Amphilinidea

Abstract

An order of large (several to many centimeters long) flatworms without proglottids ("unsegmented") and intestine. Eight species have been found in the body cavity of freshwater and marine fishes, and of freshwater turtles. (The Cestodaria, in which they were formerly included jointly with the Gyrocotylidea, is probably not a valid taxon.) The known life cycles (of three species) include crustaceans (Amphipoda, Decapoda) as intermediate hosts. Final hosts become infected by eating intermediate hosts. Larvae are characterized by anterior penetration glands, two separate posterior nephridiopores (external excretory structure openings), and five pairs of polymorphic posterior hooks, which are retained at the posterior end of the adult (see illustration). Adults are hermaphroditic. Testes are scattered throughout most of the body; the ovary is located near the posterior end and opens into the uterus, which runs forward to near the anterior end, turns backward toward the posterior end, and then forward again to open through an anterior uterine pore. The follicular vitellarium (the part of the female reproductive system that produces nutritive cells filled with yolk) extends along much of the body margins. A muscular proboscis is located at the anterior end, and is sometimes very weakly developed or absent.