The parasitic flatworms.

Monogenea, A small group of ectoparasites.

Gyrodactylus elegans lives in the gills of European Carp. Each adult G. elegans eventually gives birth to a single live young. This young animal contains within it, in embryonic form, another young animal, which in turn contains another even smaller embryo which in turn contains yet another embryo. As they grow each one of these animals gives birth to the embryo within it.

Polystoma intergerrimum is a parasite of frogs. This species times its reproductive cycle to be in harmony with the frog's reproductive cycle. Thus the larval parasites live on the gills of the tadpoles. When the frog goes through its metamorphosis and leaves the water the parasite also changes. It leaves the tadpole's gills and migrates through its digestive tract to the frog's cloaca, from there it moves to the frog's bladder. Here in a safe and still enjoyably moist environment it reaches maturity. When the frog returns to the water to breed the parasite breeds as well, starting the cycle off all over again.

Cestodes, tapeworms

These are species that are highly modified structurally for their parasitic life cycle. They have two hosts and many species parasitize humans, living as adults in the intestines.

The adult body of a Cestode, tapeworm, consists of a scolex, a type of "head", which is distinguished by the presence of suckers and hooks. Behind the head is an series of reproductive segments or proglottids. These contain testis and ovaries and are shed periodically. The fish tapeworm, Diphyllobothrium latum, can reach 20 metres in length, contain 3,000 proglottids and produce millions of eggs every day.

Top diagram is of a tapeworm, individuals consist of scolex, an anterior region that acts as a holdfast and contains suckers and spines, and a series of mature proglottids, which contains the reproductive organs.

Bottom diagram is of one proglottid or segment of a tapeworm. Mature proglottids are shed and look like moving "rice" in feces.

Cestodes have no digestive system of their own, they simply absorb nutrients from their hosts. To aid this process the entire surface of their body is covered with microscopic wrinkles or projections which greatly increase the surface area available for the absorption of nutrients.

Epidermis in Cestodes on top and that in Trematodes on bottom.

Trematodes, flukes

Structurally, flukes or trematodes resemble free living forms.

These species are high modified with regard to life cycle for their parasitic life cycle. They almost always have at least two hosts. The egg hatches into a larvae that then reproduces asexually into yet another larvae form. Some species may have as many as four larvae forms that may also be capable of asexually giving rise to the next larvae form or simply producing more of the same larval form.

 

Most flukes are hermaphrodites, but some, including a species that parasitizes humans, is sexual.