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Animals are organisms classified in the kingdom Animalia.

in the diagram below, notice bacteria, fungi, plants – and animals. The term animal refers to a specific biological grouping, called a kingdom.

Within the animal kingdom, animals are divided into various sub-groups.
Vertebrates: animals with a backbone – birds, mammals, amphibians, reptiles (*), fish.

(*) Reptiles, well, they’re kind of not really a meaningful group anymore – 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 modern species including ragworms, earthworms, and leeches.
Mollusks – clams, oysters, octopuses, squid, snails
Arthropods – millipedes, centipedes, insects, spiders, scorpions, crabs, lobsters, shrimp
Sea sponges (not on the diagram below)
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
You’re an animal! The biological definition of “animal” includes all mammals, including humans. Specifically, you are in the mammal family.

classification of animals

Typical animal cell, in three dimensions


Here is a typical animal cell, as drawn in 2D on a Regents Exam.

1. lysosome, 2. endoplasmic reticulum, 3. chromosome (DNA)

4. golgi body (apparatus), 5. vacuole, 6. mitochondria

7. ribosome, 8. nucleolus, 9. nucleus

10. centrioles, 11. plasma membrane 12. cytoplasm

How are cell diagrams typically drawn:

#1 – generally, lysosomes are illustrated as “shaded-in” circles or ovals.

#2 – Transportation channels running through the cytoplasm

#3 – Chromosomes are made of genes, which are made of DNA nucleotides.

#4 – stacks of membranes (like pancakes)

#5 – an “empty” oval or circle

#6 – recognize mitochondria by the zig-zag line drawn in them. these are where respiration reactions occur.

#7 – very small dots, sometimes on the endoplasmic reticulum, sometimes out floating in cytoplasm

#8 – drawn as a dark circle inside the nucleus

#9 – generally depicted as the largest “shaded-in” circle inside the cell

#10 – drawn in pairs, two cylinder-like structures at right angles to each other

#11 – the outer boundary

#12 – the watery fluid that everything else floats in

Animals are characterized by being:

* multicellular

* eukaryotic

* Do not have rigid cell walls (those would be found in plants, or bacteria)

* body plan becomes fixed as they develop, although some undergo metamorphosis.

* motile (can move spontaneously and independently, at some point in their lives)

* heterotrophs – must eat other organisms, or their products, for sustenance.

* Nearly all animals have bodies differentiated into separate tissues. **

** except for the simplest animal-like organisms, e.g. sponges

sponge stat 5000 species comic

Learning Standards

Massachusetts Science and Technology/Engineering Curriculum FrameworkLife

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.

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