The Animal Kingdom includes multicellular organisms that eat other organisms, develop from embryos, and have specialized cells for movement, sensing, digestion, or support. Animals range from simple sponges to insects, mollusks, fish, birds, and mammals. Studying animal diversity helps biologists understand evolution, body plans, ecosystems, and human biology.
A branching tree is useful because it shows how major animal groups are related through shared ancestry.
Key Facts
- Animals are multicellular, eukaryotic, heterotrophic organisms that usually digest food internally.
- Major animal phyla include Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Mollusca, Arthropoda, Echinodermata, and Chordata.
- Symmetry types include asymmetry, radial symmetry, and bilateral symmetry.
- Body cavity types include acoelomate, pseudocoelomate, and coelomate body plans.
- Most animals are bilaterians, meaning they have bilateral symmetry and a head to tail body axis.
- A simple diversity calculation is percent of total = group count / total count × 100.
Vocabulary
- Phylum
- A phylum is a large classification group that contains animals with a major shared body plan.
- Symmetry
- Symmetry describes how an animal body can be divided into matching parts.
- Coelom
- A coelom is a fluid filled body cavity completely lined by mesoderm tissue.
- Chordate
- A chordate is an animal that has, at least during development, a notochord, dorsal nerve cord, pharyngeal slits, and a post anal tail.
- Evolutionary tree
- An evolutionary tree is a diagram that shows relationships among groups based on common ancestry.
Common Mistakes to Avoid
- Calling every animal with a shell a mollusk is wrong because some shelled animals, such as certain arthropods, have exoskeletons rather than mollusk shells.
- Assuming all simple animals are primitive failures is wrong because sponges, cnidarians, and worms are successful modern lineages adapted to their environments.
- Confusing radial symmetry with bilateral symmetry is wrong because radial animals have body parts arranged around a central axis, while bilateral animals have left and right sides.
- Placing echinoderms near jellyfish because adults look radial is wrong because echinoderms are deuterostomes and are more closely related to chordates than to cnidarians.
Practice Questions
- 1 A survey finds 120 arthropod species, 30 mollusk species, 20 annelid species, and 10 chordate species in a habitat. What percent of the listed animal species are arthropods?
- 2 A classroom chart shows 9 major animal phyla. If 6 of them are primarily bilaterally symmetrical, what fraction and what percent of the phyla are primarily bilateral?
- 3 Explain why an evolutionary tree is a better overview of the Animal Kingdom than a simple list of phyla.