Answer:
After an organism is buried, minerals carried by water such as silica, calcite or pyrite replace the organic material in the fossil. Some common examples are most dinosaur bones, petrified wood, and many trilobite fossils.
Hunger and thirst are considered to be internal stimuli.
Hope that helped you.
Parasitic Flatworms do not have a digestive system because they do not need to digest there food. They just suck in nutrients already digested by the host they are in.
