Earworm looks like a light green cartooned worm and has an elderly appearance. It sits in a rocking chair when it listens to its music. It is very similar to Tapeworm from the episode, "Pest of the West." It wore a cowboy hat that Sandy put on it. When it was traveling from SpoongeBob to Squidward it also had a hat on, along with a suitcase.
Sandy is the first to realize SpongeBob has it and says that Earworms can be dangerous. Earworms are little worms that make themselves at home in one's brain (in the episode, it was SpongeBob's brain) and plays a recording of the song over and over, and enjoys it (in the episode, the song was Musical Doodle). The only way to get rid of it/them is to lure it/them out with "another catchy tune." Mr. Krabs and Sandy attempted to help SpongeBob get rid of the Earworm (Mr. Krabs sang High Seas Chant and Sandy sang Life on the Prairie), but they fail. Squidward actually played a catchy song on his clarinet and it leaves Spongebob and enters Squidward. The Earworm got into SpongeBob by simply crawling through one of his holes. It enters Squidward by crawling underneath his eyelids.
Earworm may be a parody of tape-worms.
Earworm is very much like the Tapeworm from the episode "Pest of the West," except it is smaller, green and has no stripes and has wrinkles.
After fleeing from SpongeBob's head, it crawled into Squidward's head as a result of him creating a catchy song on his clarinet.