Turbellarians account for about 3,000 species of flatworm in 12 orders. Most species within this group dwell in saltwater, although some species can be found in fresh water. Beautiful marine polycladids, from this group, adorn aquariums with their brilliant colors. This is the only class with some non-parasitic flatworms.
For the most part, monogeneans are parasites that live on rather than in a host. These flatworms hook, suction or clamp onto their host. Often they change hosts as they progress through the life cycle. Favored hosts during adulthood include whales, amphibians, fish, squid and octopus.
More than 6,000 species of this parasite exist. The most interesting ability of most species in this class is the ability to reproduce, by replication, while in the larval form. A single egg can generate thousands of parasitic offspring. Several species of these flatworms infect humans, causing disease in millions of people mainly in Asian, African, European and South American countries.
The class feared most by mankind is the cestodes, or tapeworms. Tapeworms have no digestive system because they cling inside the intestines of the host and simply absorb nutrients. With no need to move around, they simply eat and reproduce. Eggs leave the host through its feces. They are ingested by organisms dining on feces. They move up the food chain, taking up residence in each new patron until a large mammal or human eats raw or undercooked meat or fish. Here the offspring finds a permanent habitat inside the intestines and renews the cycle.