Gyrocotylidea

An order of flatworms, 2-3 cm (1 in.) long, in the intestine of Holocephali (the chimaeras, a subclass of the Chondrichthyes fishes). There are at least 10 species in two genera, 'Gyrocotyle' and 'Gyrocotyloides'. (The Cestodaria, in which they were formerly included along with the Amphilinidea, is probably not a valid taxon.) They lack proglottids (the segments found in tapeworms) and an intestine, and have a posterior attachment organ, the rosette (modified to form a cuplike sucker at the end of a caudal stalk in 'Gyrocotyloides') [see illustration]. Their body margins undulate. Most testes follicles are located in groups near the anterior end. The follicular vitellarium (the part of the female reproductive system that produces nutritive cells filled with yolk) is lateral, from the anterior to posterior body end. The ovary is located in the posterior third of the body. The uterus is sac-shaped or (in one species) branched. A ciliated lycophora larva about 0.3-0.5 mm (0.012-0.02 in.) long, with two separate anterior nephridiopores (external excretory structure openings) and five pairs of posterior hooks, hatches from the egg. The life cycles are unknown. Young hosts harbor many worms; large ones harbor usually no more than two. No pathological effects are known except in high-intensity infections.