Another major group of trematodes that is found in the small intestine of cats is the Heterophyids. This family of trematodes is characterized by a very small body in the adult fluke and a fusion of the male and female reproductive tracts just prior to exit from the body to form a hermaphroditic duct. There are many genera within this family, and numerous species have been reported from the cat. Cats become infected by the ingestion of fish or sometimes amphibia that contain the metacercariae. The fish are infected by a swimming cercarial stage that typically has eyespots, a long tail, and which penetrates the skin of the fish host. The adults of this trematode family seem to be able to develop in a number of different hosts, both birds and mammals, and thus the same species is often found in several different host species as an adult.
There are 10 subfamilies of heterophyids represented in cats. The differences are mainly morphologic The first, the Apophallinae, are characterized by having their vitellaria distributed throughout the posterior part of the body and no spines on the oral sucker. The Second subfamily, the Ascocotylinae, has an oral sucker that has a cone-shaped extension into the esophagus of the adult fluke and the presence of a circumoral crown of spines. This subfamily, according to Soganderes-Bernal and Lumsden (1963) is represented by a single genus Ascocotyle; this single genus incorporates many of the parasites of the cats that were originally reported under the generic names Parascocotyle and Phagicola. The third subfamily, the Centrocestinae, is characterized by a small circle of spines around the oral sucker that has no esophageal extension. The Cryptocotylinae are much like the Centrocestinae, but there are no spines around the mouth. The Euryhelminthinae are characterized by the possession of bodies that are wider than they are long. The Galactosominae are characterized by having an elongate body, the uterus extending all the way to the posterior of the body, and by relatively large oral suckers. The Haplorchiinae are identified by their possession of a single testis. The Heterophyinae are characterized by their possession of an extra ventral sucker that is next to the ventral sucker and which is used in copulation. The Metagoniminae are characterized by their having an oral sucker that is rather small and subterminal and with the genital opening and ventral sucker often being off the midline of the body. The tenth group, the Stictodoriinae, is characterized by having a subterminal oral sucker and the ventral sucker is a non-suctorial organ with numerous spines that project into the genital opening.