Turbellaria - Planarian worms. Class of free living Platyhelminthes which rarely exceed an inch in length. Most of them live in water-all of them in damp places and they can be seen crawling over the mud of most ponds. Their gliding movement is caused by the beating of the cilia which cover the body wall and propel the animal along. Planarians are carnivorous, feeding on a variety of small worms and arthropods.

Small prey are taken into the gut and surrounded with digestive enzymes. The fats are directly broken down into soluble food but proteins and carbohydrates are absorbed by the cells lining the gut and digested there.

For organisms too large to be ingested whole, turbellarians have a special device. Just inside the mouth, the first part of the gut (the pharynx) is very muscular, and can actually be protruded. It becomes attached to be victim and a backward and forward pumping motion, together with the action of digestive enzymes, breaks the food into more manageable lumps which are then taken into the gut.Deprived of food for long periods, turbellarians will actually begin to digest themselves. They shrink in size as eggs, reproductive organs, muscles and parts of the gut ate consumed. On normal feeding they return to their full size, regenerating all the lost organs.

Planarians are classified according to the structure of the gut which varies from a simple sac to a highly branched system. There is a simple nervous system showing the development of a brain at the front. Most planarians have a number of simple eyes there is no lens but pigmented cells no doubt aid the animal to distinguish light and dark. There is also a well developed chemical sense that helps the animal to find food.

Typhlosole - Infolding of the dorsal wall of the gut of earthworms that increases the surface for absorption.