Flatworms belong to the Phylum Platyhelminthes (Latin for "Flatworms," although it sounds much more official), whose characteristics shape - flat - is an evolutionary answer to a problem of multicellular life:  as an animal gets bigger, the inside cells get farther and farther away from the outside, so that oxygen gets depleted before it reaches them and wastes build up on the way out.  One answer, found in other groups than this one, is to circulate carrier fluid to bring things in to and away from the cells of the interior;  flatworms use a simpler but more limiting response.  Because a flatworm is flat, even its innermost cells are close to a surface.

Flatworms also have a feature that the sponges and cnidarians do not, a middle layer of actual organized cells.  Flatworms have three cell layers: an ectoderm, an outer covering like the other groups have;  an endoderm, a lining of the digestive spaces somewhat like a cnidarians;  and a mesoderm between the other two.  With actual tissues protected like this, another evolutionary "explosion" of features happened.  Mesoderm allows tissues to organize into organs and even connect in organ systems, so flatworms are much, much more complex than cnidarians.  The evolutionary background of the group is still being debated - are they descendants of pre-cnidaria or simplified mollusks?

The types of organs and organ systems vary among the groups of flatworms. Most have a nervous system, sometimes with a type of processing center called a ganglion, kind of a tiny "brain."  Most have a muscle system, although no skeleton - the body, with all of the cells connected to all of the others, gives resistance to the muscles.  Most have reproductive systems, producing gametes for sexual reproduction;  the vast majority of flatworm individuals are simultaneously male and female (monoecious), and many are built so that they can sexually reproduce all by themselves.  Some systems found in larger animals are not really needed by flatworms:  because of their shape, they dont need a circulation system, or a respiration system, and most dont have much of a specialized waste removal system.  The digestive system of flatworms is not usually all that different from a cnidarians:  food goes into a sac-like gut from the mouth, gets digested, and whatever is left comes back out through the mouth.  Sometimes the gut is more complex in shape, to provide more surface area for absorbing nutrients across, but it rarely has any other organs associated with it.

Flatworms are bilaterally symmetrical, with "mirror image" right and left sides, probably a development that arose during the evolution of a front end - if an animal tends to "lead" with the same end all the time, it will move most efficiently if both sides match.  A preferred front end also leads to cephalization, a concentration of senses and processors in the front.  A preferred front end becomes an actual head end.  In an interesting development, the most active flatworms do not have their mouths up front, although that will become a common feature of heads in later groups of animals.

Flatworms, like the groups of animals so far, are almost entirely confined to watery environments, either in actual water or in the wet environment found inside larger animals.  The exceptions still require lots of moisture.

This phylum includes three major subgroups of flatworms.

The Turbellaria are the only free-living group, meaning that they can be found wandering around in the general environment.  Turbellarians are aquatic (water-living), crawling around mostly on surfaces at the bottoms of bodies of water.  Planaria, a common animal in laboratory exercises, is a turbellarian.  The subgroups within this subgroup are divided mostly based upon how their digestive cavities are constructed.

The Trematodes (sometimes called flukes) are a group of parasites.  There are three subgroups within this subgroup, but two of them have relatively minor effects on the world - one is a parasite mostly on the outside of fish, the other found mostly inside mollusks such as snails.  The third group, the Digenea ("2 reproductions," or "2 lives"), include a number of serious parasites of humans and other large animals.  Some examples are covered below, with more details of their life cycles.

The Cestodes, or tapeworms, are also parasites.  In their adult forms, these worms attach with a specialized "head" called a scolex to the lining of their hosts intestines and begin to lengthen by adding proglottids, segments whose entire purpose is sexual reproduction.  Tapeworms in large animals can get quite long, with the segments farther back being progressively older.  Reproduction occurs between mature proglottids, which then fill with eggs and either drop off to be passed or break apart, so the eggs will be passed.  As adults, tapeworms tend to be very host-specific - they can only live in one or a few species of hosts - and often cause hardly any ill effects to the host.  Many species can control how many individual tapeworms are present in a host, somehow preventing new ones from attaching when a few are already there.  In their larval forms, however, tapeworms may be much more dangerous to the host, as described below.

