LinrKpan Society. SJOl 



probably slightly blackish, and also in small quantity in the river 

 Frome at Stapleton, encrusting various aquatic plants with a dark 

 brown coating, which under the microscope is resolved into a num- 

 ber of pale brown filaments that seem to adhere tolerably firmly to the 

 plant on which they are situated. When they have been for a few mo- 

 ments detached, a remarkable motion is seen to commence in them. 

 The first indication of this consists in a slight movement of a terminal 

 frustule, which begins to slide lengthwise over its contiguous frus- 

 tule, the second acts simultaneously in a similar manner with 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 ap- 

 pears 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 movement having been increased by the addition of 

 that of the independent 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 re- 

 duced 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. In the course of this movement the 

 filaments seldom resume their original Fragilaria-\i]s.Q appearance ; 

 and there are occasional interruptions to its regularity, both the ter- 

 minal frustules in some cases moving in one and the same direction 

 instead of in a direction opposite to each other. This Mr. Thwaites 

 regards as resulting from a breach in the vital or dynamical con- 

 nexion of the filament, and as not improbably indicating the place 

 where spontaneous division of the filament is about to occur. If a 

 filament, while in motion, be forcibly divided, the uninjured frustules 

 of each portion continue to move as before, proving (as the author 

 believes) 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 still extremely rigid and requires some com- 

 paratively 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 author hazards a conjecture that the action of cilia is the 

 proximate cause of the phaenomenon ; for, although he has been un- 

 able to discover cilia, he has little doubt of their presence from the 

 mode in which minute particles of indigo suspended in the water 

 were acted upon, when coming into contact with the frustules. He 

 regards the movement of each individual frustule, considered alone, 

 as closely resembling that which is seen in the detached frustules of 

 other species of Diatomacece ; namely, a so to speak alternate back- 



