12 



POND LIFE 



seemingly inert diamond or oval-shaped objects, as if prompted 

 by some unseen force, glide out of the field of view, and should 

 they meet with an obstacle, such as another of the same 

 species, the encumbrance and the visitors will often move 

 together, side by side, until one or other alters its direction. 

 Some species of diatoms live in colonies, a very interesting 



7. 



DIATOMS 



1. Pleurosigmaattenuatum. 2. Navicula cuspidata. 3. Diatoma vulgare. 

 4. Arachnoidiscua. 5. Triceratium. 6. Gomphonema geminatum. 

 7. Heliopelta. 



example being Diatoma vulgare. The members of this colony 

 form a chain, whilst the diatoms Asterionella formosa join 

 together so as to represent the " spokes of a wheel." 



Another group of great interest is the stalked diatoms, a 

 good example being G. geminatum, which reminds one of a 

 wedge fastened on to a stalk by the narrow end. 



The wedge secretes a thin stream of gelatinous substance, 

 which soon becomes solid, and acts as the stalk of the organism. 



