In leeches, acetabulum refers to the prominent posterior sucker at the extreme end of the body.

http://en.wikipedia.org/wiki/Acetabulum_(morphology)

Anelids- In leeches, acetabulum refers to the prominent posterior sucker at the extreme end of the body. In fact it forms a head-like structure, while the actual head is relatively small. It is a thick disc-shaped muscular system composed of circular, longitudinal and radial fibers.