The Anyaru is the yukkuri anus. It acts as an orifice to excrete expired filling, a byproduct of their ability to turn (barely) edible food and sweets into new filling for themselves. To avoid accumulating so much filling that they explode, yukkuris naturally expel older filling, the poo poo.
As with other animals, the anyaru is located on the bottom of the yukkuri body, behind the Mamu Mamu. Due to the constant contact with both expired filling and soil, yukkuris in the wild are often prone to dangerous mold, a condition untreatable in the wild. Thus, parent yukkuris care the most about their offspring hygiene.
Whenever a yukkuri is constipated, for natural reasons or obstruction of the anyaru, it keeps growing until it explodes.