are segments of tape worms. They actually have a symbiotic relationship with cats so not to worry. I remember a few years ago, friend who had treated a foster cat one dose or so, a couple days later, the entire tape worm was thrown up. She could see the mouth where it attaches to the inside of the intestine! (We were chatting on the phone when she found it and she suddenly screamed, "Oh my f....ing god!") when she discovered it. She was frantic in her description of the disgusting thing!
Quite unusual to see because, well, one just rarely gets the opportunity!