Monogenoidea

a class of parasitic flatworms (Platyhelminthes). In contrast to Trematoda, development in Monogenoidea is direct. The body is bilaterally symmetrical, flattened, and usually elongated (less frequently almost round). The posterior end has an attachment disk with chitinous hooks or valves that act as grips, muscular suckers, or a combination of these formations; often the disk itself is converted into a powerful sucker.

The Monogenoidea are usually no longer than 1 mm (less frequently 40–50 mm; for example, Callorhynchicola branchialis—a parasite of Chimaerae). The body is colorless or slightly pigmented by translucent internal organs, mainly the intestinal tract. The outer covering is a cuticle. Monogenoidea are acoelomate worms: the space between the integument and the gut is filled with connective tissue—parenchyma. The digestive system is well developed; the mouth is located at the anterior end of the body or on the ventral side; there is a pharynx and two branches of the gut. There are no circulatory or respiratory organs.

The excretory system is protonephridial and consists of two lateral openings. The nervous system comprises a large cerebral ganglion and two longitudinal nerve cords; the attachment disk usually has an annular nerve commissure with several ganglia. The sense organs are tactile papillae and one or two pairs of eyes at the anterior end of the body. These flatworms are generally oviparous hermaphrodites; less frequently they are ovoviviparous (species of the genus Loimos) or viviparous (species of the family Gyrodactylidae).

There are about 2,000 species of Monogenoidea. They are parasites of fish and, less frequently, of amphibians or reptiles (turtles); one species—Oculotrema hippopotami—parasitizes the eyes of hippopotamuses. The majority of Monogenoidea have a complex developmental cycle, which is adapted to the characteristics of the host. For example, the frog parasite Polystoma infects its host only in the tadpole stage, attaching itself at first to the gills; when the tadpole metamorphoses into a frog and the gills atrophy, the parasite larva goes through the gut and penetrates the urinary bladder, where it attains sexual maturity in three or four years. Monogenoidea generally live on the gills or skin of their hosts; less frequently are they endoparasites (Acolpenteron parasitizes the ureters off ish). Some Monogenoidea are destructive to fish-breeding in ponds, causing epizootic disease (Dactylogyrus vastator kills young carp).

The distribution of Monogenoidea is connected predominantly with the geographic distribution of their hosts. The class Monogenoidea is divided into the two subclasses Polyonchoinea and Oligonchoinea, which differ in the structures of their attachment apparatus, their genital systems, and their cycles of development. Monogenoidea are descended from Turbellaria.