ANIMAL NATURE OF DIATOMEjE. 371 



Ehrenberg discovered, in some Naviculae, a distinct foot, 

 similar to that of Gasteropod molluscs, projecting from 

 the median aperture and from the fissure in the inferior 

 valve. In some Surirella, again, he observed extensible 

 and contractile cilia protruding and retracting through 

 numerous perforations in the margin. Certainly such 

 organs are not to be compared with the vibratile cilia 

 recently discovered on the surface of the vegetable beings 

 before mentioned. But (independently of the presence of 

 these organs, which hitherto have only been seen by 

 Ehrenberg and Corda) the motions of Diatomese, as 

 admirably described by Kiitzing himself, are so different 

 from the motion of Oscillatoriese, Desmidiese, Proto- 

 coccoidese, and the spores of other Algse, that they must 

 certainly be referred to a different origin. Careful experi- 

 ments may exchide all exterior causes, from the mediate 

 impulse of other bodies, from evaporation, chemical and 

 other agency. There still remains admissible the sup- 

 position of currents produced by the continued inter- 

 change between the exterior and interior liquid. But, 

 with equal right we may admit the other supposition, 

 that in a being whose nature, for other reasons, we 

 believe to be animal, the movements may be effected by 

 the admirable vital powers through organs which escape 

 our sight by their minuteness. I add that, even in the 

 smallest Naviculse, observed with high magnifying 

 powers, there is seen, during their motions, an agitation 

 and a kind of sparkling, or, to speak more scientifically, 

 a rapid and indeterminate change in refraction of light at 

 their extremities, precisely in the situation of apertures 

 existing there in the shield. Hence I venture to infer 

 that though we cannot draw an absolute proof in support 

 of the animal nature of these beings from their move- 

 ments, it is still right to bestow particular attention on 

 these facts. 



Resuming what has been hitherto said, and comparing 

 the arguments which seem to indicate the vegetable 

 nature of Diatomese with those which favour their animal 



