The free-living forms generally range from less than 1 mm to over 30 cm in length, but the largest flatworms are tapeworms, some of which can attain lengths of several meters. Unlike the previous groups we have discussed, the flatworms have three tissue layers, similar to the rest of the groups we will discuss, right through vertebrates. A terrestrial flatworm, photographed in Seattle, Washington. These animals control their osmotic water balance with specialized structures called protonephridia , which function in much the same way as our own kidneys. The structure of the gut is used to classify members of the class Turbellaria, but only members of the orders Tricladida and Polycladida are likely to be encountered by the average aquarist.
Many flukes are parasitic on fishes and invertebrates, and many can infect mammals.
The term intermediate host refers to one of the animals infected by these parasites en route to the host in which they reach sexual maturity. Although these seem like relatively simple animals, most parasitic flatworms have surprisingly complex life cycles. The free-living turbellarians typically have a less complicated reproductive cycle. They are still generally hermaphroditic and copulate to engage in mutual cross-fertilization, but most of these forms simply produce a few eggs which are either brooded or laid in protective egg capsules and develop directly into tiny turbellarians which look for all the world like the adults. There are however a number of exceptions, which produce a swimming larva.