Flatworms Study Guide


Flatworm belongs to the Platyhelminthes phylum. These are the simplest of the worm group. You can find flatworms in several places. These can be parasitic flatworms or free living. The parasite lives off another living thing called a host, which is harmful. Tapeworm is one of the well-known flatworms.

The tapeworm gets into a person’s digestive tract and then grows to an enormous length. The tapeworm then eats the hosts’ food and is dangerous to the human or animal.

The Structure of Flatworms

Flatworms are usually 0.04 inches or 1 millimeter in length. However, these can also grow as long as 20 meters or 66 feet. These consist of a flat body, and this is because flatworms do not have any coelom. Flatworms also lack any respiratory system. Their cells exchange gases through diffusion with the environment directly. Flatworms do not have a complete digestive system.

Flatworms reflect many evolutionary advances in invertebrates. These have three embryonic cell layers, which also include the mesoderm. The mesoderm layers let them develop their organ system. The flatworms have an excretory and muscular system. Their muscular system lets them move from one place to another on any solid surface. The excretory system lets them maintain their water and salt balance. Flatworms also show bilateral symmetry.

Reproduction in Flatworms

Flatworms have sexual reproduction. In most flatworms reproduction, the same individual will produce both the sperm and the egg. After fertilization, the fertilized eggs pass from the adult’s body and then hatch into larvae. The larva stages are many, and in the final stage, the larva develops into adults. This life cycle keeps repeating.

Flatworms in our Environment

Flatworms survive in various environments. Some of them are free-living, which feeds on small organisms and rotting matter. These are called marine flatworms and freshwater flatworms. Other kinds of flatworms are parasitic. They live inside another organism which is the host.

Tapeworms and flukes are parasitic. Flatworms in humans are also possible. The tapeworm lives in the digestive system of the host. Usually, they need more than one kind of host to complete their life cycle. Tapeworms do not have a digestive system or a mouth. These get their nutrients from the host’s digestive system, which they take in through their suckers.

Not all are flatworm parasites. Some are free-living and carnivorous. These feed on small invertebrates and decaying animal matter. Most of these free-living species are aquatic, and some of them are also found in moist soil.

Features of the Flatworms

Listed below are the important features of a flatworm.

  • Flatworms do not have a true body cavity, but they have bilateral symmetry. Because these do not have a body cavity, they are known as acoelomates.
  • Flatworms have an incomplete digestive system. This means that their digestive tract has just one single opening. Their digestion takes place in their gastrovascular cavity.
  • Flatworms do not possess a respiratory system. These have pores through which oxygen enters the body through diffusion.
  • There are no blood vessels in flatworms. It is their gastrovascular cavity that distributes the nutrients through the body.
  • They have a nervous system that resembles a ladder.
  • Most flatworms will have a distinct head region which includes the nerve cells and the sensory organs. The head region development as cephalization.


  • Flatworms are segments, and bilaterally symmetrical worms with no coelom bit have three layers of germs’.
  • Some of their forms live freely, while others are parasitic.
  • The flatworms have a cephalized nervous system with a head ganglion attached to its nerve cord and is connected through the body with the transverse branches.
  • Osmoregulation and excretion is controlled through the flame cells
  • Flatworms do not have a circulatory or respiratory system, and these functions happen because of absorption through their body walls
  • The non-parasitic flatworm forms have an incomplete gut system
  • Their movement takes place with the circular, longitudinal and oblique muscle layers


1. Are flatworms harmful to humans?

Several parasitic flatworms take residence in the host humans. These cause infections and disease and are harmful to humans. However, not every kind of flatworm is a parasite. Certain flatworms are free-living and carnivores that do not reside in the human body and are thus not harmful.

2. What are the 3 characteristics of flatworms?

Here are the 3 characteristics of flatworms that are true of all the species:

  • Flatworms have no true body cavity; however, they have bilateral symmetry
  • Flatworms have an incomplete digestive system
  • Flatworms do not have a respiratory system.

3. What are 2 examples of flatworms?

There are many kinds of flatworms. There are today 25000 known varieties of flatworms. Liver fluke and planaria are the two examples of flatworms among the many species discovered.

4. Where are flatworms found?

Most of the freshwater variety of flatworms is free living. These can be found in ponds, streams, lakes, temporary puddles, and ditches.

5. What eats a flatworm?

Blue Velvet Nudibranch is believed to eat the flatworm.

6. What do flatworms look like?

The flatworms are of several kinds; however, mostly, these are bilaterally symmetrical animals. It means that their left and right sides are mirror images of each other.

7. What color are flatworms?

Some flatworms are colorful. These are bright blue, orange, green, or pink. You can also find flatworms in brown and some dull colours.

8. Is a tapeworm a flatworm?

Yes tapeworm is a flatworm. These stay in the human body, where the human body acts as a host.

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