18.2 Mollusks and Annelids

Lesson Objectives

  • Describe invertebrates in the phylum Mollusca.
  • Summarize the characteristics of annelids.


  • Annelida
    • invertebrate phylum of segmented worms such as earthworms
  • deposit feeder
    • animal that obtains organic matter for nutrition by eating soil or the sediments at the bottom of a body of water
  • gills
    • organs in aquatic organisms composed of thin filaments that absorb oxygen from water
  • heart
    • muscular organ in the chest that that pumps blood through blood vessels when it contracts
  • mantle
    • layer of tissue that lies between the shell and body of a mollusk and forms a cavity, called the mantle cavity, that pumps water for filter feeding
  • Mollusca
    • phylum of invertebrates that are generally characterized by a hard outer shell, a mantle, and a feeding organ called a radula
  • regeneration
    • regrowing of tissues, organs, or limbs that have been lost or damaged


Mollusks are invertebrates such as the common snail. Most mollusks have shells. Annelids are worms such as the familiar earthworm. They have segmented bodies. Annelids look like roundworms on the outside, but on the inside they are more like mollusks.


Have you ever been to the ocean or eaten seafood? If you have, then you probably have encountered members of the phylum Mollusca. Mollusks include snails, scallops, and squids, as shown in Figure below. There are more than 100,000 known species of mollusks. About 80 percent of mollusk species are gastropods.

This figure shows some of the more common and familiar mollusks.

Structure and Function of Mollusks

Mollusks are a very diverse phylum. Some mollusks are nearly microscopic. The largest mollusk, a colossal squid, may be as long as a school bus and weigh over half a ton! The basic body plan of a mollusk is shown in Figure below. The main distinguishing feature is a hard outer shell. It covers the top of the body and encloses the internal organs. Most mollusks have a distinct head region. The head may have tentacles for sensing the environment and grasping food. There is generally a muscular foot, which may be used for walking. However, the foot has modifications in many species to be used for other purposes.

Basic Mollusk Body Plan. The basic body plan shown here varies among mollusk classes. For example, several mollusk species don’t have shells. Do you know which ones?

Two unique features of mollusks are the mantle and radula (see Figure above). Themantle is a layer of tissue that lies between the shell and the body. It secretes calcium carbonate to form the shell. It forms a cavity, called the mantle cavity, between the mantle and the body. The mantle cavity pumps water for filter feeding. The radula is a feeding organ with teeth made of chitin. It is located in front of the mouth in the head region. Herbivorous mollusks use the radula to scrape food such as algae off rocks. Predatory mollusks use the radula to drill holes in the shells of their prey.

Mollusks have a coelom and a complete digestive system. Their excretory system consists of tube-shaped organs called nephridia (see Figure above). The organs filter waste from body fluids and release the waste into the coelom. Terrestrial mollusks exchange gases with the surrounding air. This occurs across the lining of the mantle cavity. Aquatic mollusks “breathe” under water with gills. Gills are thin filaments that absorb gases and exchange them between the blood and surrounding water. Mollusks have a circulatory system with one or two hearts that pump blood. The heart is a muscular organ that pumps blood through the circulatory system when its muscles contract. The circulatory system may be open or closed, depending on the species.

The major classes of mollusks vary in structure and function. You can read about some of their differences in Figure below.

Use this figure to compare and contrast gastropods, bivalves, and cephalopods

Mollusk Reproduction

Mollusks reproduce sexually. Most species have separate male and female sexes. Gametes are released into the mantle cavity. Fertilization may be internal or external, depending on the species. Fertilized eggs develop into larvae. There may be one or more larval stages. Each one is different from the adult stage. Mollusks (and annelids) have a unique larval form called a trochophore. It is a tiny organism with cilia for swimming.

Ecology of Mollusks

Mollusks live in most terrestrial, freshwater, and marine habitats. However, the majority of species live in the ocean. They can be found in both shallow and deep water and from tropical to polar latitudes. Mollusks are a major food source for other organisms, including humans. You may have eaten mollusks such as clams, oysters, scallops, or mussels. The different classes of mollusks have different ways of obtaining food.

  • Gastropods are may be herbivores, predators, or internal parasites. They live in both aquatic and terrestrial habitats. Marine species live mainly in shallow coastal waters. Gastropods use their foot to crawl slowly over rocks, reefs, or soil, looking for food.
  • Bivalves are generally sessile filter feeders. They live in both freshwater and marine habitats. They use their foot to attach themselves to rocks or reefs or to burrow into mud. Bivalves feed on plankton and nonliving organic matter. They filter the food out of the water as it flows through their mantle cavity.
  • Cephalopods are carnivores that live only in marine habitats. They may be found in the open ocean or close to shore. They are either predators or scavengers. They generally eat other invertebrates and fish.

Hands on Activity:

Cut & Assemble Paper Clam
KQED: Cool Critters: Dwarf Cuttlefish

What’s the coolest critter in the ocean under 4 inches long? The Dwarf Cuttlefish! Cuttlefish are marine animals that belong to the class Cephalopoda. Despite their name, cuttlefish are not fish but molluscs. Recent studies indicate that cuttlefish are among the most intelligent invertebrates, with one of the largest brain-to-body size ratios of all invertebrates. Cuttlefish have an internal shell called the cuttlebone and eight arms and two tentacles furnished with denticulated suckers, with which they secure their prey. For more information on the cuttlefish, see http://www.kqed.org/quest/television/cool-critters-dwarf-cuttlefish.

