Peanut Worms: Sipuncula

Most peanut worms quickly withdraw their introverts when disturbed and avoid light by retreating into their burrows or rock crevices. They use their introvert hooks and muscles to pull their bodies forward. Swimming is not common and is accomplished by simply jerking the body trunk in all directions.

Many peanut worms can replace missing tentacles and introverts. Some species can regenerate portions of the digestive tract and body trunk. Others reproduce by purposely dividing their bodies. Each body part then develops all the necessary missing parts. This type of reproduction is called budding, or asexual (ay-SEK-shuh-wuhl) reproduction. Asexual reproduction does not involve mating or male or female reproductive systems.

Most peanut worms require both males and females to reproduce. Only one species has individuals with both male and female reproductive organs. Another species is able to reproduce without fertilization (FUR-teh-lih-ZAY-shun), or the combining of egg and sperm to start development. The sexes are identical to each other, and their reproductive organs are present only while they reproduce. Eggs and sperm are released into the coelom, where they are collected by the kidneylike organs and expelled into the water. Fertilization takes place outside of the body. Peanut worms develop in a variety of ways. Some species develop from eggs directly into miniature versions of the adults, while others must first go through various free-swimming larval stages before becoming young worms.