it and the central one. 

 This lateral elongation of the filament continues until the point of 

 contact between the contiguous frustules is reduced to a very small 

 portion of their length, when the filament is again contracted by the 

 frustules sliding back again as it were over each other; and this 

 changed direction of movement proceeding, the filament is again 

 drawn out until the frustules are again only slightly in contact. The 

 direction of the movement is then again reversed, and continues to 

 alternate in opposite directions, the time occupied in passing from 

 the elongation in one direction to the opposite being generally about 

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

 uninjured frustules of each portion continue to move as before, 

 proving that the filament is a compound structure, notwithstanding 

 that its frustules move in unison. When the filament is elongated to 

 its utmost extent, it is extremely rigid, and requires some compara- 

 tively 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 temperature increases the rapidity of the move- 

 ment." 



The motion here so accurately described is not essentially different 

 from that noticeable in many of the free species of Diatomaceas, the 

 peculiarity 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 astonishment. 



The disposition of the coil in Bacillaria is the opposite of that in 

 the last genus. In Bacillaria the curve takes an upward and inward 

 direction, forming a roll as made by a sheet of paper, and not a 

 spiral as is seen in the turns of a corkscrew. 



The form arises in both cases from the unusual development of the 

 valves, in Meridion the growth being greatest at one extremity of the 



