Flatworms

Scientific name : Platyhelminthes

Platyhelminthes are an animal phylum. Better known as flat worms, they are the first animals in the evolutionary tree to have a worm shape, with a head, tail, sense organ concentrated at the head, and the capability to move in a direction, looking actively for food or for a mate. Flat worms include 3 classes: Trematoda and Cestoda are all parassites of vertebrates and invertebrates. The class Turbellaria alone includes free living species, and it is the only class that we will consider here. Marine flat worms are small, uncommon and not easy to observe. Their colour patterns are nevertheless striking, often recalling the nudibranches. As the name says, those worms have a general flattened shape. On the back there are no obvious organs. Marine species have normally head tentacles, where sense organs (mainly smell and touch) are concentrated. The mouth is in the inferior part, toward the head. There is no anus, the undigested particles are rejected through the mouth. Flatworms colour patterns are often striking, like in nudibranches. Some species mimic the warning colour pattern of some nudibranches. There are unpalatable flat worms, producing or accumulating poisonous substances. Few species, feeding on corals or hydroids, can accumulate the nematocysts preserving their stinging power. Flat worms crawl on motile tiny hair (cilia). Movement in the relatively large marine polyclads is by waves of muscular contraction propagating toward the tail, like in gastropods. Few species can swim, for short distances, undulating the whole flattened body. Most species are hermaphrodites, and fecundation is often reciprocal. Some species have armed penis that can penetrate in any part of the partner's body, piercing the body wall. Sperms migrate then toward the ovaries. Eggs are benthic, adhaere to submerged objects. From eggs a swimming larva can have birth, but often a small worm is produced directly. Marine flatworms live in all the environments, but they are more common in bay, lagoon, and along the external reef. Many species are nocturnal. For the small size and the flamboyant colour, many flatwotms could be confused with nudibranches. The main difference is that nudibranches have a higher body profile, protecting many internal organs. With a simpler anatomy, flatworms have a completely flat body, sometimes thinner than a paper sheet. The majority of nudibranches have external gills, missing in flat worms, that can breath through the skin. The rhinophores (antennae) of nudibranches have a more complex and specialized structure than the flat worm's tentacles (often simply a fold at the edge of the head).

Associated organisms
Associated with hard and soft coral and with ascidians.

	Description	Author	Place	Size
	Dance	DANIELE IOP	Indonesia, Lembeh strait	507 kB