POLYZOA OF WATERWORKS. 431 



which have been experienced at Liverpool. In order to increase 

 the supply of this city a dam was constructed at Vyrny, in 

 Wales. Filters were installed at Oswestry, thirty kilometres 

 from the dam ; and the water was then carried for about twice 

 that distance to Liverpool. In this latter part of the system no 

 difficulties have been experienced ; but in the part which carries 

 the water from Lake Vyrny to the filters, constant trouble has 

 been caused by the formation of a mucilaginous layer which 

 seriously reduces the calibre of the pipes. In the deposit thus 

 formed Kemna states (05, p. 78) that he has found Crenothrix, 

 one of the Iron-Bacteria. 



Although De Yries was principally concerned with the study 

 of Grenothrix, he did not restrict himself to the consideration 

 of the flora of the system. He points out (pp. 34, 35), as the 

 result of his observations, that the fauna in the Rotterdam 

 wa,ter-pipes is closely similar to that which had been found by 

 Kraepelin at Hamburg. Among the organisms thus recorded, 

 Cordylophora, Paludicella, a.n(\ two forms oi '■^ Plumatella'''' [one 

 of which may have been Fredericella] were important con- 

 stituents. Here, as at Hamburg, deficient filtration was the 

 real cause of the trouble. In parts of the system where the 

 conditions were favoui^able, owing to faulty construction of the 

 filters, some of the animals occurred in astonishing profusion. 

 I may refer specially to certain Crustacea {Asellus and Gam- 

 viarus), which were found in almost incredible numbers. This 

 is brought out sufiiciently clearly by the observation recorded on 

 p. 65 of the memoir cited, that the bottom of one of the filters 

 wus found to be coated by a continuous layer, of almost the 

 thickness of a finger, of the fseces of Asellus, while the animals 

 themselves occurred in " countless millions " ! 



De Yries' memoir gives references to other cases of water- 

 works which have at various times suffered seriously from the 

 occiurence of Iron-Bacteria or an unduly rich fauna, or both 

 combined. Among these I may mention the " Bei-lin water- 

 calamity," in 1878 onwards, which was completely cured by the 

 introduction of sand-filti^ation in 1883 (pp. 9, 14, 15, 37); 

 Philadelphia (p. 35); and Lille (p. 37). Further information 

 with regard to cases which have occiirred in America is given by 

 Whipple (10), who mentions Boston, where Polyzoa, especially 

 Fredericella and Phmiatella, were the most important constituents 

 of the fauna. Other cases recorded are Brooklyn (p. 169) and 

 Henderson. N.O. (p. 123). In addition to the genera of Polyzoa 

 which have already been mentioned as occurring in drinking- 

 water, Whipple gives Pectinatella and Gristatella. 



There is some reason to believe that the Corporation of Man- 

 chester has suffered from troubles of a similar nature to those 

 experienced at Hamburg and Rotterdam. Hickson (04, p. 675) 

 has stated that, two years befoi^e the date of his Address, the 

 mains had become partially blocked by the growth of an organism 

 which was described as a " moss," but which Hickson had the 



