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# 17.2: Overview of Invertebrates

Difficulty Level: At Grade Created by: CK-12

## Lesson 17.2: True or False

Name___________________ Class______________ Date________

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

_____ 1. An earthworm is an example of a segmented invertebrate.

_____ 2. Invertebrates with an incomplete digestive system starve, because their food cannot be completely digested.

_____ 3. A psuedocoelom refers to concentration of nervous tissue at one end of the animal.

_____ 4. Some invertebrates move, but cannot regulate which direction they move or how fast they move.

_____ 5. A larva is a juvenile stage of the life cycle.

_____ 6. A bilaterally symmetrical invertebrate has two identical left and right halves.

_____ 7. A radially symmetrical invertebrate can be divided into two similar halves by a line traversing the center of the animal.

_____ 8. Endoderm becomes muscle tissue.

_____ 9. All invertebrates reproduce by asexual budding.

_____ 10. Body segments make an animal less flexible.

_____ 11. Protostome and deuterostome are two main classes of invertebrates.

_____ 12. Some invertebrates have an internal skeleton.

_____ 13. The outside of an animal is formed from the ectoderm.

_____ 14. A partial coelom is called a psuedocoelom.

_____ 15. Complete digestion is less efficient than incomplete digestion because most animals eat too much food.

Name___________________ Class______________ Date________

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

Complete Digestive System

Early invertebrates had an incomplete digestive system. There was just one opening for the mouth and anus. Ancestors of modern roundworms were the first animals to evolve a complete digestive system. With a separate mouth and anus, food could move through the body in just one direction. This made digestion more efficient. An animal could keep eating while digesting food and getting rid of waste. Different parts of the digestive tract could also become specialized for different digestive functions. This led to the evolution of digestive organs.

Pseudocoelom and Coelom

Ancestors of roundworms also evolved a pseudocoelom. This is a partial body cavity that is filled with fluid. It allows room for internal organs to develop. The fluid also cushions the internal organs. The pressure of the fluid within the cavity provides stiffness. It gives the body internal support, forming a hydrostatic skeleton. It explains why roundworms are round and flatworms are flat. Later, a true coelom evolved. This is a fluid-filled body cavity, completely enclosed by mesoderm. It lies between the digestive cavity and body wall (see the figure below). Invertebrates with a true coelom include mollusks and annelids.

(Image courtesy of CK-12 Foundation and under the Creative Commons license CC-BY-NC-SA 3.0.)

Questions

1. Define complete digestive system. What are two advantages of having a complete digestive system?

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2. Define psuedocoelom.

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3. Define coelom.

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4. An organism gains what advantages by having a coelom?

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5. Do you think humans have a coelom? Why or why not?

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## Lesson 17.2: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

1. An example of an organism that has an incomplete digestive system is
1. a sea star.
2. a sponge.
3. a spider.
4. a sand dollar.
2. Animals who can control the direction they move in all have
1. a home in the ocean.
2. development as a deuterostome.
3. muscles.
3. Corals detect touch with
1. a nerve net.
2. a brain.
3. a complete digestive system.
4. a gamete.
4. ________ was one of the first animal traits to evolve.
1. cephalization
2. a complete digestive system
3. a notochord
4. multicellularity
5. Humans have
1. no symmetry.
3. bilateral symmetry.
4. none of the above.
6. The mesoderm is located
1. outside the ectoderm.
2. inside the endoderm.
3. in between the ectoderm and endoderm.
4. inside the gut.
7. The notochord evolved
1. before a bony backbone.
2. before multicellurity.
3. outside the animal's body.
4. none of the above.
8. Squids belong to the phylum
1. Porifera.
2. Nematoda.
3. Arthropoda.
4. none of the above

## Lesson 17.2: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.

Definitions

_____ 1. outer embryonic cell layer in animals

_____ 2. digestive system that consists of a digestive cavity and a single opening that serves as both mouth and anus

_____ 3. concentration of nerve tissue in one end of an animal, forming a head region

_____ 4. partial, fluid-filled cavity inside the body of some invertebrates

_____ 5. fluid-filled body cavity

_____ 6. inner embryonic cell layer in animals

_____ 7. division of an animal body into multiple segments

_____ 8. type of internal support in an animal body that results from the pressure of fluid within the body cavity known as the coelom

_____ 9. digestive system consisting of a digestive tract and two body openings (mouth and anus)

_____ 10. juvenile stage that occurs in the life cycle of many invertebrates, fish, and amphibians and that differs in form and function from the adult stage

_____ 11. embryonic cell layer in many animals that is located between the endoderm (inner cell layer) and ectoderm (outer cell layer)

_____ 12. symmetry of a body plan in which there are distinct head and tail ends, so the body can be divided into two identical right and left halves

Terms

a. bilateral symmetry

b. cephalization

c. coelom

d. complete digestive system

e. ectoderm

f. endoderm

g. hydrostatic skeleton

h. incomplete digestive system

i. larva

j. mesoderm

k. psuedocoelom

l. segmentation

## Lesson 17.2: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. Sea stars (starfish) have ________ symmetry.

2. ________ is the concentration of nerve tissue in one end of an animal, forming a head region.

3. A digestive system in which food enters and waste exits the same body opening is ________.

4. A false coelom (incomplete coelom) is called a ________.

5. In animals, the middle embryonic cell layer is the ________.

6. In animals, the inner embryonic cell layer is the ________.

7. In animals, the outer embyronic cell layer is the ________.

8. A ________ is a digestive system consisting of a digestive tract and two body openings.

9. ________ refers to division of an animal into multiple segments.

10. ________ symmetry is the type where the body can be divided into two identical right and left halves.

11. The ________ is a juvenile stage that occurs in the life cycle of many invertebrates, fish, and amphibians and that differs in form and function from the adult stage.

12. The support from a ________ results from the pressure of fluid within the body cavity known as the coelom.

## Lesson 17.2: Critical Writing

Name___________________ Class______________ Date________

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

Assume you have discovered a new invertebrate. It has multicellularity, specialized cells but no tissues, asymmetry, and an incomplete digestive system. In which phylum would you place it? Why?

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