Occasionally , a parasitic flatworm ( Ribeiroia ondatrae ) digs into the rear of a tadpole , causing a rearrangement of the limb bud cells .
The frog develops an extra leg or two .
Occasionally , a parasitic flatworm ( Ribeiroia ondatrae ) digs into the rear of a tadpole , causing a rearrangement of the limb bud cells and the frog develops an extra leg or two .