Name 	   Class 	    Date 	

25.2 Animal Body Plans and Evolution
Lesson Objectives
Discuss some trends in animal evolution.
Explain the differences among the animal phyla.
Lesson Summary
Features of Body Plans Each animal phylum has a unique organization of body structures called its “body plan.” The features of a body plan include
levels of organization: cells, tissues, organs, organ systems
body symmetry:
•	radial symmetry: body parts extend from a central point
•	bilateral symmetry: left and right sides are mirror images, with front and back ends
differentiation of germ layers:
•	endoderm, the innermost layer
•	mesoderm, the middle layer
•	ectoderm, the outermost layer
formation of a cavity, or fluid-filled space between the digestive tract and the body wall:
•	a true coelom (found in most complex animal phyla) develops in the mesoderm and is lined with tissue derived from the mesoderm
•	a pseudocoelom is only partially lined with mesoderm
•	Some invertebrates lack a body cavity and some have only a primitive, jellylike layer between the ectoderm and endoderm.
patterns of embryological development
•	Sexually reproducing animals begin life as a zygote, or fertilized egg.
•	The zygote develops into a hollow ball of cells, the blastula.
•	The blastula folds in on itself and creates a tube that becomes the digestive tract; the tube has a single opening, the blastopore:
In protostomes (most invertebrates), the blastopore becomes the mouth.
In deuterostomes (chordates and echinoderms), the blastopore becomes the anus.
segmentation: repeated parts, such as the segments of worms
cephalization: the concentration of sense organs and nerves near the anterior (head) end
limb formation: external appendages such as legs, flippers, and wings

The Cladogram of Animals The features of body plans provide the evidence needed to build a cladogram, or phylogenetic tree, of all animals. Animal phyla are usually defined by their adult body plans and patterns of embryological development.
The characteristics of animals vary within each phylum.
Each phylum may be thought of as an “evolutionary experiment.” Phyla with successful body plans have survived.

Notes:

Features of Body Plans
1.	Complete the table of main ideas and details about animal body plans. Use the boxes to list and summarize the features of animal body plans.

Features of Body Plans	
Main Idea: Feature of Body Plan	Details: Important structures or patterns 
of development	
Levels of organization		
	None, radial, or bilateral	
Germ layers		
Body cavity		
Patterns of embryological development		
	Repeated parts, such as the segments of worms	
Cephalization		
Limb formation		

THINK VISUALLY

2. 	Sketch two common objects that show the difference between radial symmetry and bilateral symmetry. Label your sketches and explain them.

  3. What type of symmetry do you have?
  _____________________________________________
4.	
THINK VISUALLY

Label each of these organisms with the kind of symmetry it exhibits.

	

	

	

	

5. Label the diagram showing the difference between deuterostomes and protostomes. Label the following structures: blastula, blastopore, ectoderm, endoderm, mesoderm, mouth, protostome.
For Questions 6–14, complete each statement by writing the correct word or words.
6. Deuterostomes that show radial symmetry in their adult form are called 	.
7. 		 are bilaterally symmetrical animals with three germ layers and no coelom.
8. 		 are protostomes with a true coelom and cephalization without segmentation.
9. Members of the 	 phylum have no body symmetry.
10. Animals in the 	 phylum have specialized cells and tissues, but no organs.
11. Both	 and	 are segmented protostomes with bilateral symmetry.
12. In addition to echinoderms, 	 are also deuterostomes.
13. An important way in which the body plan of mollusks differs from that of arthropods is that mollusks lack	.
14. Only members of the 	 phylum have a pseudocoelom.

The Cladogram of Animals
Use the cladogram to answer Questions 15–17.
15. On the lines provided, label the names of the animal phyla that correspond to the letters in  the diagram.
a. 	 	 d. 	 	g. 	
b. 	 	 e. 	 	h. 	
c. 	 	 f. 	 	i. 	
16. What characteristics define the branch of the cladogram that leads to the mollusks?

17. Is a chordate a “better” animal than a sponge? Explain your answer.

Apply the Big idea

	
18. Cows, hawks, and whales are all vertebrates. However, their forelimbs are noticeably different. Explain how “evolutionary experiments” that yield variations on a body plan have produced such diversity among vertebrates. Use forelimb structures as an example.