18.1 Sponges, Cnidarians, Flatworms, and Roundworms

Lesson Objectives

  • Describe invertebrates in the phylum Porifera.
  • Outline characteristics of cnidarians.
  • Give an overview of the platyhelminths.
  • Summarize traits of nematode invertebrates.


Chapter 18.1 workbook pages

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  • Cnidaria
    • invertebrate phylum that includes animals such as jellyfish and corals that are characterized by radial symmetry, tissues, and a stinger called a nematocyst
  • endoskeleton
    • internal skeleton that provides support and protection
  • filter feeder
    • animal that obtains organic matter for nutrition by filtering particles out of water
  • medusa (plural, medusae)
    • basic body plan in cnidarians such as jellyfish that is bell-shaped and typically motile
  • Nematoda
    • phylum of invertebrates called roundworms, which have a pseudocoelom and complete digestive system
  • Platyhelminthes
    • invertebrate phylum of flatworms that are characterized by a flat body because they lack a coelom or pseudocoelom
  • polyp
    • basic body plan in cnidarians such as jellyfish that is tubular in shape and typically sessile
  • Porifera
    • invertebrate phylum of sponges, which have a non-bony endoskeleton and are sessile as adults
  • sessile
    • of or relating to an animal that is unable to move from place to place


Invertebrates are animals without a backbone. They are the most numerous animals on Earth. Most invertebrates are insects.


Sponges are aquatic invertebrates that make up the phylum Porifera. The wordporifera means pore-bearing. The phylum is aptly named. As you can see from Figure below, a sponge has a porous body. There are at least 5,000 living species of sponges. Almost all of them inhabit the ocean, living mainly on coral reefs or the ocean floor.

Sponge on a Coral Reef. This orange sponge is covered with pores. Can you predict the function of the pores?

Structure and Function of Sponges

Sponges come in a variety of shapes and sizes. For example, they may be shaped like tubes, fans, cones, or just blobs. They range in diameter from about a centimeter (0.4 inches) to over a meter (3.3 feet). Many species live in colonies that may be quite large. Adult sponges are sessile. This means they are unable to move from place to place. Root-like projections anchor them to solid surfaces such as rocks and reefs.

Sponges have an internal skeleton that gives them support and protection. An internal skeleton is called an endoskeleton. A sponge endoskeleton consists of short, sharp rods called spicules (see Figure below). Spicules are made of silica, calcium carbonate, or spongin, a tough protein. They grow from specialized cells in the body of the sponge.

Sponge Anatomy. A sponge lacks tissues and organs, but it has several types of specialized cells.

Sponges are filter feeders. They pump water into their body through their pores. The water flows through a large central cavity called the spongocoel (see Figure above). As the water flows by, specialized collar cells filter out food particles such as bacteria. Collar cells have tiny hairs that trap the particles. They also have a flagellum that whips the water and keeps it moving. Once the food is trapped, the collar cells digest it (see Figure below). Cells called amebocytes also help digest the food. They distribute the nutrients to the rest of the body as well. Finally, the water flows back out of the body through an opening called the osculum. As water flows through the sponge, oxygen diffuses from the water to the sponge’s cells. The cells also expel wastes into the water for removal through the osculum.

Jonathan Bird’s Blue World: Sponges

Collar Cell. The collar cells of sponges trap and digest food.

Sponge Reproduction

Sponges reproduce both asexually and sexually. Asexual reproduction occurs by budding. Figure below shows the sponge life cycle when sexual reproduction is involved. Adult sponges produce eggs and sperm. In many species, the same individuals produce both. However, they don’t produce eggs and sperm at the same time. As a result, self-fertilization is unlikely to occur. What is an advantage of avoiding self-fertilization?

Sponge Life Cycle. When sponges reproduce sexually, they have this life cycle.

Sperm are released into the surrounding water through the osculum. If they enter a female sponge through a pore, they may be trapped by collar cells. Trapped sperm are delivered to eggs inside the female body, where fertilization takes place. The resulting zygote develops into a larva. Unlike the adult, the larva is motile. It is covered with cilia that propel it through the water. As the larva grows, it becomes more similar to an adult sponge and loses its ability to swim.

