DIATOMACE.E 449 



DIATOMACE^E 



Observations should also be made on some of the 

 Diatomacece, a family of somewhat doubtful affinity, 

 and remarkable for the variety and beauty of form of 

 its members. 



Almost any sediment from a fresh- water pool, 

 mounted on a slide and examined under a high power, 

 will show specimens of Navicula : it is to be recognized 

 as a unicellular organism pointed at both ends, and 

 showing active movement. Note the yellowish brown 

 colour characteristic of Diatoms, and observe carefully 

 that within the cell-wall there is a basis of colourless 

 protoplasm, in which are embedded a nucleus, and a 

 yellowish brown coloured chromatophore, of variable 

 shape and position. 



Specimens of Pinnularia, which is also a common 

 form, should be observed, since in it the peculiar 

 structure of the cell-wall is to be seen : it is composed 

 of two halves, tests, which fit one inside the other like 

 a pill-box and its lid : this structure is characteristic of 

 the Diatoms, and evidence of it may be found in the 

 various forms examined : but it is only under high 

 powers that the peculiarity can be clearly made out. 

 Compare the structures above noted in the Desmids. 



A third form, Gomphonema, is to be found attached 

 by a thin transparent stalk to the surface of almost any 

 submerged plant : in this the characters above noted 

 may be again seen. 



Attention is to be paid to the varieties of surface- 

 marking in the cell-walls of these and other Diatoms : 



G G 



