POLYZOA OF AVATEinVORKS. 429 



purpose of removing many of them. But any good they may do 

 in this way is more than counterbalanced by other inconveiJences. 

 One of the most obviovis of these is the diminution of the calibre 

 of the water-pipes, which mray take place to such an extent ns 

 seriously to interfere with the circulation. Tlius Kemna (99, 

 p. 46) mentions a case which had come under his own observation 

 of a pipe 60 cm. (about 24 inches) in diameter, which was en- 

 circled, on its inner side, by a layer of Polyzoa 15 cm. (6 inches) 

 in thickness. It has further been found by Water Engineers 

 who have been confronted with this difficulty that, as the Polyzoa 

 disintegrate, masses of the tubes are torn off from the walls of 

 the pipes and pass into the circulation. They thus pass to the 

 smaller pipes of the domestic supply, where they give endless 

 trouble by choking the ball-taps and strainers of the meters to 

 such an extent as to stop the supply of water. 



These are, however, by no means the only troubles which 

 result from the presence of Polyzoa and other organisms in the 

 water-supply. It is the habit of Freshwater Polyzoa, in these 

 latitudes at least and in the state of nature, to die towards the 

 approach of winter. There is evidence that even in the sheltered 

 conditions under which they live in water-pipes protected from 

 the weather, they are not entirely insensible to seasons ; although 

 it may be noted that Kraepelin (85, p. 12) points out that many 

 of the constituents of the pipe-fauna were found, in the depth of 

 winter, at a time when the water containing organisms living in 

 exposed places was covered with ice and snow, in a condition 

 differing but slightly from that in which he had found them in 

 the summer. But he adds tha.t the polypides of Plumatella were 

 all dead at this time (December), although in March the hiber- 

 nacula of Paludicella were splitting (as a preliminary to germina- 

 tion), and the statoblast-" embryos" of Fredericella were already 

 escaping from their shells. Owing to the death of the polypides 

 of the Polyzoa, at the approach of winter, a certain amount of 

 decomposition of organic material naturally occurs. This sets up 

 further troubles, partly by tainting the water and partly by j^ro- 

 viding material which encourages the growth of Bacteria. The 

 tainting of the watei-, which may be due to a combina.tion of 

 these two factors, is sometimes veiy serious. Thus De Yries (90, 

 p. 38) records a case of this kind in which even horses would 

 not drink the water. 



It is not my purpose to discuss the flora of the filtering- beds 

 and water-pipes — a subject of great biological interest with 

 which I do not feel myself competent to deal — but it is neces- 

 sary to refer to an illuminating study of this question which was 

 made, in 1890, by Hugo De Vries, in the memoir just referred 

 to. The town of Rotterdam is supplied by water from the Maas. 

 The waterworks were established in 1874, and at first gave no 

 trouble. In the spring of 1877 serious difficulties began to be 

 experienced as the result of a sudden invasion of " Iron- 

 Bacteria," and in paiticular of a form described by De Yries 



