CHICAGO — Northwestern University acknowledged Wednesday that an unusual demonstration was held on campus last week in which students observed a woman being penetrated by a sex toy.

The university will pay the man who arranged for the demonstration, which was held in front of about 100 students attending psychology professor J. Michael Bailey’s human sexuality class. The demonstration occurred after class and was optional.

Bailey occasionally holds lectures, demonstrations and other activities after class, and on Feb. 21, he invited Ken Melvoin-Berg, a Chicago tour guide, to discuss human sexuality, fetishes and female orgasm.

After an initial discussion at Ryan Auditorium, the class was told that a couple were going to demonstrate the use of a sex toy. “”Both Professor Bailey and myself gave them five or six warnings about what was about to happen and it would be graphic,”” Melvoin-Berg told The Chicago Tribune.

The woman undressed and got on stage with her male partner, who then penetrated the woman with the device.

“”It was a fun and educational experience,”” Melvoin-Berg said. The students, he said, “”seemed to be incredibly pleased. We had a number of them that got closer and closer.””

Melvoin-Berg said he typically gets paid $300 to $500 for an hour-long lecture, but he declined to say how much Northwestern has agreed to pay him. Spokesman Al Cubbage confirmed that the university has agreed to pay.

There are 567 students registered for Bailey’s class, Cubbage said. Bailey did not return calls or e-mails seeking comment, and Cubbage said Bailey told him he would not comment.

Northwestern issued the following statement: “”Northwestern University faculty members engage in teaching and research on a wide variety of topics, some of them controversial and at the leading edge of their respective disciplines.

“”The university supports the efforts of its faculty to further the advancement of knowledge.””