Dugesia are flatworms, belonging to the invertebrate phylum Platyhelminthes. These worms are the simplest forms of animals that have three cell layers: an endoderm (gut) layer, ectoderm (skin) on the surface and mesoderm (muscle) in between. Flatworms are unsegmented, bilaterally symmetrical and worm-shaped, with a distinct head end and tail. The heads of Dugesia and other freshwater flatworms have eyespots that allow them to react to light, usually by moving away (negative phototropism) and these are connected to a simple nervous system with a head ganglion and nerve nets.
Higher worms and animals have a body cavity called a coelom. This allows the gut to move independently of the muscle tissue. Flatworms do not have a coelom. They also do not have an anus. There is only one opening at the head end and this is used for both ingesting food and excreting waste. Thus the gut is described as being a two-way gut, since food moves one way and wastes back the other way. Flatworms have no respiratory or circulatory systems. Air flows across the tissues directly and this limits how big these animals can get.
Dugesia are easily kept in the laboratory and so are suitable for study. They also make interesting pets. I had a colony in a petri dish as a teenager. They were easy to keep: give them clean water and a slice of liver to munch on and they were happy. I must admit to having performed surgery on several of them. I had read that it was possible to cut their heads down the middle and both halves would grow. I had a number of two headed flatworms in my collection. I was not a good enough surgeon to beat the record though. There was a case of someone cutting a flatworm head 18 times and growing an 18 headed worm.
As well as free living flatworms (Turbellaria), there are also two groups of parasitic flatworms: the flukes (Trematodes) and the tapeworms (Cestodes). These worms share the same basic characteristics with Dugesia: bilateral symmetry, no coelom, three cell layers, two way guts, one mouth/anal opening, but the flukes and tapeworms are specialised for parasitism in vertebrate tissues and guts. They are economically, medically and agriculturally important for the conditions that they cause. All three groups were lumped together in the Platyhelminthes but recent genetic studies indicate that they may need to be separated as they may not be descended from common ancestors.
Free-living flatworms aren't as important but they are certainly nicer looking animals. Dugesia is rather plain but its relations, the marine flatworms can be quite colourful. There are about 3000 species world-wide, found in fresh water, marine habitats and moist soils, especially in the tropics. However because they are small and economically unimportant, most people are unaware of their presence. To evolutionary biologists though, they show important steps in the evolution of multicellular animals.