User blog comment:MrAnonymous/Was Naraku, Onigumo?/@comment-3237614-20120417040031

Yep, what Rowan said is correct. Onigumo was in love with Kikyo but he was severely burned and could not move. He came to know about the Shikon jewel and wished to use for his greedy needs (and in turn, have Kikyo for himself). So he allowed demons to take over his body, thus becoming Naraku. Naraku is, in fact, hundreds and thousands of demons in one human body. As Onigumo wasn't dead when the demons took over, his heart is still inside the body [of Naraku]. In chapter 211, Muso reveals that Onigumo wanted to move like before and so he asked the demons to merge with him, but the demons wanted to kill Kikyo (obviously, as she is a miko). So, Naraku has one human heart and many demon souls and one body, of Onigumo.