Worm

Round Worm
A round worm is a type of enemy which will periodically burrow into the ground and pop up in different part of the room. If Isaac is in their line of sight when that happens, it will spit a blood shot in his direction.

Roundy
Like the Round Worm, a Roundy will burrow into the ground and appear somewhere else. When it surfaces, it releases a small rock wave around it and fires an shot towards Isaac.
 * If multiple Roundy's surface in the radius of their rock wave, they will kill each other.

Ulcer
Like the Round Worm, the Ulcer will burrow into the ground and appear somewhere else. When it surfaces, if Isaac is within its line of sight, it will spawn a Dart Fly, but if there are 6 of them in the current room, it will start firing a blood shot instead. Ulcers burrow underground at a slightly slower rate than most other Worms, usually making them easier to attack.

Night Crawler
Like the Round Worm, the Night Crawler will burrow into the ground and appear somewhere else. When Isaac is within their line of sight, they will fire four short-range projectiles that spiral outwards.

Tube Worm
Tube Worms were added in Booster Pack #4.

Like the Round Worm, the Tube Worm will burrow into the ground and appear somewhere else. Upon popping out of the ground, they will fire a long-ranged arcing spray of shots at Isaac, attempting to intercept Isaac's future position based on his movement. Unlike standard Round Worms, they can fire at Isaac even if they do not have a line of sight.

Fire Worm
Fire Worms were introduced in The Binding of Isaac: Repentance.

Like the Round Worm, the Fire Worm will burrow into the ground and appear somewhere else. If Isaac is in their line of sight, they will fire a single flame at him. They can only appear in lava-filled gaps.

Tainted Round Worm
Tainted Round Worms are much stronger and tankier versions of Round Worms. Like Round Worms, they will pop out of random locations in the floor, but the Tainted Round Worm does so alongside multiple tail ends. The head and all the tail ends will shoot Isaac together when he is in sight; while the tail ends each shoot a small blood shot, the head spits a large blood shot which will split into 8 blood shots upon obstacle collision.

They are only found on the Ascent to.

Tainted Tube Worm
Tainted Tube Worms are a stronger variant of Tube Worms. After popping up in a random location, they spit 4 spread-out shots in the direction of Isaac.

They are only found in during the Ascent.

Trivia

 * Round Worms are parasitic creatures in real life that can be found inside cats or dogs.
 * Nightcrawlers in real life could refer to earthworms, although they don't look similar to the Nightcrawlers from The Binding of Isaac.
 * Ulcers are a reference to ulcers, breaks in bodily membranes caused by sloughing out of inflamed necrotic tissue.
 * Tube worms are creatures that live underwater and anchor their tails underground, which may be where the Tube Worm from The Binding of Isaac got its name from. However, they don't resemble real-life tube worms.
 * If a Tube Worm is spawned in a floor other than the Flooded Caves and Downpour, it will have a more Round Worm-like appearance with white skin (this can be seen naturally in Dross).
 * Same thing occurs with their tainted variant.