Trematoda is a class of parasitic worms. Trematoda is one of three classes from the phylum Platyhelminthes, the other two being Cestoda and Monogenea. Trematodes have also been called flukes due to their flat, oval shaped bodies. Trematodes are estimated to include between 18,000 - 24,000 species of parasitic worms. Nearly all of these worms infect mollusks and vertebrates, with a select few that infect turtles, fish, and solely mollusks.
The eggs of Trematoda are usually ingested by the host. The eggs hatch into miracidia, or a motile form of the parasite that is cilia covered and settles in the host to create sporocysts. The sporocyst is a sac that can either create more sporocysts or rediae. The rediae is the larval form with an oral sucker that can then produce more rediae or cercariae. Then free swimming parasitic larvae (cercariae) hatches from the eggs that were ingested by the host and encyst on the skin of another host. The metacercariae is the cercariae encysted and resting on the skin of the host, which can then be ingested by a different host. The parasite is usally passed in the feces of the host.
Trematodes have flattened, oval shaped or worm like bodies and can range in length from a millimeter to several centimeters. The characteristic that distinguishes Trematoda the most from other parasites is the presence of two suckers, one close to the mouth of the parasite and the other located on the underside. The body of the parasite is covered in a tegument, which is a protective cell covering of proteins, carbohydrates, RNA, and lipids to keep the parasite from being dissolved by stomach acid. The tegument also allows for cellular gas exchange, since the parasite does not have respiratory organelles. Trematodes do not have an anus to excrete waste (like other flat worms), so their waste is egested through a single bladder and excreted through two pores at the posterior end of the parasite. Trematoda are hermaphroditic, containing both testes, a single ovary, and a uterus they can reproduce asexually or sexually.