120 A NURSERY FOR THE MICROSCOPE. 



species are Pinnularia, Stauroneis, Pleurosigma, and 

 Navicula. The latter is especially abundant, and 

 may be seen moving about a spectral craft, with- 

 out oars or crew amid the tangled mass of living 



Fig. 79- 



Fig. 80. 



Cocconeis insida. 



Cocconeis major. 



and decomposing vegetation to be seen in a drop 

 of water taken from the bottom of the tank ! The 

 boat-like outline (whence the name of Navicula, 

 and others which some of the genera bear) is then 

 seen to be admirably adapted to move in and out 

 of such interstices. It is indeed most instructive 

 to see these diatoms going this way and that, as 

 if gifted with sense, backing in and out, turning 

 one way and another, as if they possessed volition. 

 And yet we know they are merely very lowly orga- 

 nised cells. Division takes place by one half of the 

 glassy shell, with its contents, separating from the 

 other, just as we should take the lid off a full pill-box. 



