170 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



pollution of the mussels would be very slight, and would 

 be due only to the slight amount of general pollution of 

 the sea caused by the sewage from the towns at the 

 entrance to the Menai Straits." 



These conclusions, at which Mr. Johnstone arrived 

 from an examination of the conditions on the ground 

 quite as much as from the bacteriological investigation in 

 the laboratory, illustrate the value of a personal topo- 

 graphical inspection by a competent marine biologist. 



In continuation of the experiments which I had 

 begun with Prof. Boyce, on cleansing shell-fish from 

 sewage bacteria by means of currents of water, I asked Mr. 

 Andrew Scott, in 1904, to re-investigate the matter with 

 polluted mussels at the Piel laboratory in the Barrow 

 Channel. Mr. Scott used mussels taken from a sewage 

 polluted area, and after ascertaining that they contained 

 large numbers of sewage bacteria, he exposed them to 

 currents of 1'75, 3'3 and 6 gallons of clean sea-water per 

 hour. The results of a number of experiments showed a 

 rapid diminution in the number of bacteria as measured 

 by the colonies produced in Petri dishes of neutral-red 

 bile salt agar. At the commencement of the experiment 

 the control mussels showed numbers like 1200, 1500 and 

 2000 colonies, and at the end of from 24 to 48 hours 

 numbers such as 30, 20, and 10 colonies only were found. 



In illustration of the importance of understanding 

 the tidal and other currents before collecting samples of 

 water for bacteriological examination, take the following 

 series of observations made by Mr. Johnstone. In May, 

 1908, he made comparative cultures of samples of water 

 taken from the Barrow Channel every two hours, and 

 obtained the results given in the following table, stated in 

 number of intestinal bacteria in 2 c.c. of water : — 



