That is a tapeworm.
It is a very common parasite of both warm and cold blooded animals.
They CAN grow to be 20 feet or more long in humans!!!
They reproduce by allowing segments of themselves that contain something akin to a fertilized egg to drop off.
In Victorian times, women would purposely swallow a single tapeworm (swallow 2 and you get babies, ycuk!) to lose weight.
Another worm parasite is the nematode. There are TONS of species of thse little things, and they are much more painful to host than a tapeworm, which is pretty much painless. NEmatodes can bore through tissues and be very dangerous. You can often find them in the flesh of fish. I used to find them in Pacific cod all the time, and tapeworms too actually, but the tapeworms are confined to the gut.
Anyway that's all I've got, lol.