Platyhelminthes

__**Platyhelminthes**__ Flatworms are invertebrates with soft bodies and bilateral symmetry. They have three cell layers. The outer layer is the ectoderm, the middle layer is the mesoderm, and the inner layer is the endoderm. The three layers of tissues are organized into organs and organ systems. The roundworms have a definite front end (head) and back end (tail). They have a top side (dorsal) and a bottom side (ventral). They live in many different environments. Some are free-living and don not require a host organism to obtain food and a place to live. Others are parasitic and do require a host in order to live.

Flatworms have flattened bodies, and they are the simplest worms. They are members of the phylum Platyhelminthes and include three groups. One group lives in ponds and streams; they are free-living and the most common flatworms, the planarians. Planarians do not depend on another organism or host to survive. The planarian is one step up on the ladder of complexity from the cnidarians. They have specialized organs that are organized into simple systems. They have only one body opening for food to come in and wastes to leave. Planarians are a little more advanced, however, in that they have nerve organs called eyespots on their triangular shaped heads for detecting and reacting to light. The eyespots are found on the dorsal anterior head. The body of the planaria is covered with cilia or little hairs that help the worm move along the mucus path that it lays down. Planarian eat small organisms and the dead bodies of larger organisms. They are both predators and scavengers. They live in freshwater or moist habitats under rocks and plants. Planarians can reproduce asexually if they are cut in two. Each part will grow into a new flatworm. There are some free-living flatworms that live in marine environments.

The other two groups of flatworms are parasites that require a host in order to live. They feed on and usually cause harm to the host. The tapeworm and the fluke are both parasites. They have special body parts, hooks and suckers, that help them live inside another organism. Tapeworms do not have a mouth or digestive system. The tapeworm lives in the intestine of almost every kind of animal. In the intestine, they absorb food that is already digested by the host.