Britannica Sources used in this answer

Why do leeches lack setae?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

annelid *The structure of polychaetes.* (Left) Free-moving polychaetes. (A) *Neanthes,* (B) *Nereis.* (Right) Tube-dwelling (sedentary) polychaetes. (C) *Amphitrite,* (D) *Sabella.*

Leeches (subclass Hirudinea) generally lack setae, which are the chitinous, bristle-like structures typically found in other annelids such as polychaetes and oligochaetes.123

While most leeches are characterized by the absence of these bristles, there is a notable exception: the primitive order Acanthobdellida, which possesses setae on its five anterior segments. For the vast majority of other leeches, locomotion is achieved through the use of anterior and posterior suckers rather than the anchoring mechanism provided by setae in other segmented worms.24