Phylum Plathelminthes. Flatworms

Flatworms are more complex than sponges and coelenterates. Flatworms and the rest of the animal phyla are organized into three distinct layers of tissues. Tissues from these layers form the organs of a flatworm. Flatworms are the simplest worms. They have flat bodies with definite right and left sides, a head, a back end, and top and bottom sides. All parasites, including parasitic flatworms, are believed to have origin as free-living forms and to have lost certain tissues and organs, as secondary effect of their parasitic existence, while developing adaptations of advantage to the parasitic way of life.

Some flatworms live in the freshwater while others live in seawater. However, most flatworms, such as the fluke and tapeworm, are parasites. They live in or on other organisms that supply their food. Free-living flatworm does not live in or on other organisms for its food. It uses its digestive system to break down food and has sense organs in its head to help it find food. The two eyespots on the planarian’s head are sensitive to light. The bumps on the side of its head are sensitive to touch.

Flatworms make up the phylum Platyhelminthes. The phylum name is derived from two Greek words: platy, which means flat, and helminthes, which means worm. The 13,000 species of flatworms belong to three major classes. Members of the class Turbellaria are called free living, because they can live independently. Members of the class Trematoda, the flukes, and the class Cestoda, the tapeworms, are parasites. They derive nutrition from living hosts.

Planarians

The most common flatworm is the planarian, which belongs to the class of Turbellaria.Planarians live in both aquatic and terrestrial environments. Some kinds grow to be 30 cm lon, and many are brightly colored.

Planarians have a spade-shaped head and a body covered with cilia. The planarian’s mouth is in the middle of its body on the bottom side. The mouth, a tube-like pharynx and a large digestive cavity are parts of its digestive system. The planarian has a good way to eat food that is too large to fit in its mouth. It simply extends its pharynx through its mouth opening. The pharynx then draws the food inside the body to be digested in the digestive track. The planarian does not have an opening for removing wastes. Therefore, wastes must also pass through the mouth. Planarians eat dead animals or slow-moving animal organisms, including smaller planarians. Planarians move by laying down on the trail or slime from mucus-producing cells and using their cilia to propel themselves along the trail. A planarian will move away from light and toward food. These responses are made possible by the planarian’s simple nervous system.

Planarians are hermaphrodites – that is, each animal has both female and male reproductive structures. Sexual reproduction occurs by a mutual exchange of sperm. In most animals females have ovaries, which are organs that make eggs. Males have testes which make sperm. Planarians have both ovaries and testes in the same individual. Usually the sperm do not fertilize eggs from the same worm. Instead, two worms mate and exchange sperm cells. After fertilization, the eggs pass out of the body to develop into new planarians. Asexual reproduction occurs if you cut a planarian - each half would regenerate a new half.