Anatomy and Physiology of Animals/Classification

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Objectives[edit | edit source]

After completing this section, you should know:

  • how to write the scientific name of animals correctly
  • know that animals belong to the Animal kingdom and that this is divided into phyla, classes, orders, families
  • know the definition of a species
  • know the phylum and class of the more common animals dealt with in this course

Classification is the process used by scientists to make sense of the 1.5 million or so different kinds of living organisms on the planet. It does this by describing, naming and placing them in different groups. As veterinary nurses you are mainly concerned with the Animal Kingdom but don’t forget that animals rely on the Plant Kingdom for food to survive. Also many diseases that animals are affected by are members of the other Kingdoms—fungi, bacteria and single celled animals.

Naming And Classifying Animals[edit | edit source]

There are more than 1.5 million different kinds of living organism on Earth ranging from small and simple bacteria to large, complex mammals. From the earliest time that humans have studied the natural world they have named these living organisms and placed them in different groups on the basis of their similarities and differences.

Naming Animals[edit | edit source]

Of course we know what a cat, a dog and a whale are but, in some situations using the common names for animals can be confusing. Problems arise because people in different countries, and even sometimes in the same country, have different common names for the same animals. For example a cat can be a chat, a Katze, gato, katt, or a moggie, depending on which language you use. To add to the confusion sometimes the same name is used for different animals. For example, the name ‘gopher’ is used for ground squirrels, rodents (pocket gophers), for moles and in the south-eastern United States for a turtle. This is the reason why all animals have been given an official scientific or binomial name. Unfortunately these names are always in Latin. For example:

  • Common rat: Rattus rattus
  • Human: Homo sapiens
  • Domestic cat: Felis domesticus
  • Domestic dog: Canis familiaris

As you can see from the above there are certain rules about writing scientific names:

  • They always have 2 parts to them.
  • The first part is the genus name and is always written with a capital first letter.
  • The second name is the species name and is always written in lower case.
  • The name is always underlined or printed in italics.

The first time you refer to an organism you should write the whole name in full. If you need to keep referring to the same organism you can then abbreviate the genus name to just the initial. Thus “Canis familiaris” becomes “C. familiaris” the second and subsequent times you refer to it.

Classification Of Living Organisms[edit | edit source]

To make some sense of the multitude of living organisms they have been placed in different groups. The method that has been agreed by biologists for doing this is called the classification system. The system is based on the assumption that the process of evolution has, over the millennia, brought about slow changes that have converted simple one-celled organisms to complex multi-celled ones and generated the earth’s incredible diversity of life forms. The classification system attempts to reflect the evolutionary relationships between organisms.

Initially this classification was based only on the appearance of the organism. However, the development of new techniques has advanced our scientific knowledge. The light microscope and later the electron microscope have enabled us to view the smallest structures, and now techniques for comparing DNA have begun to clarify still further the relationships between organisms. In the light of the advances in knowledge the classification has undergone numerous revisions over time.

At present most biologists divide the living world into 5 kingdoms, namely:

  • bacteria
  • protists
  • fungi
  • plants
  • animal

We are concerned here almost entirely with the Animal Kingdom. However, we must not forget that bacteria, protists, and fungi cause many of the serious diseases that affect animals, and all animals rely either directly or indirectly on the plant world for their nourishment.

The Animal Kingdom[edit | edit source]

So what are animals? If we were suddenly confronted with an animal we had never seen in our lives before, how would we know it was not a plant or even a fungus? We all intuitively know part of the answer to this.


  • eat organic material (plants or other animals)
  • move to find food
  • take the food into their bodies and then digest it
  • and most reproduce by fertilizing eggs by sperm

If you were tempted to add that animals are furry, run around on four legs and give birth to young that they feed on milk you were thinking only of mammals and forgetting temporarily that frogs, snakes and crocodiles, birds as well as fish, are also animals.

