310 THE MICROSCOPE. 



motionless. Another striking motion is perceptible in 

 these particles when they come in contact with the sutures 

 of the siliceous shield, being forced rapidly up and down 

 upon it. Those particles which are propelled from the 

 terminal towards the two central eminences, are never 

 observed to pass beyond the latter ; at this point there is 

 always a quiet space, from which the particles are again 

 repelled towards the extremities. This proves that the 

 linear sutures do not extend over the central eminences 

 of the shield. At these clefts the current is sometimes 

 so strong that comparatively large bodies are set in motion 

 by it." 



M. Kiitzing believes that every diatomean is formed 

 of a siliceous shield, and a soft substance, sarcode ; the 

 shield consists of pure silica, or in some cases, perhaps, 

 of silica combined with alumina. Nageli thinks the silica 

 is deposited in the outside organic membrane. In fact, an 

 organic membrane ought to exist ; for the silica could 

 not become solid, except by crystallising or depositing 

 itself on some pre-existing substance. Nageli believes that 

 it is also deposited externally ; for in many genera, and 

 especially in the Achnanthidia, the siliceous shield is 

 covered with a very delicate dilatable membrane, itself 

 containing silica, as is proved by its sustaining unchanged 

 the action of fire and acids. Therefore, comparing this 

 shield with other organic formations, whether animal or 

 vegetable, containing, in like manner, either silica or some 

 other so-called mineral element, we might reasonably 

 consider it to be formed of an organic tissue permeated 

 with silica. 



"Comparing," says Kiitzing, "the arguments which 

 seem to indicate the vegetable nature of Diatomacece with 

 those which favour their animal nature, we are of neces- 

 sity led to the latter opinion. If we suppose them to be 

 plants, we must admit every frustule, every Navicula, to be 

 a cell. We must suppose this cell with walls penetrated 

 by silica, developed within another cell of a different 

 nature, at least in every case where there is a distinct 

 peduncle, or investing tube. In this siliceous wall we 

 must recognise a complication certainly unequalled in the 

 vegetable kingdom. It would still remain to be proved 



