DIATOMACE^l. 



Observations should also be made on some of the 

 DiatomacecB, 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 recognised 

 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. Try to find examples of cell- 

 division, which takes place longitudinally, and observe 

 the details of the process. 



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, which fit one inside the other like a 

 pill-box and its lid. 



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. 



The formation of auxospores may be observed in 

 these or other forms, especially in summer. Attention 

 is to be paid to the varieties of surface-marking in the 

 cell-walls of these and other Diatoms : for further 

 details reference is to be made to special treatises. 