Ecology of Sponges

Sponges that live on coral reefs have symbiotic relationships with other reef species. They provide shelter for algae, shrimp, and crabs. In return, they get nutrients from the metabolism of the organisms they shelter. Sponges are a source of food for many species of fish. Because sponges are sessile, they cannot flee from predators. Their sharp spicules provide some defense. They also produce toxins that may poison predators that try to eat them.

BBC: The Wonderful World of the Sea Sponges


Cnidarians are invertebrates such as jellyfish and corals. They belong to the phylum Cnidaria. All cnidarians are aquatic. Most of them live in the ocean. Cnidarians are a little more complex than sponges. They have radial symmetry and tissues. There are more than 10,000 cnidarian species. They are very diverse, as shown in Figure below.

Cnidarian Diversity. Cnidarians show a lot of variability.

Structure and Function of Cnidarians

All cnidarians have something in common. It’s a nematocyst, like the one shown in Figure below. A nematocyst is a long, thin, coiled stinger. It has a barb that may inject poison. These tiny poison darts are propelled out of special cells. They are used to attack prey or defend against predators.

Cnidarian Nematocyst. A cnidarian nematocyst is like a poison dart. It is ejected from a specialized cell (shown here in yellow).

TED Ed: How does a jellyfish sting?

There are two basic body plans in cnidarians. They are called the polyp and medusa. Both are shown in Figure below. The polyp has a tubular body and is usually sessile. The medusa (plural, medusae) has a bell-shaped body and is typically motile. Some cnidarian species alternate between polyp and medusa forms. Other species exist in just one form or the other.

Cnidarian Body Plans. Cnidarians may exist in the polyp (left) or medusa (right) form.

The body of a cnidarian consists of two cell layers, ectoderm and endoderm. The cells surround a digestive cavity called the coelenteron (see Figure below). Cnidarians have a simple digestive system. The single opening is surrounded by tentacles, which are used to capture prey. The tentacles are covered with nematocyst cells. Digestion takes place in the coelenteron. Nutrients are absorbed and gases exchanged through the cells lining this cavity. Fluid in the coelenteron creates a hydrostatic skeleton. Cnidarians have a simple nervous system consisting of a nerve net that can detect touch. They may also have other sensory structures. For example, jellyfish have light-sensing structures and gravity-sensing structures. These senses give them a sense of up versus down. It also helps them balance.

Cnidarian Reproduction

Figure below shows a general cnidarian life cycle. Polyps usually reproduce asexually. One type of asexual reproduction in polyps leads to the formation of new medusae. Medusae usually reproduce sexually. Sexual reproduction forms a zygote. The zygote develops into a larva called a planula. The planula, in turn, develops into a polyp. There are many variations on the general life cycle. Obviously, species that exist only as polyps or medusae have a life cycle without the other form.

General Cnidarian Life Cycle. Cnidarians may reproduce both asexually and sexually.

Ecology of Cnidarians

Cnidarians can be found in almost all ocean habitats. They may live in water that is shallow or deep, warm or cold. A few species live in freshwater. Some cnidarians live alone, while others live in colonies. Corals form large colonies in shallow tropical water. They are confined to shallow water because they have a mutualistic relationship with algae that live inside them. The algae need sunlight for photosynthesis, so they must be relatively close to the surface of the water. Corals exist only as polyps. They catch plankton with their tentacles. Many secrete a calcium carbonate exoskeleton. Over time, this builds up to become a coral reef (see Figure below). Coral reefs provide food and shelter to many ocean organisms. They also help protect shorelines from erosion by absorbing some of the energy of waves. Coral reefs are at risk of destruction today.

Great Barrier Reef. The Great Barrier Reef is a coral reef off the coast of Australia.

Discovery News: What are Coral Reefs?

Unlike corals, jellyfish spend most of their lives as medusae. They live virtually everywhere in the ocean. They are typically carnivores. They prey on zooplankton, other invertebrates, and the eggs and larvae of fish.

