Platyhelminthes

The phylum Platyhelminthes (flatworms) is divided into four classes: the Turbellaria, or non parasitic flatworms; the Monogenea, or parasitic flatworms; the Trematoda, or flukes; and the Cestoda, or tapeworms. As their common name suggests, platyhelminthes are flattened, soft-bodied organisms. Mostturbellarians are free-living. The members of the other three classes are exclusively parasitic. Most flukes are microscopic. Intestinal tapeworms, on the other hand, may grow up to 100 feet (30 meters) long.

The blood fluke breeds in the host’s intestine or bladder. Fertilized eggs pass out in the feces, develop into larvae (miracidia), and are released in water. In an intermediary host they breed cercariae, which burrow into a third host.

General features

Compared with the simpler cnidarians, flatworms show several advanced characteristics. For example, the body is bilaterally symmetrical and has a definite head end. Free-living flatworms are active animals, and in many species the head carries pairs of eyes, as well as organs that sense chemicals. There is also a concentration of nerve cells at the front end of the body that forms a primitive brain, in addition to a nerve net similar to that of the cnidarians.
The flatworm body is composed of three layers of cells. A layer of mesodermal cells lies between the epidermis and the gastrodermis, which lines the digestive cavity. Because these animals are so highly flattened, a specialized breathing system is not required and oxygen reaches this middle cellular layer by diffusion. The mesoderm contains the complex reproductive organs, which are made up of different types of cells. This differentiation represents a higher level of organization than that found in cnidarians and similar phyla, which possess tissues, but not organs.
As in hydra, the digestive cavity in flatworms (other than tapeworms) has one entrance only and no separate exit, with the result that undigested food is ejected through the mouth. In turbellarians, this digestive cavity is often highly branched, so that food can be distributed to all parts of the body.

Turbellarians

The turbellarians include all free-living flatworms. Some species grow up to 25 inches (60 centimeters) long, but most are about one-half inch (13 millimeters) long. Most freshwater species are drab and inconspicuous, whereas tropical marine species may be very colorful.

Although they live in water, turbellarians do not swim freely, but creep along the bottom. The epidermis is equipped with tiny, hairlike cilia and glandular cells, which secrete mucus in which the cilia beat, enabling the turbellar-ian to glide along. Turbellarians also have bands of muscles that allow complex bending movements of the body.

Most turbellarians are carnivorous, feeding on small animals, or necrophagous, feeding on dead animals. Many secrete mucus and an adhesive to entangle their prey, then break it up into small particles using a part of their gut, which they can extend out of their mouth. In some cases, the prey is ingested whole.

Compared with parasitic flatworms, turbellarians have a simple life cycle. Most species are hermaphroditic, with male and female sex organs in the same individual. Self-fertilization does not usually occur, however, and a partner is needed for fertilization to take place. Sperm is exchanged between the two individuals, and both lay fertilized eggs in egg capsules or gelatinous masses. After two or three weeks, young flatworms resembling their parents hatch from the eggs.
Some turbellarians also reproduce asexu-ally; the body constricts in the middle and thehind portion breaks off to form a complete new individual. Using the same principle, if an individual is cut in two, the front half will form a new tail and the rear half a new head.

Flukes

These flatworms occur as parasites that live inside other organisms (order Digenea) and parasites that live outside their hosts (class Mono-genea). Flukes generally have leaf-shaped bodies with suckers or, rarely, hooks to attach to their hosts. As a result of their parasitic habits, adult flukes lack the sense organs and layer of cilia that are found in turbellarians.

Another feature found in flukes and typical of most parasites is a high reproductive ability, which ensures that at least some offspring reach suitable hosts. Not only do the adult flukes of the order Digenea produce large numbers of eggs, but the larvae of these flukes also reproduce within their hosts.

Monogenea are often found in the gill cavities of fish, where they have to withstand strong water currents. They feed on blood and gill tissue. Internally parasitic flukes may be found in many vertebrates as well as in other invertebrates. The liver fluke is a well-known parasite found in the bile passage of the liver of sheep and cattle, where it inflicts fatal damage. This fluke has a complex life cycle that involves several hosts. The eggs leave the vertebrate host in the feces and hatch in water into free-swimming larvae called miracidial. These larvae swim until they encounter a snail the intermediate host into which they burrow. Inside this host the larvae multiply and eventually leave the snail as another larval form called cercaria. Up to 600 cercariae may be produced from a single miracidium. These then enclose themselves within cysts on grass, where they are eaten by the vertebrate host, in which they develop into sexually mature flukes.

Tapeworms

The adults of this parasitic group are found as internal parasites in the digestive tract of vertebrates. Like the flukes, tapeworms have no obvious sense organs, and their complex life cycle usually involves two more hosts. The beef tapeworm, for example, has two hosts— cattle and man.

Tapeworms need no digestive system because they are surrounded by digested food in the gut of their host, and they simply absorb nutrients through their skin. These worms consist of a head, or scolex. Segments, or proglottids, bud off from the neck. The head usually bears hooks and suckers for attachment to the gut lining of the host. The proglottids remain attached in a long chain as they mature, finally breaking off and leaving the host in the feces. The eggs hatch from the proglottids once they have been eaten by the intermediate host.

The reproductive segment (proglottid) of a tapeworm contains male and female reproductive organs. During copulation, the cirrus of the male is inserted into the genital pore of another proglottid, and the fertilized eggs are stored in the uterus. Cross-fertilization usually occurs with a proglottid from another worm in the same host or with another proglottid on the same worm if the worm has twisted around itself. The proglottid breaks off the parent organism, and when the eggs are mature the proglottid wall ruptures, releasing them.