Peanut worms are small worms found in the sea that look like unshelled peanuts. They are called sipunculid worms and, unlike many other worms, their bodies are not divided into segments. They are generally cylindrical in shape and have an introvert, which is a segment which can be retracted into the body. Peanut worms generally stay in one spot as they have no appendages.
There are many different species of peanut worms. They consist of a cylindrical body into which the introvert can be retracted using the retractor muscle. This action works on a similar principle to when the finger of a glove is pushed out. The mouth of peanut worms is found at the end of this extension. The mouth is surrounded by a ring of tiny tentacles and they eat organic matter and detritus found in the mud in which they burrow.
Unlike many other marine animals, peanut worms do not have a respiratory system at all. They do not have a circulatory, or blood system, either. Nutrients and oxygen are transported in the body cavity, called the coelom, by the fluid that fills it, called coelomic fluid.
The intestines of the peanut worm are U-shaped, and form a loop in the body cavity. The descending part of the intestine is where digestion occurs. After the loop, in the ascending part of the intestine, fecal pellets are formed. These are expelled through the anus, which exits the body on the side of the worm.
Within the coelomic fluid are a number of tiny structures called ciliated urns. These are like tiny vacuum cleaners that ingest organic matter in the fluid. When they are filled up they are sucked up into the kidneys. After processing they are eliminated from the body.
Peanut worms generally live under rocks or in crevices between rocks in the ocean. Some may burrow into mud or sand on the ocean floor. They are generally brown or tan-colored which provides camouflage. Their strong, muscular walls keep their peanut-like shape.
These worms are not the only marine worms — there are many others which occur in all the different oceans and seas of the world. One of the most well-known marine worms is the tube worm, which builds itself, and lives in, a tube, thus protecting itself from predators. Marine worms are a source of food for many marine species.