Flatworms

The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously. Flatworms are hermaphroditic (having both male and female sex organs) and they typically reproduce both sexually and asexually. 

Most freshwater flatworms are free-living and can be found in ponds, lakes, streams, ditches, and temporary puddles. They live under rocks, plants, and debris to avoid direct sunlight. They can be found on hard and soft substrates, but are more common on the hard surfaces.