Platyhelminthes- Flatworms

Prostheceraeus giesbrechtii
Prostheceraeus giesbrechtii

Platyhelminthes is a phylum of the kingdom Animalia. There are approximately 20,000 different species, of which the majority are parasitic. They live in marine, freshwater, and damp terrestrial environments. Platyhelminthes can be divided into four classes:
1) Turbellaria- tubellarians (nonparasitic)
Tubellarians are mostly free-living; they are found in the oceans, fresh water, and in moist terrestrial habitats. Only a few are parasitic.
2) Monogenea- Monogeans
Monogeans are mostly ectoparasites; therefore most have attachment organs such as suckers, hooks, or clamps. Monogeneans generally have cephalopods (squid, octopus), fish, reptiles, or cetaceans (whales) as their primary host.
3) Trematoda- trematodes/ flukes
Trematodes/flukes are mainly endoparasitic (live within another organism).
4) Cestodian- tapeworms
Tapeworms are mainly intestinal parasites in vertebrates.

Diagnostic Characteristics

Platyhelminthes are characterized as having very thin bodies between the dorsal (back) and ventral (stomach) surfaces. They range in size from practically microscopic to over 20 meters long. Platyhelminthes have a spongy body structure because they are acoelomates and only possess a mouth. They also lack a complete digestive system. Flatworms are triptoblastic, meaning there are three primary germs (tissues with organelles and organs). Flat worms exhibit bilateral symmetry, have no internal cavity, have Protonephridial excretory organs rather than an anus, and have a nervous system of longitudinal fibers. Flatworms do not have skeletons; however they do have an endoderm, mesoderm, and ectoderm as well as a head region.

The following table sums up the basic characteristics of flatworms:

Bilateral symmetry
Number of main cell layers
Distinct brain
Specialized digestive system
Specialized excretory system
Yes (flame cells)
Body cavity containing internal organs
Specialized circulatory system
Specialized respiratory system

The exterior and interior anatomy of a flatworm
Acquiring and Digesting Food

Tubellarians are carnivores that prey on smaller organisms. Mongeans, flukes, and tapeworms are parasitic and obtain nutrients from their hosts. Most have suckers and/or hooks that allow them to attach to the hosts’ exterior or internal organs and suck nutrients into the gastrovascular cavity. Digestion occurs in the gastrovascular cavity, which has three branches, each with smaller sub-branches. The branched structure of the gastrovascular cavity increases its surface area and therefore the rate at which flatworm cells absorb nutrients. The highly branched structure also allows food to be transported to all parts of the body. Since flatworms lack a circulatory system, each cell must be close enough to a branch of the gastrovascular cavity that vital materials can diffuse into it. Flatworms are also parasites and have been known to infect humans. In the year 2000 an estimated 45 million people were infected with Taenia saginata, or beef tapeworm.

Sensing the Environment

On the platyhelminthes’ “heads,” there is a pair of ganglia (dense clusters of nerve cells). A pair of ventral nerve cords extends from the ganglia to the other end of the body. There are also two eyespots to help the flatworm "see," or sense light.
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How platyhelminthes sense the environment
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Platyhelminthes have a hydrostatic skeleton, which consists of a closed body compartment filled with fluid. They move mainly by contracting muscles against the hydrostatic skeleton, which changes their shape and form. Flatworms have cilia (extentions of the plasma membrane, made of microtubules) and two layers of muscles under their skin, which assist with movement.


Platyhelminthes lack respiratory organs. However, they absorb oxygen through their skin by diffusion. Respiration occurs throughout the length of the body of a flatworm, making them susceptible to a loss of important body fluids. Flatworms, therefore, are restricted to living in fresh water, salt water, or moist terrestrial habitats.

Metabolic Waste Removal

Platyhelminthes also have a simple excretory system. Ammonia is excreted directly through their skin by diffusion.

Platyhelminthes have a special excretory cell called a "flame cell". Flame cells are hollow and have tufts of cilia; when the celia move, they resemble flames and force out waste products.


Platyhelminthes are acoelomates, meaning they lack body cavities and circulatory organs. Circulation, like respiration, occurs through diffusion.


Although flatworms have relatively simple systems, their reproductive systems are among the most complex in the animal kingdom. Platyhelminthes are hermaphroditic, meaning they have both male and female reproductive organs. One or two ovaries and numerous testes are usually present in flatworms. Flatworms lay eggs that either hatch into tiny worms or a larval stage with cilia.
Platyhelminthes reproduction system
Platyhelminthes reproduction system

Self Protection

Regeneration is achieved by planaria due to their maintenance and utilization of embryonic stem cells called neoblasts, which make up 30% of an adult worm's cells. When a worm is amputated, epithelial cells close over the wound in as little as 15 minutes. Then the neoblasts accumulate under the epithelium and through cell regeneration, newly differentiated structures are noticeable and the body part grows back within 10 days of amputation.
The results of a failed feeding attempt by most probably a fish. But this flatworm's not crying; the wound will close up and the part will be regenerated in no time!

Osmotic Balance

Platyhelminthes’ excretory system functions mainly to sustain osmotic balance, allowing them to live in aquatic and moist habitats. The excretory system is made up of flame cells, ciliated cells, which help fluid permeate the skin. Platyhelminthes are near osmotic balance with their environment, making them isotonic or isomotic.

Review Questions

1. What implications of Platyhelminthes' lack of a body cavity play on its circulatory and respiratory systems?
2. How do Platyhelminthes perform basic functions such as osmotic balance, oxygen circulation, and waste removal?
3. What is the nervous system of the Platyhelminthes composed of?

4. Describe how Platyhelminthes acquire and digest food.


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  7. "The Class Monogenea." Web. 25 Oct. 2009. < inverts/monogenea.html>.
  8. "Flame cell." Wikipedia, the free encyclopedia. Web. 26 Oct. 2009. <>.
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  11. "Introduction to Platyhelminthes." 4 Nov. 2009. <>.
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Hannah Leff