KQED: Amazing Jellies

Jellyfish. They are otherworldly creatures that glow in the dark, without brains or bones, some more than 100 feet long. And there are many different types. Jellyfish are free-swimming members of the phylum Cnidaria. Jellyfish are found in every ocean, from the surface to the deep sea. To find out more about jellyfish, see http://www.kqed.org/quest/television/amazing-jellies–siphonophores2.


Flatworms belong to the phylum Platyhelminthes. Examples of flatworms are shown in Figure below. There are more than 25,000 species in the flatworm phylum.

Platyhelminthes. Platyhelminths include flatworms, tapeworms, and flukes.

Structure and Function of Flatworms

Flatworms range in length from about 1 millimeter (0.04 inches) to more than 20 meters (66 feet). They have a flat body because they do not have a coelom or even a pseudocoelom. They also lack a respiratory system. Instead, their cells exchange gases by diffusion directly with the environment. Their digestive system is incomplete.

Flatworms have three embryonic cell layers, including mesoderm. The mesoderm layer allows them to develop organ systems. For example, they have muscular and excretory systems. The muscular system allows them to move from place to place over solid surfaces. The excretory system lets them maintain a proper balance of water and salts. Flatworms also show cephalization and bilateral symmetry.

Flatworm Reproduction

Flatworms reproduce sexually. In most species, the same individuals produce both eggs and sperm. After fertilization occurs, the fertilized eggs pass out of the adult’s body and hatch into larvae. There may be several different larval stages. The final larval stage develops into the adult form, and the life cycle repeats.

NatGeoWild – World’s Weirdest
Note: This video mentions reproductive terminology.

Ecology of Flatworms

Both flukes and tapeworms are parasites with vertebrate hosts, including human hosts. Flukes live in the host’s circulatory system or liver. Tapeworms live in the host’s digestive system. Usually, more than one type of host is required to complete the parasite’s life cycle. Look at the life cycle of the liver fluke in Figure below. As an adult, the fluke has a vertebrate host. As a larva, it has an invertebrate host. If you follow the life cycle, you can see how each host becomes infected so the fluke can continue its life cycle.

Life Cycle of the Sheep Liver Fluke. The sheep liver fluke has a complicated life cycle with two hosts.

Tapeworms and flukes have suckers and other structures for feeding on a host. Tapeworms also have a ring of hooks on their head to attach themselves to the host (see Figure below). Unlike other invertebrates, tapeworms lack a mouth and digestive system. Instead, they absorb nutrients directly from the host’s digestive system with their suckers.

Tapeworm Suckers and Hooks. The head of a tapeworm has several suckers. At the very top of the head is a “crown” of hooks called a scolex.

Hold on just one second. Am I the only one who thinks that picture above is the stuff of nightmares???!!!!

Not all flatworms are parasites. Some are free-living carnivores. They eat other small invertebrates and decaying animals. Most of the free-living species live in aquatic habitats, but some live in moist soil.

Tapeworms video:


Roundworms make up the phylum Nematoda. This is a very diverse animal phyla. It has more than 80,000 known species.

Discovery TV: Monsters Inside Me – Deadly Roundworm

Structure and Function of Roundworms

Roundworms range in length from less than 1 millimeter to over 7 meters (23 feet) in length. As their name suggests, they have a round body. This is because they have a pseudocoelom. This is one way they differ from flatworms. Another way is their complete digestive system. It allows them to take in food, digest food, and eliminate wastes all at the same time.

Roundworms have a tough covering of cuticle on the surface of their body. It prevents their body from expanding. This allows the buildup of fluid pressure in the pseudocoelom. As a result, roundworms have a hydrostatic skeleton. This provides a counterforce for the contraction of muscles lining the pseudocoelom. This allows the worms to move efficiently along solid surfaces.

Roundworm Reproduction

Roundworms reproduce sexually. Sperm and eggs are produced by separate male and female adults. Fertilization takes place inside the female organism. Females lay huge numbers of eggs, sometimes as many as 100,000 per day! The eggs hatch into larvae, which develop into adults. Then the cycle repeats.

