EVERYDAY SCIENCE

Detailed explanation-1: -Flatworms have no circulatory system. Animals without a circulatory system have limited abilities to deliver oxygen and nutrients to their body cells because of the way that molecules behave. As molecules spread through water, they become less concentrated as they move away from their source.

Detailed explanation-2: -Planarians have no apparent circulatory or respiratory systems; however, they do possess muscles which line the inner wall of the body in longitudinal, diagonal, and circular orientations. The mesenchyme of the planarian body is populated with numerous undifferentiated pluripotent cells, known as neoblasts.

Detailed explanation-3: -The Platyhelminthes are acoelomate flatworms: their bodies are solid between the outer surface and the cavity of the digestive system. Most flatworms have a gastrovascular cavity rather than a complete digestive system; the same cavity used to bring in food is used to expel waste materials.

Detailed explanation-4: -In animals, there are two types of circulatory systems: open and closed.