SEA-FISHERIES LABORATORY. 149 



is free, or nearly so, from sewage bacteria, depends on two 

 things, (a) The ingested bacteria are merely washed out 

 from the mantle cavities, and internal cavities, of the animals 

 by the stream of water which is continually being circulated 

 through these passages. (6) The bacteria rapidly die out 

 in a medium which they are unable to use as a source of 

 energy. 



(2) Truly intestinal bacteria are probably highly specialised 

 organisms. They are not so much saprophytes as parasites. 

 Their optimum temperature is about 37° C, that is, the 

 temperature of the interior of the body of mammals, while 

 the temperature of sea-water, and that of the interior of the 

 body of marine shellfish, varies from about 3° C. to 15° C. 

 It is doubtful whether there is any initial multiplication of the 

 bacteria on entering the shellfish — to prove that there is we 

 should have to show that the numbers of bacteria per unit 

 volume of shellfish was generally greater than the numbers 

 per unit volume of the surrounding water. The concentration 

 of bacteria in the latter varies enormously with state of tide 

 and other conditions, and the condition of greatest concentra- 

 tion would be that which ought to be compared with the 

 concentration of bacteria in the shellfish. 



(3) Faecal micro-organisms may disappear very rapidly 

 when introduced into sea-water, the destruction of 90 % 

 of the organisms originally present being effected during the 

 first two days. 



(4) Mussels may be cleansed from ingested sewage bacteria 

 by keeping them in water sterilised by the addition of chlorine. 

 A concentration of chlorine in sea- water of 5 parts per million 

 is sufficient, practically, to sterilise the water, while it does 

 not interfere with the ordinary functioning of the shellfish. 



The action of chlorine is twofold. It may simply render 

 a polluted sea-water practically free from sewage bacteria, 

 and then this sterile water washes out the ingested bacteria 

 from the alimentary canals and mantle cavities of the shellfish, 



