90 TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 







showing a Triceratium Favus cut into 48 successive sections, have 

 furnished conclusive results, a summary of which I shall now give : 



1. In the simplest cases : Navicula, Pleurosigma, etc., the valve 

 consists of two plates or membranes and an intermediate grating. 



By grating is meant a plate pierced with holes like a sieve. 



The plates are sometimes but slightly united to the grating and 

 may be easily detached from it ; this may often be observed in large 

 diatoms : Coscinodiscus, etc. ; at other times, e.g., in many Navicula they 

 are so completely united to the grating that they appear to be 

 incorporated with it. 



The upper membrane may be of excessive delicacy, as is the case 

 in the Coscinodiscus, it is then easily destroyed in the treatment with 

 acids, in cleaning by rubbing, etc. 



The lower plate is simple or complex. 



It is simple when it appears in the form of a more or less thick 

 homogeneous plate. 



It is complex when it shows internal cavities within itself, and 

 appears like the valve of a Navicula. 



The lower plate of Triceratium Favus is constructed in this manner. 

 Figure 24, which is a reproduction of one of the sections of Triceratium 



, v u us 



Fig. 24. Section of Triceratium (Flogel). 



made by Dr. Flogel, which have already been described, exhibits these 

 cavities. 



Sometimes the lower plate may exhibit genuine holes, as is the case 

 in Coscinodiscus Oculus-Iridis. Perhaps in the living diatom, there may 

 exist a delicate lamina of silica over these openings, but in prepared 

 diatoms, if successfully manipulated, genuine apertures may be seen, 

 for coloured solutions may easily be injected into the alveoles formed 

 by the openings of the grating. 



The most experienced diatomists agree in believing that the membranes 

 may be sufficiently permeable to allow the valvular contents and the 

 water outside to change places by a process of endosmosis, but that 

 there is no direct communication, so long as the diatom is living and 



