Turbellarians: Turbellaria

Some species of turbellarians secrete mucus that may contain poisonous or narcotic chemicals that slow or entangle prey. Turbellarians use a number of behaviors that prevent them from straying beyond their normal habitats and to keep themselves adjusted to their surroundings. For example, most turbellarians move toward something touching their belly and away from something touching their back. This ability allows bottom-dwelling forms to keep their bottom side down. Freely swimming turbellarians have special sense organs for adjusting themselves to gravity. Most species move away from light. This characteristic prevents the worms from coming out in the daylight, when water-dwelling species may be eaten and land-dwelling species may dry out.

All turbellarians have a strong sense of smell that can be used to find food and mates. Some of these worms swing their head back and forth to help determine the proper direction of the food source. Others use trial and error to determine the proper direction to find food. They move in one direction until the signal becomes weaker and then continue switching direction until the signal is strongest.

Turbellarians use both asexual and sexual reproduction. Asexual (ay-SEK-shuh-wuhl) means without and sexual means with the uniting of egg and sperm for the transfer of DNA from two parents. Many species divide asexually by splitting in two from side to side behind the mouth, and each part generates the rest of a body. The rear portion attaches to the material on which the worm lives, and the front portion crawls away. In some species the cells vary in their ability to regrow. The cells in the middle of the body have the strongest ability to regrow. If just the tail is cut off, it will not grow a new body, whereas the main portion of the body will grow a new tail. In some species several crosswise breaks develop that lead to a train of individuals that do not detach until they reach a certain stage. Other species detach fragments that form capsules and eventually develop into new individuals.

Individual turbellarians make both sperm and eggs, and their sexual reproductive systems are quite complicated. Fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development, usually occurs when worms align themselves with each other, and the penis of each worm is inserted into the female opening of the other worm and deposits sperm. The worms then go their separate ways with the sperm stored inside. Turbellarians either are born resembling adults and then grow to maturity or produce freely swimming larvae (LAR-vee), or animals in an early stage that change form before becoming adults.