' Navicula serians^ N. rhomboides, and Pinnularia gibba. 169 



extent, as it is probably composed of a coalescence of pseudo- 

 podia extruded, through apertures of the frustule, from the 

 sarcode internally, to meet the requirements of the organism ; 

 but its existence may be inferred from an attentive observation 

 for a few moments of a living active Diatom, when the following 

 phenomena may be witnessed : — (1) the power of attaching to 

 itself, retaining, and casting off a particle of foreign matter, 

 which particle may be transported over its surface, hither and 

 thither, backwards and forwards, from one end of the frustule 

 to the other, retarded in its course, arrested, returned, and again 

 advanced forward, more or less quivering on its way, indeed just 

 as a granule or oil-globule in cyclosis is seen to be carried about 

 by the currents of protoplasm in Closterium, in the threads sus- 

 pending the nucleus in Spirogyray &c., in the pseudopodia of 

 the Rhizopoda, in the typical mass of actively moving protoplasm 

 of yEihalium, and in the threads which flow from the nucleus 

 in the interior of some of the Diatomese themselves (ex. gr. >S^m- 

 rirella). (2) The locomotive power of the Diatom itself, which 

 is of the same order of movements. (3) The presence of pseudo- 

 podial prolongations from this layer, as evinced by the sudden 

 jerk which a body previously attached to the Diatom often pre- 

 sents just after the Diatom appears to have cast off its union 

 with, and is already some, little distance from it. 



Here it should be remembered that our magnifying powers 

 still, and perhaps ever will, fall far short of demonstrating the 

 slowest locomotion and change of form, the most transparent 

 structures, and the mode of formation of parts in the Infusoria, 

 which, in higher and more visible beings, can be seen to be the 

 product of organs specialized for the purpose. The leg of a 

 Plcesconia has, in all probability, as complicated a locomotive 

 apparatus as that of a large Crustacean ; and yet it is as trans- 

 parent and appears to be as structureless as glass under the 

 microscope. Many instances of this occur in Infusoria which 

 are almost large enough to be seen with the unassisted eye; 

 and therefore when we observe the phenomena in the Diatomese 

 to which I have alluded, it seems better to let the inferential 

 explanation come from analogy with vital than with physical or 

 chemical phenomena only. 



Viewing, then, the movements, &c., of the Diatomese as de- 

 pendent on the presence of an external sarcode, together with 

 their organology internally, there is no class of living beings to 

 which they are so nearly allied as the Rhizopoda, on the animal, 

 and the Desmidiese on the vegetable side of the imaginary line 

 of demarcation which is supposed to separate these two king- 

 doms. All the elements which enter into the composition of a 

 Diatom enter into that of certain species of freshwater Rhizo- 



