Home » Animals » What are animals

What are animals

Start here


What are we learning?

  Classification of animals

  Characteristics of animals

  The major animal groups

Why are we learning this?

The study of animals is essential to the understanding of life on Earth. Animals are one of the many branches of earth’s life.

Animal kingdom is just one part of the tree of life

We see it here on the far right.

Animals include mammals, including humans, insects, birds, fish and more.

Phylogenetic Tree of Life by Ciccarelli in March 2006 Science

Image by Madeleine Price Ball. Simplified universal phylogenetic tree, made using information from the Interactive Tree of Life. Ciccarelli, et al., Mar 3 2006, Science Vol. 311

Characteristics of animals

Multicellular – animals are made of many cells.

Animals are differentiated into separate tissues. *

* except for the simplest forms, e.g. sea sponges.

Eukaryotic – cells have a nucleus, and many organelles.

Each organelle has its own job.


Cell have flexible cell membranes

(only plants and bacteria have rigid cell walls)

Cell membrane lipid bilayer animation

Animals have a body plan that becomes fixed as they develop.
It’s not just random growth of cells.

Animals are motile – they can move (as opposed to plants, which can’t)

penguin low friction GIF

Animals are heterotrophs – they must eat other organisms for sustenance.

Autotroph Heterotroph

Classification of animals

Animals are divided into sub-groups.

classification of animals

Vertebrates: animals with a backbone

birds, mammals, amphibians, reptiles (*), fish.

(*) Reptiles, well, they’re kind of not really a meaningful group – we’ll learn about that later.

Invertebrates: animals without a backbone

Coelenterata – comb jellies, coral animals, true jellies (“jellyfish), sea anemones, etc.

Flatworms – Planarians, flukes and tapeworms

Annelids – over 17,000 species including ragworms, earthworms, and leeches.

Mollusks – clams, oysters, octopuses, squid, snails

Arthropods – millipedes, centipedes, insects, spiders, scorpions, crabs, lobsters, shrimp

Arachnids – 100,000 species of spiders, scorpions, ticks, mites, etc.

Crustacean – 17,000 species of crabs, lobsters, crayfish, shrimp, krill and barnacles.

Insects – over a million different species!

Myriapoda – Over 13,000 species of centipedes and millipedes

Sea sponges (not shown on the diagram above)


Learning Standards

Massachusetts Science and Technology/Engineering Curriculum Framework

Life Science (Biology), Grades 6–8.
Classify organisms into the currently recognized kingdoms according to characteristics that they share. Be familiar with organisms from each kingdom.

Biology, High School
5.2 Describe species as reproductively distinct groups of organisms. Recognize that species are further classified into a hierarchical taxonomic system (kingdom, phylum, class, order, family, genus, species) based on morphological, behavioral, and molecular similarities.

Benchmarks for Science Literacy, American Association for the Advancement of Science

Students should begin to extend their attention from external anatomy to internal structures and functions. Patterns of development may be brought in to further illustrate similarities and differences among organisms. Also, they should move from their invented classification systems to those used in modern biology… A classification system is a framework created by scientists for describing the vast diversity of organisms, indicating the degree of relatedness between organisms, and framing research questions.

SAT Biology Subject Area Test

Evolution and diversity: Origin of life, evidence of evolution, patterns of evolution, natural selection, speciation, classification and diversity of organisms.

Teaching About Evolution and the Nature of Science, National Academy Press (1998)

Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities which reflect their evolutionary relationships. Species is the most fundamental unit of classification.

%d bloggers like this: