All the species are usually infested with Cercariae and Rediae, the larval forms of Trematode parasites of vertebrates.
It must, however, be borne in mind that a Trematode may develop in an "aberrant" manner in one host and "normally" in another; and unless we knew the initial stock, the two forms would be regarded as distinct species,.
The position of the Trematode on its host is of far-reaching importance.
If ectoparasitic and attached to the skin, apertures or gills, the Trematode adopts more elaborate adhesive organs and undergoes a less complex development than are required for the endoparasitic members of the class.
A different but still more interesting result is produced by these Trematode larvae on certain lamellibranchs.
In the case of the pearl oyster this parasite is a cestode larva, but in the less valuable but no less genuine pearl produced by Mytilus, &c., the nucleus is a Trematode-larva (Jameson).
Here it gives rise by a peculiar process to numerous individuals of a second larval form, and these usually produce a third form from which the minute immature Trematode is developed.
The larvae usually live in Molluscs, the mature worm in vertebrates, and the immature but metamorphosed Trematode in either host and also in pelagic and littoral marine and fresh-water invertebrates.