Slime moulds are a type of primitive organisms with some really interesting behaviors. For example, if there in a good supply of food, the Dictyostelids will live as individual amoebae, but if food is in short supply they will join together to form multi-cellular assemblies. When there is little food the amoebae will produce and release a chemical. They will also move toward higher concentrations of this chemical (Chemotaxis), and in this way the amoebae will be able to find each other and form multi-cellular assemblies.
In the java simulation below the amoebae release a signal chemical as they move. The concentration of the chemical is displayed in color from black (lowest concentration) through red and yellow to white (highest concentration). The chemical will diffuse and also decay at an adjustable rate. The amoebae will tend to turn toward the the highest concentration of the chemical. This will allow the amoebae to form various structures with shapes that depend on the decay rate and density of amoebae.