The Caryophyllideans are a group of tapeworms that infect fish and annelids (segmented worms) with a simple scolex or "head." Worms in this order only have one proglottid, which is believed to be the primitive condition for tapeworms. They are generally less than 10 centimetres long.
In the Caryophyllidean life cycle, adults live in fish, who pass the tapeworm eggs in their feces. Annelids eat the eggs, where they hatch into oncospherelarvae that move through intestinal wall and become procercoid larvae in the body cavity. When the annelid is eaten by a fish, the tapeworm matures.