Platyhelminthes- A.K.A- Flatworms. (Gili Resnick)

A Flatworm (12)

Characteristics of Platyhelminthes

Flatworms are flat organisms that have three cell layers: an ectoderm (outer layer), a mesoderm (middle layer), and an endoderm (inner layer that lines the digestive tract). Flatworms are complex because they have a mesoderm layer, which allows tissues to form into organs. Most flatworms have a nervous system, a muscle system (without a skeleton), and a reproductive system. In addition, the majority of flatworms are monoecious (have both male and female reproductive systems) and can sexually reproduce by themselves. (MT)

Flatworms are characterized by a lack of respiratory, circular, or skeletal system, an incomplete digestive system, and skin covered with suckers or hooks to attach to a host. They have no body cavity, spaces between organs are filled with loose tissue. (JS) (24)

What defines a flatworm: Flatworms are called flatworms because their bodies are between the dorsal and ventral areas. This means that they have very thin bodies and that they take in food and excrete waste through just one opening. The Platyhelminthes can be further divided into four subcategories: Turbellaria (a free-living species), Monogenea, Trematoda, and Cestoidea (tapeworms).


These organsims are free- living, which means they can be found in the environment like any other organism. They mostly can be found at the bottom of bodies of war because they are aquatic. (NG)

There are a few terrestrial species which are mostly nocturnal and live in humid locations, for example, leaf littler or rotting wood. Also some turbellarians live symbiotically with other animals, and some are parasites. Some aquatic species can burrow for safety. In the burrow they anchor their rear end to the bottom and stretch their head up when it's time to feed. (RL) (8)

Crazy fun fact: Turbellarians digest themselves when deprived of food for too long. When they begin feeding normally again, turbellarians can return to normal size and regenerate any organs they may have digested. (CSR, 6)

Two turbellarian flatworms performing the mating behavior known as "penis fencing" (TM)(13)
Two turbellarian flatworms performing the mating behavior known as "penis fencing" (TM)(13)


Monogea are usually ectoparasites, meaning they live and reproduce on the skin or outside of their hosts. However, they may also become endoparasites. They have hooks called opishaptors which help them latch on to the host. Strange reproduction: One type of monogea retains its larva until the larva reaches the pre-adult stage. While in utero the a second larva forms inside the first and third inside the secound and a fourth inside the third. As the pre-adult larva feeds on the host, it can mature and give birth to the larva inside. Only after the pre-adult larva has given birth to the second larva, may it have on of its own eggs be fertilized. (ORS 11)


These flatworms are a type of parasite. The schistosomes, a main subgroup, usually feeds off of snail and will move into a human if given the opportunity. The other main type, the chinese liver fluke, also begins its life on a snail, but instead will move on to a fish as oppose to a human. (NG)

Trematoda usually have one or more intermediate hosts in their juvenile stages. When the reach adulthood, the stay in a definitive host.(ORS 10)

Known as Flukes, these have flattened bodies like leaves. Unlike tapeworms, whose suckers are present only for attachment purposes, flukes have one or two suckers through which they feed. (ZXU)(15)


This type of flatworm is also a parasite and uses a scolex, which is a specialized type of head, to attatch to the body of its host cell. From this starting point, it continuously adds segments called proglottids, of which are only there to reproduce and create more. Because tapeworms have no digestive system, they tend to be host specific in choosing hosts that offer them the most nutrients to feed off of. Tapeworms are more common in pets such as cats and dogs than in humans, but can be obtained by consuming undercooked pork or beef. (NG)

external image anatomy-of-planarian.jpeg

This image demonstrates the various systems of the flatworm. On the bottom left is a model of the cnidarian excretory system. On the top left is a model of the cnidarian digestive system. In the middle is a model of the cnidarian reproductive system. On the right is a model of the cnidarian nervous system. (LW) (14)

The extraordinary ability to regenerate lost body parts and to undergo tissue regeneration, beyond simple wound healing, occurs in two classes of Platyhelminthes: Turbellaria and Cestoda. There is evidence that a special type of cell, a neoblast, is involved in planarian regeneration. Neoblasts, rich in ribonucleic acid (RNA), which plays an essential role in cell division, appear in great numbers during regeneration. For example, a planarian, non-parasitic Platyhelminthes, split lengthwise or crosswise will regenerate into two separate individuals. (ZS)(16-19)

Planarian Regeneration (ZS) (20)

Acquisition and digestion of food: The Flatworms differ greatly amongst the four categories in terms of acquisition of food. Turbellarians are marine free-living species; they are also the only type of flatworm that is not parasitic. They are carnivorous and feed on smaller animals or dead animals. They take the food in, and then secrete the waste out of the same hole. At the other end of the spectrum, Cestoidea or tapeworms are parasitic worms that prey on vertebrates, they latch on inside the intestine of animal, and absorb the food through their bodies as they do not have a “mouth” (they essentially eat the food that would otherwise be digested by their host). In the middle lie the Monogenea and Tremadota species. These species latch on to either the internal organs or outer part of their host and feed off them.

