FOOD 



127 



479. Blanket- weed 



1, Anabsena flos-aquse. 2, Coelosphierium 

 Kutzingianum, with detached cells. 



Stagnant pools undergo a considerable amount of purification owing to 

 the presence of living vegetation, particularly when the plants belong to 

 the flowering order. It can be observed, 

 even in so small a space as that furnished 

 by an ordinary aquarium, that bubbles of 

 gas are constantly being emitted from 

 growing water plants or weeds, as they 

 are called. In addition to the action 

 of plants in furnishing oxygen, aquatic 

 animals also contribute very largely 

 towards the destruction of organic bodies. 

 Myriads of minute creatures belonging 

 to the infusoria spend their lives in the 

 assimilation of organic substances; in 

 fact, the presence of these animalculge 

 and plants may be accepted as a proof 

 that the water is capable of supporting 

 animal and plant life. On the other 

 hand, however, it has been observed by 



Bennett, Rafter, and other writers referred to by the author of Water 

 in Relation to Health and Disease, that there are numerous living beings 

 contained in water which tend rather to add 

 to its pollution than to remove it. It would 

 appear that nearly all the varieties of aquatic 

 fungi derive their sustenance from decompos- 

 ing substances, and their presence in water is 

 proof in itself of the existence of septic material. 

 Such organisms as live on decomposing bodies 

 are described as saprophytes. The sewage 

 fungus (fig. 478) is a notable instance of an 

 organism of the class referred to, and it is 

 most easily recognized in the description which 

 is given of it as "a dirty-looking, jelly-like 

 layer covering the bottom and sides of the 

 water-course in which it occurs ". Its presence 

 may always be taken as proof of the existence 

 of sewage contamination. 



Besides the various fungi which are found 

 in streams, and water-courses, and drain -pipes, there are numerous algae, 

 of which the blanket- weed (fig. 479) is a well-known example. Mr. 

 Bennett describes algse in two distinct forms, the blue-green alga (fig. 480), 



Fig. 480. Blue-green Algae 



1, Spirulina Jenneri. 2, Oscillaria 

 insignis. 