Ecology of Roundworms

Roundworms may be free-living or parasitic. Free-living worms are found mainly in freshwater habitats. Some live in soil. They generally feed on bacteria, fungi, protozoans, or decaying organic matter. By breaking down organic matter, they play an important role in the carbon cycle.

Parasitic roundworms may have plant, vertebrate, or invertebrate hosts. Several species have human hosts. For example, hookworms, like the one in Figure below, are human parasites. They infect the human intestine. They are named for the hooks they use to grab onto the host’s tissues. Hookworm larvae enter the host through the skin. They migrate to the intestine, where they mature into adults. Adults lay eggs, which pass out of the host in feces. Then the cycle repeats.

Hookworm Parasite. Hookworms like this one are common human parasites.

What? Is this the nightmare chapter? Who is putting these creepy pictures in here!? I think this picture is worse than the last! Hurry up and read, so you can scroll down and not have to look at that gaping mouth!

Tiny pinworms are the most common roundworm parasites of people in the U.S. In some areas, as many as one out of three children are infected. Humans become infected when they ingest the nearly microscopic pinworm eggs. The eggs hatch and develop into adults in the host’s digestive tract. Adults lay eggs that pass out of the host’s body to continue the cycle. Pinworms have a fairly simple life cycle with only one host.

Lesson Summary

  • Sponges are aquatic invertebrates. They make up the phylum Porifera. Sponges have specialized cells and an endoskeleton. They lack tissues and body symmetry. Adult sponges are sessile filter feeders. Sponge larvae have cilia for swimming.
  • Cnidarians include jellyfish and corals. They are aquatic invertebrates. They have tissues and radial symmetry. They also have tentacles with stingers. There are two cnidarian body plans: the polyp and the medusa. They differ in several ways. Many corals secrete an exoskeleton that builds up to become a coral reef.
  • Platyhelminths are flatworms such as tapeworms and flukes. They have a mesoderm cell layer and simple organ systems. They also show cephalization and bilateral symmetry. Many flatworms are parasites with vertebrate hosts. Some are free-living carnivores that live mainly in aquatic habitats.
  • Roundworms make up the phylum Nematoda. They have a pseudocoelom and hydrostatic skeleton. Their body is covered with tough cuticle. Free-living roundworms are found mainly in freshwater habitats. Parasitic roundworms have a variety of hosts, including humans.

Lesson Review Questions


1. Define sessile. Name an invertebrate with a sessile adult stage.

2. Describe the skeleton of a sponge.

3. Sponges have specialized cells called collar cells. Describe how collar cells are specialized for the functions they serve.

4. What is a nematocyst? What is its function?

5. How do coral reefs form?

6. Describe specialized feeding structures of parasitic platyhelminths.

7. How do free-living nematodes contribute to the carbon cycle?

Apply Concepts

8. Create a diagram of an adult sponge body plan that shows how sponges obtain food.

9. Apply what you know about pinworms to develop one or more recommendations for preventing pinworm infections in humans.

Think Critically

10. Compare and contrast cnidarian polyps and medusae.

11. Platyhelminths and nematodes are both worms. Justify classifying them in different invertebrate phyla.

12. Some parasitic flatworms have a very complicated life cycle with more than one host. Infer why this might be adaptive.

Points to Consider

In this lesson, you read about flatworms and roundworms. In the next lesson, you’ll read about worms called annelids. Mollusks such as snails are also described in the next lesson.

  • How are annelids different from flatworms and roundworms?
  • Why do you think annelids are placed in a lesson with mollusks instead of with flatworms and roundworms?

Previous: Overview of invertebrates

Next: Mollusks and Annelids

3 thoughts on “18.1 Sponges, Cnidarians, Flatworms, and Roundworms

  1. I deeply regret watching that National Geographic video…

    1. LOL, yeah it’s kind of gross/weird. God’s creation can certainly be interesting!

    2. Yeah… imagine having to be the narrator though…

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