What is the function of the peristomium?

The first section behind the prostomium, called the peristomium (Greek περι- meaning "around" and στομα meaning "mouth"), is regarded by some zoologists as not a true segment, but in some polychaetes the peristomium has chetae and appendages like those of other segments.[3]

The segments develop one at a time from a growth zone just ahead of the pygidium, so that an annelid's youngest segment is just in front of the growth zone while the peristomium is the oldest. This pattern is called teloblastic growth.[3] Some groups of annelids, including all leeches,[10] have fixed maximum numbers of segments, while others add segments throughout their lives.[7]

hOpE tHiS hElP ! ! ! :))