The line in the middle of the poop looks like a long piece of tapeworm! If you google images of tapeworms, you will see they are made up of many segments strung together. They test parasites by putting them in liquid, and parasites float to the top. Tapeworm doesn't float, and so that test can't be used for tapeworm. If a vet wanted to know what it was, the vet would have to send it to a lab.