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18.2: Mollusks and Annelids

Difficulty Level: At Grade Created by: CK-12
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Lesson 18.2: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. Mollusks were the first vertebrates to evolve.

_____ 2. Earthworms are mollusks.

_____ 3. The hard outer shell of mollusks is made by the mantle.

_____ 4. Mollusks can be filter feeders.

_____ 5. The radula of mollusks excretes waste from the mollusk's body.

_____ 6. Mollusks have an incomplete digestive system with one opening.

_____ 7. Gills function to digest food.

_____ 8. Mollusks have one or two hearts.

_____ 9. Bivalves are typically very fast moving.

_____ 10. The phylum Annelida consists of unsegmented worms.

_____ 11. Some annelids live in the ocean.

_____ 12. Annelids have a true coelom.

_____ 13. Annelids have an open circulatory system.

_____ 14. Snails are annelids.

_____ 15. In some species of annelids, the same individual can make both female and male gametes.

Lesson 18.2: Critical Reading

Name___________________ Class______________ Date________

Read these passages from the text and answer the questions that follow.

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).


1. How large are annelids?

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2. Do annelids have a coelom?

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3. What kind of circulatory system do annelids have?

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4. How do annelids exchange gases with their environment?

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5. What is the function of gills?

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Lesson 18.2: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

  1. Which of the following is not a mollusk?
    1. hydra
    2. clam
    3. snail
    4. squid
  2. The outer shell of mollusks is made of
    1. chitin.
    2. spicules.
    3. calcium carbonate.
    4. muscle.
  3. Wastes are excreted from mollusks via the
    1. outer shell.
    2. nerve fibers.
    3. radula.
    4. nephridia.
  4. Which of the following classes of mollusks has a closed circulatory system?
    1. bivalves
    2. cephalopods
    3. gastropods
    4. poriferans
  5. A trochophore is
    1. an excretory organ of annelids.
    2. an excretory organ of mollusks.
    3. a larval form of mollusks.
    4. a larval form of annelids.
  6. The class of mollusks that contains only ocean-living organisms is
    1. bivalves.
    2. cephalopods.
    3. gastropods.
    4. segmented worms.
  7. The ability of segmented worms to replace broken off segments by growing new ones is called
    1. regeneration.
    2. suckers.
    3. tentacles.
    4. degeneration.
  8. An example of a deposit feeder is __________.
    1. a squid
    2. a clam
    3. a polychaete
    4. an earthworm

Lesson 18.2: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.


_____ 1. regrowing of tissues, organs, or limbs that have been lost or damaged

_____ 2. type of circulatory system in which blood flows only within a network of blood vessels and not through body cavities

_____ 3. an excretory organ in mollusks and annelids, has functions similar to a kidney

_____ 4. mollusks having two shells hinged together

_____ 5. invertebrate phylum of segmented worms, such as earthworms

_____ 6. 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

_____ 7. muscular organ that that pumps blood through blood vessels when it contracts

_____ 8. a class of mollusk with a brain and a closed circulatory system

_____ 9. phylum of invertebrates that are generally characterized by a hard outer shell, a mantle, and a feeding organ called a radula

_____ 10. a class of mollusks with muscular food and typically one shell

_____ 11. type of circulatory system in which blood flows not only through blood vessels but also through a body cavity

_____ 12. organ in aquatic organisms composed of thin filaments that absorb oxygen from water


a. Annelida

b. bivalve

c. cephalopod

d. closed circulatory system

e. gastropod

f. gill

g. heart

h. mantle

i. Mollusca

j. nephridia

k. open circulatory system

l. regeneration

Lesson 18.2: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. The ability to replace a damaged or removed body part is called __________.

2. A circulatory system in which the blood enters the body cavity is a(n) __________.

3. A circulatory system in which the blood stays within blood vessels is a(n) __________.

4. A mollusk with a single shell and a muscular foot is a(n) __________.

5. A __________ gets nutrients by eating soil or sediments at the bottom of a body of water.

6. __________ is the phylum containing segmented worms.

7. The phylum __________ is comprised of gastropods, bivalves, and cephalopods.

8. Mollusks and annelids have excretory organs similar in function to kidneys called __________.

9. The __________ pumps blood throughout the body.

10. The organ of gas exchange in mollusks and annelids is the __________.

11. A mollusk with two hinged shells is a __________.

12. The __________ is a layer of tissue between the shell and body of a mollusk and forms a cavity that pumps water for filter feeding.

Lesson 18.2: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Describe the structure and function of the mantle and mantle cavity in a mollusk.

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