Lab: The Origin and Evolution of Animals
Lab Overview
Download and print the lab before you begin. Complete the required work on the printed pages, using the illustrations on this webpage as visual guides for the drawings and labels.
Your drawings do not need to match the illustrations exactly. Focus on the overall form and the important structures. Drawing requires you to actively identify shapes, relationships, and labels instead of simply looking at them, which helps you learn and remember the organisms more effectively.
Drawings are evaluated for completion, not artistic quality. The phylogeny exercises are evaluated for accuracy.
The phylogeny exercises require you to apply what you learn throughout the lab. Read the lab text carefully and use the organisms, structures, and evolutionary traits you encounter to determine where each characteristic belongs on the phylogenies. The completed phylogenies are not provided on this webpage.
Learning Goals
- Recognize major animal groups and the body features used to distinguish them.
- Connect animal body plans and developmental traits with evolutionary relationships.
- Use morphological characteristics as synapomorphies when interpreting an animal phylogeny.
Jump to Exercise
Exercise A: The Origin of Animals
Animals are multicellular opisthokonts, and choanoflagellates are their closest living relatives. In this exercise, you will compare single and colonial choanoflagellates and use a phylogeny to examine characteristics shared by major opisthokont groups.
Step 1. Draw a unicellular choanoflagellate.
Step 2. Draw a colony of choanoflagellates.
Step 3. Label the synapomorphies.
A synapomorphy is a shared characteristic that helps identify a branch of a phylogeny. Use the descriptions in your printed lab to compare Fungi, Choanozoa, and Animalia. For each branch, identify which group or groups descend from it, then choose the characteristic from the word bank that matches all of those descendants.
Fill all five boxes before checking your answers.
Exercise B: Porifera
Sponges are multicellular filter-feeding animals with porous bodies that move water through internal spaces. Specialized cells called choanocytes line parts of the internal water-flow system and help capture food from the water.
Step 4a. Draw a sponge.
Step 4b. Draw a cross section of a sponge. Label the choanocytes.
Exercise C: Diploblastic Animals
Science Note
The printed lab introduces Cnidaria and Ctenophora together as animals with a primarily two-layered developmental organization. Current evidence treats Cnidaria and Ctenophora as separate evolutionary lineages, and their developmental biology is not identical.
For this exercise, focus on comparing their body forms, symmetry, feeding structures, and nervous organization while using the terminology and sequence provided in your printed lab.
Phylum Cnidaria
Cnidarians include sea anemones, corals, hydras, and jellyfishes. Their body plans are organized around a central oral–aboral axis, and they capture prey with specialized cells called cnidocytes.
5. Draw a sea anemone.
Step 6. Draw a jellyfish from top-down highlighting its radial symmetry.
Step 7. Draw a representation of a nerve net in a hydra.
8. Draw a coral.
Step 9. Draw a comb jelly.
Step 10. Draw a triploblastic blastula.
Step 11. Draw a representation of bilateral symmetry.
Step 12. Draw cephalization. Label a ganglium and a nerve tract.
Exercise E: Acoelomorpha
Acoelomorphs are small, early-diverging bilaterians with flattened, unsegmented bodies and no coelom. Many live in marine sediments and move using cilia along the outer surface.
Science Note
The printed lab describes Acoelomorpha as “primitive.” Modern biology avoids this wording because living lineages are not intermediate stages on a ladder of evolution.
Acoelomorphs are better understood as an early-diverging bilaterian lineage with their own evolutionary history. Their exact position near the base of Bilateria is still investigated.
Step 13. Draw an acoelomorphate.
Exercise F: Coelomates (Protostomes and Deuterostomes)
This exercise compares the classic developmental patterns used in the printed lab to distinguish protostomes and deuterostomes. Focus on two features: what the blastopore becomes and how the coelom forms during development.
Step 14. Draw protostome and deuterostomes development of the blastopore and coelom.
Exercise G: Protostomes Overview
Protostomes include several major evolutionary lineages with very different body plans. In this exercise, the printed lab uses a clam and a molting insect to compare two examples of growth: adding material to an existing shell and shedding an external covering during molting.
Science Note
The printed lab describes incremental skeletal growth as a characteristic of all Lophotrochozoa. That is too broad. Lophotrochozoa contains many animals with very different body plans and methods of growth, including animals that do not have a rigid skeleton or shell.
The clam in this exercise should therefore be treated as an example of shell growth, not as a growth pattern that defines all lophotrochozoans. In contrast, molting (ecdysis) is a defining feature of Ecdysozoa.
Step 15. Draw a clam. Label its growth rings.
Step 16. Draw a molting insect. Label its exoskeleton.
Exercise H: Protostomes – Lophotrochozoa
This exercise surveys several very different protostome body plans. As you work through the drawings, focus on the structures that distinguish each group, including feeding structures, body shape, segmentation, and the characteristic features of molluscs.
Phylum Bryozoa and Phylum Rotifera
Bryozoans and rotifers both use cilia to move food toward the mouth, but their feeding structures are different. Bryozoans use a lophophore, a crown of ciliated tentacles, while rotifers use a ciliated corona at the anterior end of the body.
Phylum Platyhelminthes
Flatworms have broad, flattened, unsegmented bodies. In this section, compare a free-living planarian with two parasitic forms: tapeworms and flukes.
