The phylum Platyhelminthes (Greek: platy, flat, helminthes, worm) is a large and diverse one.

We think usually of flatworms as being planarians, the familiar little freshwater beasts so common in biology demonstrations, but the planarians (Class Turbellaria, Order Tricladida) are members of only one of the four Classes in the phylum. Various Turbellarians may be free-living or parasitic, and inhabit both fresh and salt water; the other three Classes are all exclusively parasitic.

Flatworms, so various among themselves, share certain characteristics. First, they all have a definite head end, usually with simple eyes, a definite tail end, and are bilaterally symmetrical. And they are, as the name implies, flat. Most are small, on the order of 1 to 3 centimeters, but a polyclad (Class Turbellaria, Order Polycladida) may be 25 or more centimeters long, and a tapeworm (Class Cestoidea, Order Cestoda) may be many yards long. The digestive cavity, if there is one, has only one opening, the mouth, indigestible particles must leave by the same route. There is a simple central nervous system, but not much by way of a complex circulatory system. Many are free-living, but members of this phylum do show a tendency to become parasitic.

The flatworms have managed quite a bit of variability on this simple plan.

Class Turbellaria

Turbellarians comprise some 3,000 species in nine Orders. These include the familiar planarians as well as the large, flat, sometimes colorful polyclads, which browse on coral and whose colors are for camouflage. Almost all are hermaphroditic. Most are marine, but some live in fresh water, and a few are terrestrial. Most are free-living carnivores.

As noted, the most famous members of this Class are the little fresh-water, free living worms upon which so many regeneration experiments have been done (cut the head in half, it grows two heads, etc.) These are of the Order Tricladida, genus Dugesia.

For an analysis of the famous experiment concerning planarian learning, see the ability of planarian worms to run a maze more successfully after being fed the remains of a successful worm

For a fascinating detail about mating among the Turbellarians, please see penis fencing. Don't miss that one.

Class Monogenea

Monogeneans have, as the name implies, one host and a relatively simple life history. There are about 400 species in two subclasses (12 Orders between them). All are parasites. Most live on the skin or gills of fishes. They are quite small, 1 to 2 centimeters, and cling to the slippery surfaces of their fast-moving hosts with a complex adhesive organ, usually a cluster of suckers. They are hermaphroditic, but each individual fertilizes another's eggs rather than its own. For an interesting but fairly trivial detail about the sex life of one kind of Monogenean, see Love only me, until we die: Marriage among the flatworms.

Class Trematoda

All are parasites; most of mollusks, but we are very aware of these worms because many prey on vertebrates, often human beings.

Almost all start out as parasites of a mollusk. But almost all have at least 2, and sometimes as many as 4, hosts in their lives.

A simple example is the Chinese liver fluke, Clonorchis sinensis, common in China, Korea and Japan. The adult lives in the liver of a vertebrate, which may be a dog, cat, wild carnivore or a human being. It feeds there on blood and tissue, shedding eggs into the liver, which pass through bile passages into the intestine and thence out into (usually) fresh water. There the eggs, if successful, are found and eaten by a fresh water snail. The eggs hatch in the snail, reproduce asexually there, and myriads of a young larval form make their way out of the snail and back into the water. There the lucky ones locate a fish or crustacean, and digest their way into it. There they encyst. When inadequately cooked infected fish is eaten by a human being, the worms are released in his or her intestine and make their way to the liver to start the story over. Large numbers of these parasites may produce severe liver disease or death; and they have been implicated in the development of liver cancer. All in all an unattractive animal.

Schistosomes are similar (infesting the veins of the intestines rather than the liver), and produce in human beings a debilitating illness. They are considered a health problem in the tropics second only to malaria. The building of the Aswan High Dam in Egypt triggered outbreaks in that country, since the standing water provided an ideal habitat for the intermediate host snail. Control focuses on good sanitation and on the elimination, where possible, of the snails.

There are some 6,000 species of Trematodes in two subclasses. All have multiple hosts.

Class Cestoidea

The ever-popular tapeworms.

Adults are parasites of vertebrate intestinal tracts; few vertebrates are free of them. The young live in various tissues of invertebrates, often insects. They have no mouths or digestive cavities at any stage; the body is divided into repeated units, each with one or more complete reproductive systems.

Lacking digestive organs of their own, tapeworms absorb the handy, already-digested food available in the host's intestines through their outer covering. Little is known about how they resist the attacks of the host's immune system, but from what we can tell it's a constant battle. Some species of hosts (mice are an example) are better at resisting tapeworms than others.

Each segment of a tapeworm (and there may be thousands) contains male and female reproductive organs, and when the segment is mature millions of eggs are shed into the intestines and pass out with the feces of the host. An illustrative species is Diphyllobothrium latum, a large tapeworm of human beings. The eggs must reach water to survive: if they do, they develop into 6-hooked larvae, which swim about until eaten by a copepod. There the larva uses its hooks to penetrate into the copepod's body cavity and changes into a juvenile stage, and there it waits until the copepod is eaten by a fish, where it similarly burrows into the tissues of the fish. It may go through several stages of fish (as smaller fish are eaten by larger ones), but what it is waiting for is the day that the host fish is eaten by any one of a number of suitable mammals, including a human being. It becomes a reproductive adult in the intestine of the final mammal, and the story starts over.

Diphyllobothrium latum has been known for centuries in the Baltics and in Scandinavia; in some regions nearly the whole population is infected. Europeans transported the parasite to the Americas.

Infection may be asymptomatic, but may also cause pain, nausea, weakness and other symptoms, and may trigger anemia. Treatment can be difficult; commonly used medicines for tapeworms are praziquantel (Biltricide) and albendazole (Albenza). Untreated, a large tapeworm (this species can reach 20 meters) may live as long as its host. To avoid infection, it is recommended that one avoid raw fish and meat, thoroughly cook all meats, and be especially careful when traveling to undeveloped areas.

Life histories vary a good deal among tapeworms; intermediate hosts may be fleas, beetles, and a variety of worms. Several species infect human beings, doing various sorts of damage.

There are several thousand species of these unattractive animals, in two subclasses which contain, between them, four Orders.

From a variety of sources, of which Living Invertebrates (Pearse/Buchsbaum, Blackwell Scientific Publications, Boston, 1987) was the most comprehensive.