Animals: Overview

Big Picture

Animals are a group of living organisms with much variety. Despite their differences, animals have certain traits in com-mon. For example, all animals have a similar cell that does not include a cell wall. Their similarities show that animals descended from a common ancestor. Animals originated from chordates, which were organisms that have a notochord. Animals are classified into phyla, depending on characteristics such as having a vertebral column or not.

Key Terms

Animals: A kingdom of multicellular eukaryotes.

Invertebrates: Animals that lack a vertebral column.

Vertebrates: Animals that have a vertebral column.

Vertebral Column: Another name for backbone.

Exoskeleton: Non-bony skeleton that forms on the outside of the body.

Chordate: An animal with a notochord.

Notochord: A rigid rod that runs down the entire body.

Cranium: A bony skull that protects and encloses the brain.

Amniotes: Animals that produce eggs with internal membranes, which allow them to stay on land to reproduce as opposed to finding water.

Synapsids: One of the two groups that evolved from amniotes and eventually evolved into mammals.

Sauropsids: One of the two groups that evolved from amniotes. Sauropsids evolved into reptiles, dinosaurs, and birds.

Characteristics of Animals

Animals are heterotrophs because they do not make their own food or nutrients. They get their energy and nutrients from consuming other living things.

  • Can detect environmental stimuli through their sensory nerve cells
  • Can control movement at some stage during their life
  • Have an internal digestion system for food
  • Most animals spend most of their life as diploids
  • Most animals reproduce sexually
  • A sperm and an egg fuse to form a zygote, which develops into an embryo
  • The embryo develops into an adult

Animal Cells

Animal cells have a nucleus and other membrane-bound organelles, but unlike plants and fungi, they do not have a cell wall.
This is what gives animal cells flexibility to take on different shapes to complete different jobs.

Animal's Cell
Image credit: Mariana Ruiz Villarreal (LadyofHats),
Public Domain
Animals: Overview cont.

Classfication of Animals

There are more than 30 phyla under the animal kingdom, but below are the 9 largest (each phylum has more than 10,000 species).

  • Porifera - sponges
  • Cnidaria - jellyfish, corals
  • Platyhelminthes - flatworms, tapeworms, flukes
  • Nematoda - roundworms
  • Mollusca - snails, clams, squids
  • Annelida - earthworms, leeches, marine worms
  • Anthropoda - insects, spiders, crustaceans, centipedes
  • Echinodermata - sea stars, sea urchins, sand dollars, sea cucumbers
  • Chordata-tunicates, lancelets, fish, amphibians, reptiles, birds, mammal

Animals can be classified as invertebrates or vertebrates depending on the presence of a vertebral column.

  • 95% of all animals are invertebrates.
  • All vertebrates belong in the phylum Chordata. This includes fish, amphibians, reptiles, birds, and mammals.
Types of Vertebrates
Figure: Pie chart displaying the proportions of different kinds of vertebrates
Image Credit: Jin, CC-BY-NC-SA 3.0


Major Trends in Animal Evolution

  • Ancestor of animals thought to be protists that live in colonies in the ocean. Eventually, the cells specialized and needed each other to survive.
  • Early invertebrates developed exoskeleton that allowed them to move on land. The exoskeleton keeps the body from drying out.
  • Chordates with notochords evolved. The notochord supports the body, gives it shape, and provides a place for muscles to anchor. Chordates also have a hollow nerve cord that runs along the top of the body, and possibly gill slits and a tail.
  • Vertebrates evolved from chordates. The notochord was replaced with a backbone. They also developed a cranium.
  • Early vertebrates still needed water to reproduce. Amniotes evolved and removed the dependency on water. Two important amniote groups evolved:
  • Synapsids, which later evolved into mammals
  • Sauropsids, which later evolved into reptiles, dinosaurs, and birds.