Different phylum. Flatworms are Platyhelmintes characterized by not having a through-put gut and a rather simple ladder type nervous system. The one that most people is Planaria that is often used in biology class to demonstrate regeneration. (You cut it in half and the tail grows a new head - the head a new tail.) You may have found similar species on your lettuce. There are marine species, some quite large and attractive - up to 3 inches long and an inch and a half wide. I must say I have no idea if a stomatopod will eat them. Nutritionally, the marine species at least should be alright.