KQED: The Fierce Humboldt Squid

The Humboldt squid is a large, predatory invertebrate found in the waters of the Pacific Ocean. A mysterious sea creature up to 7 feet long, with 10 arms, a sharp beak and a ravenous appetite, packs of fierce Humboldt Squid attack nearly everything they see, from fish to scuba divers. Traveling in groups of 1,000 or more and swimming at speeds of more than 15 miles an hour, these animals hunt and feed together, and use jet propulsion to shoot out of the water to escape predators. Humboldt squid live at depths of between 600 and about 2,000 feet, coming to the surface at night to feed. They live for approximately two years and spend much of their short life in the ocean’s oxygen-minimum zone, where very little other life exists. Because they live at such depths, little is known about these mysterious sea creatures. The Humboldt squid usually lives in the waters of the Humboldt Current, ranging from the southern tip of South America north to California, but in recent years, this squid has been found as far north as Alaska. Marine biologists are working to discover why they have headed north from their traditional homes off South America. See http://www.kqed.org/quest/television/the-fierce-humboldt-squid for additional information.

 Where’s the Octopus?

When marine biologist Roger Hanlon captured the first scene in this video he started screaming. Hanlon, senior scientist at the Marine Biological Laboratory in Woods Hole, studies camouflage in cephalopods–squid, cuttlefish and octopus. They are masters of optical illusion. The video at http://www.sciencefriday.com/videos/watch/10397 shows some of Hanlon’s top video picks of sea creatures going in and out of hiding.


The phylum Annelida is made up of segmented worms such as earthworms. Segmented worms are divided into many repeating segments. There are roughly 15,000 species of annelids. Most belong to one of three classes. A species in each class is pictured in Figure below.

Classes of Annelids. The majority of annelids are polychaetes. They live on the ocean floor, so you may not be familiar with them.

Structure and Function of Annelids

Annelids range in length from less than 1 millimeter to over 3 meters. They never attain the large size of some mollusks. Like mollusks, however, they have a coelom. In fact, the annelid coelom is even larger, allowing greater development of internal organs. Annelids have other similarities with mollusks, including:

  • A closed circulatory system (like cephalopods).
  • An excretory system consisting of tubular nephridia.
  • A complete digestive system.
  • A brain.
  • Sensory organs for detecting light and other stimuli.
  • Gills for gas exchange (but many exchange gas through their skin).

The segmentation of annelids is highly adaptive. For one thing, it allows more efficient movement. Each segment generally has its own nerve and muscle tissues. Thus, localized muscle contractions can move just those segments needed for a particular motion. Segmentation also allows an animal to have specialized segments to carry out particular functions. This allows the whole animal to be more efficient. Annelids have the amazing capacity to regrow segments that break off. This is called regeneration.

Annelids have a variety of structures on the surface of their body for movement and other functions. These vary, depending on the species. Several of the structures are described in Figure below.

Annelid External Structures. Many annelids have bristles and other types of external structures. Each structure is not present in all species.

Annelid Reproduction

Most species of annelids can reproduce both asexually and sexually. However, leeches can reproduce only sexually. Asexual reproduction may occur by budding or fission. Sexual reproduction varies by species.

  • In some species, the same individual produces both sperm and eggs. But worms mate to exchange sperm, rather than self-fertilizing their own eggs. Fertilized eggs are deposited in a mucous cocoon. Offspring emerge from the cocoon looking like small adults. They grow to adult size without going through a larval stage.
  • In polychaete species, there are separate sexes. Adult worms go through a major transformation to develop reproductive organs. This occurs in many adults at once. Then they all swim to the surface and release their gametes in the water, where fertilization takes place. Offspring go through a larval stage before developing into adults.

Ecology of Annelids

Annelids live in a diversity of freshwater, marine, and terrestrial habitats. They vary in what they feed on and how they obtain their food.

  • Earthworms are deposit feeders. They burrow through the ground, eating soil and extracting organic matter from it. Earthworm feces, called worm casts, are very rich in plant nutrients. Earthworm burrows help aerate soil, which is also good for plants.
  • Polychaete worms live on the ocean floor. They may be sedentary filter feeders or active predators or scavengers. Active species crawl along the ocean floor in search of food.
  • Leeches are either predators or parasites. As predators, they capture and eat other invertebrates. As parasites, they feed off the blood of vertebrate hosts. They have a tubular organ, called a proboscis, for feeding.

Lesson Summary

  • Mollusks are invertebrates such as snails, scallops, and squids. They have a hard outer shell. There is a layer of tissue called the mantle between the shell and the body. Most mollusks have tentacles for feeding and sensing, and many have a muscular foot. Mollusks also have a coelom, a complete digestive system, and specialized organs for excretion. The majority of mollusks live in the ocean. Different classes of mollusks have different ways of obtaining food.
  • Annelids are segmented worms such as earthworms and leeches. Annelids have a coelom, closed circulatory system, excretory system, and complete digestive system. They also have a brain. Earthworms are important deposit feeders that help form and enrich soil. Leeches are either predators or parasites. Parasitic leeches feed off the blood of vertebrate hosts.

Lesson Review Questions


1. Describe the basic body plan of a mollusk.

2. What are gills? What is their function?

3. What is the difference between an open and a closed circulatory system?

4. What is a radula? What is it used for?

5. Define regeneration.

Apply Concepts

6. Create a Venn diagram to show important similarities and differences among the three major classes of mollusks.

Think Critically

7. Explain the advantages of a segmented body.

8. Polychaete worms have an interesting reproductive strategy. Describe this strategy.

Points to Consider

Most invertebrates you have read about so far live in aquatic habitats. Many of those that are not aquatic live inside other organisms as parasites. In the next lesson you will read about invertebrates that live mainly on land. They are the arthropods, such as insects.

  • Compared with aquatic invertebrates, what challenges do you think terrestrial invertebrates might face?
  • How might terrestrial invertebrates meet these challenges? What special tissues, organs, or appendages help them live life on land?

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