Planarian
A planarian is one of many non-parasitic flatworms in the Turbellaria class. They can be found all over the world, living in both saltwater and freshwater environments. Some are terrestrial and can be found under logs, in soil, and on plants in humid areas. The digestive system consists of a mouth, pharynx and a gastrovascular cavity, which functions in both digestion and the diffusion of nutrients throughout the organism. The mouth can be found in the center of the underside of the body. Enzymes are secreted from the mouth to help digestion and the pharynx is what connects the mouth to the gastrovascular cavity, and pushes the food to the intestines. Food is not digested here, but nutrients are sent to the rest of the extremities. Planaria eat both small living and small dead animals, in which they suck with their muscular mouths.
Cestoda (i.e.Tapeworm)
Parasites of the Class Cestoda have no digestion system at all. Nutrients are thus absorbed through the outer tegument or skin when primarily attached to it’s host. When absorbing the nutrients, the tail begins to grow, creating various different segments with it’s own reproductive and digestive system. As new segments begin to grow, older segments are pushed further and further back until they reach the end in which they do not have a digestion system anymore. In this case it is probable that the ancestral groups of cestoda had digestive systems, but these have degenerated and been lost over time.
Convoluta
This is an organism in the Order Acoela (Class Turbellaria). It has a mouth and a pharynx, but no digestive cavity. Endoderm cells distribute the food from the mouth throughout the organism. This group is very primitive, the majority of them being extremely tiny at only a few millimeters in length, and live among sand grains in the sea. They behave in a very similar way to the protozoans that are also in their environment. Some may act as symbionts with other invertebrates, and a few members have symbiotic algae living beneath their epidermis.