Shelled Oysters

A Kerryman went to the doctor and complained of a severe pain in his stomach.
'Have you eaten anything unusual recently?' asked the doctor.
'I had oysters for dinner yesterday', said the Kerryman.
'Were they fresh?'
'I don't know'.
'What do you mean, you don't know?' asked the doctor,
'Surely you examined them when you removed the shells?'
'Nobody told me that you had to remove the shells', said the Kerryman.