Is this a roundworm? Or am I treating the wrong thing? (pics)

I noticed last night that my 12 week old kitten had a little white worm on his fur. Maybe 1/4 of an inch long. I stupidly flushed it right away instead of saving it to be identified. My sister had a full bottle of liquid dewormer. Made to treat roundworms. I gave him some of it as per the instructions. Today, I found this in his stool. What type of worm is it? It's about 4 or 5 inches long.. Also, does this mean the liquid dewormer is working?

Looks like a roundworm, just today treated my 4 month old rescue after finding similar worm in vomit.. Should see vet for a prescribed med. to weight. Vet said post treatment you will see dead worms shed in feces. Also there should be a follow up treatment  a few weeks later.