The only parasitic worms that have seem to have predictably reliable shapes are those appearing in old laboratory photos in textbooks, etc. In real life, inside real mammalian hosts, they have been known to have become shapeshifters from hell. Many papers have been written over the past decade or two describing their newly discovered phenotypic plasticities and polymorphisms, which can really make identification difficult from specimens that have spent time inside hosts and expelled via various chemicals taken to try to get rid of them, which can damage and distort what they look like too.