84 SHELLFISH 



Buchan found that a mussel of average size has a bulk of 

 about 15 c.c. but considerable variations occur with individual 

 mussels. 



When the only opinion required is as to the relative purity of 

 mussels from different sources, and not a quantitative enumera- 

 tion, the following method of Johnstone (loc. cit.) may be found 

 useful. 



The mussels are washed under the tap and opened so 

 that the adductor muscles of the shell, the pedal muscles, and 

 the muscles of the mantle, are alone cut through. The soft 

 parts of the mollusc are retained in the right-hand valve of the 

 shell. Sterilized knives are prepared, two for each shellfish. A 

 slit is made in the body of the animal immediately over the 

 stomach and through the dark green " digestive gland " really 

 an extension of the lumen of the stomach and a small quantity 

 of the stomach juices is withdrawn by a sterile pipette with 

 rubber teat (previously made in batches and drawn out to deliver 

 approximately the same volume of liquid) and distributed over 

 the surface of an L.B.A. plate. The volume of fluid taken 

 amounts to about O'l c.c. and usually a mussel contains enough 

 to make two or three separate inoculations. In this way a com- 

 parative estimate of the number of B. coli group organisms is 

 obtained. 



Cockles. These small shellfish should be treated in the same 

 way as oysters. A sample of cockles consisting of ten animals 

 should be made up to IOOC.G. One c.c. of the cockle mixture 

 then contains O'l animal. 



C. Bacteriological procedures. The number of organisms 

 developing upon gelatine and agar plates can be obtained in 

 the ordinary way but the enumeration is of no value and is 

 scarcely worth doing. 



The determinations of value are those for the ordinary ex- 

 cretal indicators, B. coli group organisms, spores of B. enteritidis 

 sporogenes and, to a less extent, streptococci. 



For B. coli Houston used cxoooi, O'ooi, 0*01, 0*1, I, 10 and 

 100 c.c. the amounts being added to bile salt broth, while for 

 B. enteritidis sporogenes the two extremes were omitted. 