Sensing the environment and Locomotion: Flatworms all live in aquatic or very moist environments and most move around and sense where they are based on the moistness around them, detected by the cells on the outside of their body (that essentially make up their bodies). One Subspecies, the Turbellaria, has small eyes and can see light and darkness, as well as a sense of smell. All flatworms move around by “swimming” using their tail like bodies to get around in the water. The scientific way to explain this, is that the flatworms use a film of mucus that they secrete themselves to help their cilia (hair like substances that help it move) to move around.

The longitudinal, circular, and oblique muscles control the movement of some flatworms. Flatworms can also move by beating their epidermal cilia along slime trails. Flatworms use cephalization to correlate their movement. Some flatworms develop a head region with ocelli, light sensitive organs, which help the process of cephalization. Flatworms can also have chemoreceptors, statocysts, and rheoreceptors to help in the process of cephalization. (7)(MF)

Flatworms have extremely simple nervous systems to help them sense the environment around them. They have two nerve cords running the entire lenght of their body. These nerve cords are used for sensory. Flatworms also have two simple brains called ganglia, which are simply a bundle of nerves. For sensing light, flatworms have two "eyespots" which are a simple form of eyes. (RG) (22)

These are the major components of a Planarian nervous system. C is an image of a Planarian eyespot. (AK) (21)

Respiration and Circulation: The Platyhelminthes lack any respiratory or circulatory organs. Their flat bodies and the fact that their skin is always in contact with water allows them to diffuse the oxygen into their bodies.

Platyhelminthes respire using diffusion, making the flat body necessary, to allow each cell to be in contact or close to the surface with the environment. mostly Nitrogenous waste is lost through diffusion also. (MLK) (23)

Metabolic waste removal: The flatworm spills digestive juices on its prey before sucking it in to the Gastrovascular cavity. After it is done digesting the food along the lining of the gastrovascular cavity, the undigested food exists out through the mouth. Nitrogenous waste is excreted through as ammonia through its cells into the surrounding water.

Self Protection: Since most species of Platyhelminthes are parasitic, meaning they feed off a host animal; they use the host as a means of protection. Tapeworms, for example, latch on to the intestines of humans and are unharmed by the acids and natural bacteria (they are protected by a type of soft outer shell) and can therefore thrive there.

Osmotic Balance: The Flatworms use a type of excretory system to maintain osmotic balance. Any excess waste is secreted into the surrounding water and helps them maintain their osmotic balance that allows them to live in both freshwater and salt-water environments.Flatworms have primitive excretory systems, made up of flame cells, excretory ducts and excretory pores. The flame cells are located on the ends of a branched tubule system. Cilia are attached to the end of every flame cell. When interstitial fluid, the fluid that bathes tissues, enters the branched system through the flame cells, cilia propel the fluid into the rest of the system. Planaria have protonephridiopores coming off of the tubule system, which are excretory ducts that open into the excretory environment. The flame cell system is used for osmoregulation, though the specific mechanisms of osmoregulation in flatworms are unknown. It is hypothesized that the tubules are lined with cells that absorb specific salts before excretion. Freshwater flatworms have very dilute excreted fluid, due to their hypoosmotic environments. (CSR,4, 5)

Temperature Balance: There is a similarity in the way flatworms use their hosts for protection and temperature. In the same way they use their host organism for protection, they use them to maintain the temperature, whatever the temperature of the host, that is, the temperature (approximately) of the flatworm.


So how do these creatures reproduce and survive?

If you enjoy eating sushi…. Watch out!

The eggs originally exit with the primary host's feces and then contaminate water, where they are ingested by another organism such as a snail (the first intermediate host). The eggs then hatch within the snail's gut and develop into miracidia that stay in the walls of the snail's digestive tract. Within these tissues the miracidium is transformed into a sporocyst. The sporcyst develops internal chambers; and, within each chamber, a redia develops asexually. Each of the redia is also chambered and asexually produces a number of internalized cercaria. The cercaria have finished their development within the first intermediate host, so they then burrow out of the snail and become free-swimming. Thus, if a single miracidium is ingested by a snail, several hundred cercaria can be liberated. The cercaria then seek out a second intermediate host (a fish) and burrow through the fish's skin. Once in the fish's muscles they lose their tails and encyst (the encysted form is called a metacercaria). When raw or poorly cooked fish is eaten the young flukes are released from their cysts by digestive juices in the stomach. Once out of the stomach they make their way up the common bile duct to the liver where they attach and suck the host's blood. (YA)(9)

Review Questions:

Describe the structure of a flatworm and it's locomotion? (SR)

What are the types of flatworm and what are their defining characteristics? (MC)

How do tubellarians acquire food? Why is this method different than all other flat worms? (SD)

How does living within a host effect a flatworm? What systems does the host have over the flatworm?(CW)


Campbell, N.C., Reece, J.R. (2002). (Sixth Edition). (NG) (MT) (CSR, 4) (CSR, 5) (CSR, 6) (7)(MF) (RL) (8) 10) 11) (SR 12) (TM)(13) (LW) (14) (ZXU)(15) (AK) (21) (RG) (22) (24) (JS)