In flatworms, Acetabulum is the ventral sucker situated towards the anterior part of the Body.

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

 Trematode-
In flatworms, acetabulum is the ventral sucker situated towards the anterior part of the body, but behind the anterior oral sucker. It is composed of numerous spines for penetrating and gripping the host tissue. The location and structure of the acetabulum, and the pattern of the spine alignment are important diagnostic tool among trematode species.