4i0 DR. S. F. HAKMER ON THE 



the Metropolitan Water Board, has kindly given me some addi- 

 tional information with regard to this case. Without certifying 

 the exactness of the above recorded computation of the weight 

 of the Molluscs removed, he admits the existence of large 

 numbers of these animals in the main. He emphasizes the fact, 

 however, that no trouble of this kind has been experienced in 

 the water which has been admitted to the pipes after filtration. 

 The Molluscs are found only in the mains which are used for 

 conveying water from the Thames to the storage reservoirs, or 

 from the reservoirs to the filter-beds. The importance of filteiing 

 the water before admitting it to the pipes is admirably brought 

 out by this case, since the Molluscs in question have been unable 

 to get past the barrier of the filter-beds. 



The consideration of the above cases is sufficient to show that 

 the risks which might be caused by the entry of animal life into 

 the pipes of a water-system are not confined to any particular 

 part of the country. The cases which I have examined come 

 fi"oni Aberdeen in the north to Torquay in the south-west. It 

 is obvious that if the water is unfiltered and thus contains micro- 

 scopic organisms in sufficient quantity, the chance inti'oduction 

 of a few Polyzoon statoblasts or embiyos might be followed by a 

 jyrowth in the pipes sufficient to interfere very seriously with the 

 circulation, and ultimately to choke the taps. It is important 

 to notice, as has been so well shown by Kraepelin, that the 

 Polyzoa play a leadin^:;' part in promoting the growth of animal 

 life in the pipes. They are specially adapted by the disposition of 

 their ciliated tentacles to catch the diatoms and other microscopic 

 organisms on which they habitually feed, or, as Kemna, * has 

 expressed it, they are " microphagous." They serve in their turn 

 as the food of larger animals ; and, unless they prepared the food 

 in this way, it is hardly likely that animals such as Eels would be 

 capable of flourishing in the pipes of a water-system. The great 

 quantity of Polyzoa found at Hamburg, Torquay, and elsewhere 

 justifies the conclusion that in any discussion of this question 

 the occurrence of Polyzoa is a fact of cardinal importance. 

 Sponges are perhaps the organisms which rank next in im- 

 portance to the Pol^yzoa in this respect. 



The practical and theoretical interest of this subject has 

 received very little attention in this country. Professor S. J. 

 Hickson, in his Presidential Address to Section D of the South- 

 port Meeting of the British Association (04, p. 675), has, how- 

 ever, used Avords to wliich I can completely subscribe. He states 

 " that our ignoi^ance of the general balance of animal and vege- 

 table life in the large reservoirs is profound, and that a systematic 

 inquiry conducted by competent persons would most certainly 

 lead to knowledge which would be of great scientific importance, 

 and in the long run remunerative to the community." 



* Kemna, A., " Octacnemus, Une Ascidie ni6gopliage," Ann. Soc. roy. Zool. et 

 Malacol. de Belgique, xli. 1906, p. 57. 



