13 
Wherever animals or vegetables are crowded, disease appears. 
This is true of molluscs, and it is seldom worth while looking for 
a specimen fit for a collection where any particular kind of shell- 
fish lives in great numbers. Animal and vegetable parasites will 
be found wherever shellfish are crowded together. For instance, 
a perfect cockle, or one good enough for a collection, will not be 
found on a cockle bank, but solitary ones must be looked for else- 
where. 