Tapeworms have no digestive systems as adults, which makes sense for an animal that already absorbs several materials through its surface and lives inside an intestine.  Interestingly, tapeworms surface cells often have the same sort of surface-increasing structures as the lining cells of the intestine.

Both trematodes and cestodes exhibit alternation of generations, that switching back-and-forth between sexual and asexual reproduction.  As parasites in all stages, the asexual stages take place in intermediate hosts and the sexual stages take place in final or definitive hosts.  One would think since this approach produces both of the reproductive types advantages (high variation plus ability to copy successful forms), it would be fairly common, but in practice it becomes an extremely complicated life cycle.  Following are a few flatworm parasites of humans, just a fraction of the full list.
 

TREMATODES OR FLUKES

There are many types of trematodes / flukes that cause human disease.  They include Schistosomes, which infect perhaps a quarter-billion people, with well over twice that at risk for the disease.  There are three different species of schistosomes, which range across the world.  The intermediate hosts for schistosomes are snails - each fluke species has particular snails that they must infect in order to complete their life cycles.  After reproducing in the snails, larvae swim about seeking skin to burrow through.  Once under a humans skin, the larvae migrate eventually through the blood to the linings of either the intestine or the bladder (depending upon the species of fluke).  In the linings, they reproduce sexually and the females (schistosomes are unusual flukes in that they are dioecious, with the male individuals wrapped around the females) lay eggs.  If all goes well for the worm, the eggs pass into the space of the intestine or bladder and are passed into a water body where they can hatch and the early larvae can infect more snails.  In many cases, though, the eggs dont make it out, and circulate in the body.  Sometimes the eggs have spines (for tearing through the linings), which make them very damaging as they circulate;  sometimes the body, in an attempt to fight the invaders, just makes matters worse.

In North America, a schistosome of water birds can invade humans by mistake and cause an irritation and rash called swimmer's itch (cercarial dermatitis).  These worms, evolved to live in birds, cannot survive long in mammals like us, though.

The Chinese liver fluke, Clonorchis sinensis, is common in eastern Asia.  Its life cycle starts like schistosomes, with a snail, but the emerging larvae attach to fish (or to crustaceans which are then eaten by fish), and it is through the eating of uncooked Asian freshwater fish that it passes to humans.  The adult flukes live in the liver, eating the tissues;  their eggs travel down the bile duct to the intestines and out.

CESTODES / TAPEWORMS

As mentioned above, adult tapeworms in the intestine rarely cause serious health problems.  It is when humans become the intermediate hosts that real problems can happen.

The beef and pork tapeworms, Genus Taenia, rarely use humans as definitive hosts.  If eggs are picked up, from eating poorly-cooked infected meat or from contaminated vegetation (watercress is a common source), the larvae bore out of the digestive system and use the blood to get to various places in the body.  Asexual reproduction produces a larvae that resembles a cyst, which can damage surrounding tissues.

A similar type of infections can be caused by larvae of a tapeworm of carnivorous mammals, including dogs and cats:  Echinococcus.  Eggs picked up from the environment and accidentally swallowed produce larvae that grow in humans as hydatids, sometimes called bladder worms.  These may become quite large if the structures around them allow it.  The hydatids can both press on neighboring organs and cause trouble, or cause allergic reactions to the worm fluid inside them.  If you were to be eaten by your dog, the hydatids would break up into many tapeworms - Echinococcus is one of the smaller tapeworms, limited to about three proglottids at a time, and so can invade canaine intestines in large numbers (sometimes dangerously so).

 A different, somewhat more technical introduction to the flatworms.

Pictures of several marine flatworms from the Caribbean.