Sources : Gudgeon

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.41): The gobio [gudgeon] is a small fish, as he says [i.e. Liber rerum], and although it is small, where it has waters with sandy bottoms and pebbles to feed in, it finds more suitable food. Some say that this little fish, in hot summer days, has some white worms in its belly, and that it is corrupted by their ugliness. The shape of this fish is almost round with small silvery scales. - [Badke translation/paraphrase]