Photo courtesy of Univ. of Cali. Museum of Paleontology
Monogenetic flukes are small. They do not have a well developed sucker. At their rear end, there is a bulbous structure called an opisthaptor with tiny hooks covering it. The life cycle of monogeneans involvles only one host. The eggs hatch into larvae with cilia (the adults do not have cilia) and may attach themselves to a host or swim freely for a while before finding a host. Most of the monogenean class are ectoparasites, meaning the live on the outside of their host, although some may live in the urinary bladders of frogs and turtles. Monogeneans are acoelomate, which means they are without a body cavity and their organs are encased in parenchyma. Monogeneans can be divided into two other classes; the mono-opisthocotylea and the polyopisthocotylea, which are based on how the opisthaptor is structured.
Monogeneans contain a simple nervous system focused around the cerebral ganglia from which come the eventrial and dorso-lateral nerve trunks. These are brought together through transverse ganglia in a ladder like form. The digestive system of a monogenean is a comprised of a simple blind sack. The sack may be branched, and opens to the exterior through the mouth and is often accompanied by a muscular pharynx. There is a simple excretory system throughout the phylum platyhelminthes which is known as the flame cell-protonephridial. This excretory system consists of a system of canals which terminate in a flame cell at the end and empty into a series of ducts that open on the outside through exterior pores.
Photo courtesy of Univ. of Cali. Museum of Paleontology
There are somwhere around 9,000 species of trematodes. Most of them have a leaf type shape. All of them are parasitic and often the adults parasitize vertebrates. On their body is a certain kind of epidermal arrangement by which the main cell bodies are deep and seperated from the cytoplasm that is next to a layer of muscle near the exterior. This particular epidermal arrangement is called tegument. Some species of trematoda have a very strong pharynx in the anterior sucker. It is used to suck in cells, blood, and fluids.
Trematodes can be compared to turbellarians in that they have a well developed alimentary canal. Their muscular, excretory, and reproductive systems are also relatively complete. Also, the way that trematoda have adapted has caused a loss in quite a bit of the digestive system. The food that the trematoda needs to live off of has already been digested for them.
Most of the trematoda class have a very complex life cycle. The larval stages parasitize many or a single species that are different from the adult hosts. Trematoda are mostly endoparasites and include several parasites that have a huge effect on the human population. The human liver flukes and blood flukes that cause schistosomiasis are good examples of how they could affect the human population.