Worm Phyla: Platyhelminthes, Nematoda and Annelida

The next three phyla we will discuss are all “worms.” By appearance, they seem similar, however their internal anatomy is very different. Let’s take a brief detour and discuss the coelom because knowing what it is will help us distinguish among each of these worm Phyla.

The Coelom: An Evolutionary Milestone for Animals


coelom (from Greek, meaning “hollow or “cavity”) is a fluid-filled body cavity that surrounds an animal’s digestive tract. None of the animals we have discussed thus far have coeloms.

Humans do have a coelom. We have a tube that runs from our mouth to our anus that serves as our digestive system. It is derived from endodermal tissue during our development. There is a cavity surrounding this “tube” where our organs reside. Sometimes this body plan is called “tube within a tube.”

What are the advantages to having fluid-filled coelom?

  1. absorbs shock
  2. transports gas, nutrients and waste
  3. allows advanced types of motility (swimming, burrowing, musculature)
  4. allows organs in the cavity the ability to move and grow
  5. storage site for sperm and eggs

Phylum Platyhelminthes: Flatworms


The Phylum Platyhelminthes includes the first bilaterally symmetrical animals. Members of this Phylum (Planaria, tapeworms and flukes) are dorsoventrally flattened and have a definite anterior and posterior end.

Flatworm bodies are solid with all three tissue layers. Because there is no cavity that surrounds the endoderm (digestive system), flatworms do not have a coelom and are called acoelomate.

Watch the following movie clip from The Shape of Life to learn more about the flatworms. While viewing, record important ideas on your Fact Sheet.

Some flatworms are parasites, while others are free-living carnivores or scavengers. Examples of parasitic flatworms are flukes and tapeworms. Flatworms have a single opening (pharynx) where both food and waste pass (This is similar to Cnidarians).

Gas exchange in this phylum occurs by simple diffusion; there are no circulatory or respiratory organs. This restricts them to moist environments. Specialized flame cells help get rid of wastes.

The nervous system of Platyhelminthes is composed of a nerve net near the head. Some have light-sensitive eyespots.

Planarians are hermaphrodites producing both eggs and sperm, but they exchange sperm with each other during sexual reproduction. Planarians can also reproduce asexually by fragmentation.

Flukes and tapeworms often live in their host’s digestive tract and are resistant to the host’s enzymes. They do not have a digestive system so they absorb nutrients from their hosts.


Flukes have complex life cycles that involve more than one host. A fluke causes Schistosomiasis, a disease that affects 250 million people worldwide (mostly children) who are exposed by contact with contaminated water. This blood fluke attacks the kidneys, liver, and intestines causing progressive weakness. It often takes 20 years to die from Schistosomiasis, and there is no cure.


Tapeworms are divided into sections called proglottids. Each section has a complete reproductive system producing fertilized eggs. Tapeworms are hermaphroditic (one body having both sexual parts), and they fertilize their own eggs. Ripe proglottids with their eggs pass out with the host’s feces. Tapeworms anterior end is called the scolex and is modified with both hooks and suckers to attach to the host’s intestines.

Humans most often get tapeworms from infected, undercooked pork, beef, or fish. Tapeworm eggs can withstand boiling water so it is important to cook these meats well enough to destroy the eggs. The longest tapeworm ever passed by a person was 39 meters!

Phylum Nematoda: Roundworms


Members of the Phylum Nematoda are called roundworms because they are cylindrical in shape, tapered at both ends, and vary in length from being microscopic to 20 inches long. They are covered with a thick protective cuticle that is flexible and can be molted (shed, allowing for growth). Most roundworms are parasites and are found in all habitats. They are bilaterally symmetrical and unsegmented.


Roundworms are pseudocoelomates, meaning they having a body cavity that is not completely lined in mesoderm on all sides. The body cavity or pseudocoel serves as a hydrostatic skeleton against which muscles can contract.

Unlike flatworms, roundworms have a complete gut. This means that they have a one-way digestive tract with a gut that begins with a mouth and ends with an anus. Therefore, they are usually able to digest food and pass waste in one continuous direction.

Roundworms have no blood or heart. Nutrients are distributed by a non-blood fluid that is not pumped, but flows freely, in the body cavity.

They have separate sexes generally and reproduce sexually.

Hover over each picture to learn more about representative Nematodes.

trichinella.png FilariaWorm.png Ascaris.png Heartworm.png

Phylum Annelida: Segmented Worms


Members of the Phylum Annelida are the segmented worms and are abundant in all habitats. Members include the earthworms, leeches and sandworms. Annelids have a coelom that is fully lined and contains the body organs. The coelom runs from the mouth to the anus. Annelids have bilateral symmetry, a well-developed brain and diverse sense organs showing cephalization.

Let’s talk a little about segmentation in Annelids. External segments are characterized by ring-like structures called metameres along the body, and corresponding internal segments are called septa. Segmentation gives worms more flexibility in movement. Also, if one segment is damaged, it isn’t usually fatal to the animal because their organs are duplicated in other segments.

Watch the following movie clip from The Shape of Life to learn more about the Annelids.While viewing, record important ideas on your Fact Sheet.