DESMIDS AND DIATOMS. 515 
resemblance to true plants is very marked, and as they 
bear the closest analogy to the Desmids, which all ob- 
servers now admit to be vegetables, it can scarcely be 
doubted that they, too, are of the same nature. 
One of the greatest obstacles to a belief in the vegeta- 
ble nature of the Diatoms has always been the wonder- 
fully curious motions which nearly all of them exhibit in 
their living state. This is not a merely mechanical mo- 
tion, due to light or other external agents, although they 
share this property with other known plants, but they 
seem to have a certain internal principle of locomotion 
peculiar to themselves. They may constantly be seen 
swimming through the water, with a motion slow, to be 
sure, when we consider how much that motion is mag- 
nified, yet certainly as rapid as that of many undoubted 
animals among which they dwell. As a general rule 
these motions are simply backwards and forwards, any 
interposing obstacle being pushed aside, but not avoided ; 
at other times, the motion is a slow rolling from side to 
side. In one species, however (the Bacillaria paradoxa, 
(PL I3, fig. 18), so singular are the movements exhib- 
ited, and so unlike anything that occurs either in the 
animal or vegetable kingdom, that they never fail to ex- 
cite astonishment in those who, for the first time, behold 
the curious phenomenon. 
Like a lengthened ribbon, crossed by numerous close 
and parallel bands or bars, the Bacillaria frequently 
attains a length of an inch or more. Hanging from some 
green confervoid plant, or floating freely in the water, 
this fragile form, transparent and lustrous as the finest 
spun glass, is at first quite motionless. Slowly detaching 
itself, however, at one end, a strange activity will soon 
become apparent throughout its entire length. Each 
