Phylum Platyhelminthes The Flatworms

The flatworms are characterized by……. three layers of cells a linear nervous system with a ganglion dorsoventrally flat, bilaterally symmetrical body cephalization – the development of an anterior end incomplete digestive system (one opening) an excretory system for removing cellular wastes

There are four classes of flatworms, three of which are important to us… Class Turbellaria – free-living flatworms. These include freshwater species like planaria, marine flatworms, and a few terrestrial species that are sometimes called ribbon worms. These worms are scavengers. Class Cestoda – tapeworms. Tapeworms are parasites that live in the digestive systems of vertebrate animals. Class Trematoda – flukes. These are also parasites whose hosts are vertebrate animals. Unlike tapeworms, flukes may parasitize tissues other than the digestive system. For example there are species that live on the liver, in the brain, and on the kidneys.

Flatworms, with their three cell layers, are much more complex than earlier animals like Cnidarians and sponges. In cross section, you can see that inside the ectoderm (epidermis) of a flatworm there is an extensive muscular system. The “pharynx” is the equivalent of the gastrovascular cavity whose lining is called endoderm. The white material, is not a gel-like material as it is in earlier animals. Instead it is a layer of cells called the mesoderm layer. These additional cells allow for a greater diversity of complex tissues that do not occur in earlier invertebrates.

You are probably familiar with Planarians from Biology. They are frequently used to demonstrate the regenerative capabilities of lower invertebrate animals. If cut in half, two Planaria will result. If the head is split lengthwise, two heads will result after healing is complete. Not all Planarians will look like those you worked with in Biology. There are hundreds of species and they all vary in color, shape and size. Planarians live on land in tropical rainforests and in freshwater lakes, rivers, and streams.

Eyespot Planaria’s eyespots allow them to detect light but they cannot focus like your eyes. The ganglion, their primitive brain is visible along the inside margins of each eyespot, giving Planarians a “cross-eyed” appearance. Two long nerves that extend from the Planarian’s head to posterior end fuse at the anterior end to form the ganglion. Having sensory organs concentrated at the anterior end of an animal is called cephalization. Cephalus means head. Therefore flatworms are the first animals to have a head. Consequently if they have a head, they then have a right and left side. Cephalization then, leads to a new kind of organization among animals called bilateral symmetry. Animals with bilateral symmetry have two sides. Bi = two and lateral = sides.

There is also a diversity of marine flatworms. Many are brightly colored warning fish that they taste bad. Others are cryptically colored allowing them to camouflage with their surroundings. Marine flatworms, like Planaria, are in the class Turbellaria.

This is a very simplified drawing of a tapeworm. I’ve labeled the scolex (head) and the proglottids (segments). The entire length of the tapeworm’s body, excluding the scolex, is called the strobillus. The slides that follow will show these structures in detail. Scolex Proglottids

These are real tapeworms. Scolex Proglottids

Since tapeworms live in the intestines of their host, they must be able to hold on to the intestinal wall. If they don’t they will move out of the host’s intestines with its wastes. A series of four suckers and ring of hooks on the scolex anchor the tapeworm to the host’s intestines. The photographs at the left and right show the suckers and hooks on the scolexes of living tapeworms.

Although tapeworms exhibit cephalization, they have no eyespots. They also have lost their digestive system to evolutionary change. Because they live in their host’s intestine, they are surrounded by food. Molecules of food simply diffuse into the tapeworm’s cells from its environment. Some tapeworms grow quite long. One species that parasitizes horses has been known to attain a length of 75 feet. The length is determined by the number of proglottids that remain attached to the body. Mature proglottids break off and pass with the host’s feces. Living inside a digestive system can be hazardous. Digestive enzymes would simply dissolve most worms. However, tapeworms have a waxy cuticle that covers their ectoderm. It is impermeable to the enzymes.

I included the drawing to facilitate your recognizing structures in the photograph on the right. Flatworms are hermaphroditic. Each proglottid is one complete reproductive system. In these illustrations you can see both male and female reproductive structures. Sexual reproduction occurs when two tapeworms align, side by side, exchanging sperm through the genital pore (atrium). Eggs and sperm usually mature at different times so that a tapeworm doesn’t fertilize its own eggs. This insures genetic variation. When the eggs in a proglottid are fertilized, the proglottid is considered “gravid”.

When a gravid proglottid is at the end of the worm, it breaks off and wiggles out the host’s rectum. It then dries, splits open and spills out its eggs which are injested by a new host. The red and yellow object drawn on this photograph is a wooden match stick to show the size of proglottids that have broken free from the body of the tapeworm from which they came. If your dog has tapeworms, the best way to detect them is to look around its rectum for the dried rice-like proglottids that get caught in the dogs fur.

Flukes, like tapeworms, are parasites of vertebrate animals. The anterior end of this one is at the top of the photograph. The black “squiggly” lines are excretory ducts that collect wastes from cells and excrete them. The dark bow-shaped lines that extend from anterior to posterior, are longitudinal nerves. Flukes are round to football-shaped, but the head is always a bit pointed.

1 2 3 Ganglion – it is the flatworm’s primitive brain and is formed by the fusion of the two longitudinal nerves. Longitudinal nerves extend the length of the flatworm’s body on both sides. Transverse nerves branch towards the ectoderm from the transverse nerve.

Fluke life cycles are complex and usually involve two hosts, one is for the adult fluke (in this diagram a cow or sheep) and is called the definitive or primary host. The intermediate host is infested by the fluke’s larvae (a snail in the case of the sheep liver fluke). Liver flukes live and reproduce in the bile duct (duct from gall bladder to liver). Notice that there are two larval stages (the miracidium and the cercariae). This fluke evolved with sheep and snails. All flukes have similarly complex life cycles that include two larval forms and multiple hosts.

The Chinese liver fluke is a human parasite. Its intermediate hosts include snails and fish. Humans become infected by eating raw or poorly cooked fish. In some fishing villages in southeast Asia, medical tests have proven that 100% of the human residents are infested because the people’s diet includes raw fish. In such villages, human feces are often dumped into the same waters that fishes are taken from as a food source.

The End