A hurdy-gurdy is a type of street organ or barrel organ. This is a mechanical musical instrument which is played by turning a handle. Inside the barrel organ is a cylinder studded with pins and staples. As this cylinder is turned, different tunes are played automatically.
Those who played barrel organs in public to earn money used to be known as organ grinders. Sometimes their barrel organs were free standing; sometimes they had to be carried. Some organ grinders had performing monkeys as an extra attraction.
Perhaps the 'extra attraction' of the little boy in Frankfurt was the tortoise.