Flukes are flatworms modified for a life of parasitism. They belong to the Phylum Platyhelminthes along wtih free-living flatworms such as Dugesia and the parastic cestodes or tapeworms. All platyhelminths, including the flukes, share a number of characteristics. They are unsegmented and bilaterally symmetrical, meaning if they are cut in two down the middle, the left half is like the right half. They have a definite head end with anterior eyespots, chemorecptive organs and a brain. There is a complete third germ layer, the mesoderm, but there is no body cavity or coelom.
Flukes are roughly leaf shaped with a fatter head and and a pointed tail. They have lost their epidermis but are covered by a cuticle and have no external cilia. Eyes and other sense organs are poorly developed or lost. They have one or more suckers for attachment to their host. Adults are parasites of all classes of vertebrates but the most common hosts are fish. They are a successful group with about 9000 species described and possibly as many as 20,000 total. Some are external parasites, clinging to body surfaces. Others are semi-external, living in gills, mouths or sexual openings, while others are internal parasites, usually in the gut.
Trematodes are divided into two subclasses by the number of suckers they have. Monogeneans have suckers on a large posterior disk while the mouth lacks or has a very small oral sucker. These animals are usually external or semi-external parasites of aquatic animals such as fish and frogs. They have relatively direct life cycles in which there is only one larval stage that does not reproduce and metamorphoses directly into the adult form. Usually both larval and adult stages inhabit only one host.
Digeneans have well developed oral and posterior suckers. The adults of this group are internal parasites of vertebrates and have a complicated indirect life style, with a succession of different larval stages. Some of these reproduce but only the last larval stage can metamorphose into the adult form. As with any parasites, the emphasis is on numbers and the vast majority die without reaching the adult reproductive stage. In these species there may be several hosts, including one or more intermediate hosts and then a final host. First intermediate hosts are usually molluscs. Second intermediate hosts are often fish, and the final hosts, where the adult lives and reproduces, is a vertebrate of some sort. Getting from one host to another often involves predation where the first intermediate host is eaten by the second which is in turn eaten by the final host such as a human.
Trematodes have large complex reproductive systems and produce huge numbers of eggs and larvae to overcome one of the problems of parasitism, which is the need to find a new host. Eggs are often capable of surviving in unsuitable habitats for long periods of time until they are consumed by the next host. Most can die and the parents are still successful if only one or two larvae finally make it to the next host. The larvae then have to adapt ecologically to the habits of the host. Meglitsch (p185) states: "As a good hunter knows the habits of the game he seeks, a good parasite adapts its behavior to conform to the habits of suitable host animals. This is most commonly achieved by a larva's fitting itself into the food chain of the final host. A larva which lives in a copepod, often eaten by fish, has partly solved the problem of finding its way to a fish-eating bird or mammal... These adaptations are important for the maintenance of many parasitic life cycles."
One of the consequences of a parasitic life style is the loss of organs that were only useful in the free living forms. Parasites are like cave animals that have lost the need for eyes and other sense organs. Locomotory organs are not needed so become poorly developed. The digestive tract may be useless if the parasite can simply absorb what it needs from the host. So trematodes have huge reproductive systems and large attachment devices such as suckers, but very few other organs.
There are a number of economically important flukes that parasitise humans and their domesticated animals, such as the Chinese liver fluke, Opisthorchis sinensis. This animal's first host is a snail, which is then eaten by a fish and then is passed to humans by eating the fish raw or undercooked. Another common fluke which causes agricultural losses is the sheep liver fluke, Fasciola hepatica. Schistosomiasis is caused by the blood fluke Schistosoma mansoni, which infects some 250 million people. Paragonimus westermani is a fluke that lives in and can damage the lungs. There are several important intestinal flukes such as the giant intestinal fluke, found in Asia, Fasciolopsis buski. Flukes like these can cause millions of dollars of damage to livestock and untold suffering to millions of people. For these reasons alone, they are worth studying.
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press. http://pathmicro.med.sc.edu/parasitology/trematodes.htm