These are all members of the group called the vertebrates (or animals with a backbone) and mammals make up only about 8% of this group. The diagram on the next page shows the percentage of the different kinds of vertebrates.

Proportions of different kinds of vertebrate.JPG

However, the term animal includes much more than just the Vertebrates. In fact this group makes up only a very small portion of all animals. Take a look at the diagram below, which shows the size of the different groups of animals in the Animal Kingdom as proportions of the total number of different animal species. Notice the small size of the segment representing vertebrates! All the other animals in the Animal Kingdom are animals with no backbone, or invertebrates. This includes the worms, sea anemones, starfish, snails, crabs, spiders and insects. As more than 90% of the invertebrates are insects, no wonder people worry that insects may take over the world one day!

Fraction of vertebrates within the animal kingdom.jpg

The Classification Of Vertebrates[edit | edit source]

As we have seen above the Vertebrates are divided into 5 groups or classes namely:

  • Fish
  • Amphibia (frogs and toads)
  • Reptiles (snakes and crocodiles)
  • Birds
  • Mammals

These classes are all based on similarities. For instance all mammals have a similar skeleton, hair on their bodies, are warm bodied and suckle their young.

The class Mammalia (the mammals) contains 3 subclasses:

  • Duck billed platypus and the spiny anteater
  • Marsupials (animals like the kangaroo with pouches)
  • True mammals (with a placenta)

Within the subclass containing the true mammals, there are groupings called orders that contain mammals that are more closely similar or related, than others. Examples of six mammalian orders are given below:

  • Rodents (Rodentia) (rats and mice)
  • Carnivores (Carnivora) (cats, dogs, bears and seals)
  • Even-toed grazers (Artiodactyla) (pigs, sheep, cattle, antelopes)
  • Odd-toed grazers (Perissodactyla) (horses, donkeys, zebras)
  • Marine mammals (Cetacea) (whales, sea cows)
  • Primates (monkeys, apes, humans)

Within each order there are various families. For example within the carnivore mammals are the families:

  • Canidae (dog-like carnivores)
  • Felidae (cat-like carnivores)

Even at this point it is possible to find groupings that are more closely related than others. These groups are called genera (singular genus). For instance within the cat family Felidae is the genus Felis containing the cats, as well as genera containing panthers, lynxes, and sabre toothed tigers!

The final groups within the system are the species. The definition of a species is a group of animals that can mate successfully and produce fertile offspring. This means that all domestic cats belong to the species Felis domesticus, because all breeds of cat whether Siamese, Manx or ordinary House hold cat can cross breed. However, domestic cats can not mate successfully with lions, tigers or jaguars, so these are placed in separate species, e.g. Felis leo, Felis tigris and Felis onca.

Even within the same species, there can be animals with quite wide variations in appearance that still breed successfully. We call these different breeds, races or varieties. For example there are many different breeds of dogs from Dalmatian to Chihuahua and of cats, from Siamese to Manx and domestic short-hairs, but all can cross breed. Often these breeds have been produced by selective breeding but varieties can arise in the wild when groups of animals are separated by a mountain range or sea and have developed different characteristics over long periods of time.

To summarise, the classification system consists of:

The Animal Kingdom which is divided into

Phyla which are divided into

Classes which are divided into

Orders which are divided into

Families which are divided into

Genera which are divided into


Kings Play Cricket On Flat Green Surfaces” OR “Kindly Professors Cannot Often Fail Good Students” are just two of the phrases students use to remind themselves of the order of these categories - on the other hand you might like to invent your own.

Summary[edit | edit source]

  • The scientific name of an animal has two parts, the genus and the species, and must be written in italics or underlined.
  • Animals are divided into vertebrates and invertebrates.
  • The classification system has groupings called phyla, classes, orders, families, genera and species.
  • Furry, milk-producing animals are all in the class Mammalia.
  • Members within a species can mate and produce fertile offspring.
  • Sub-groups within a species include breeds, races and varieties.

Worksheet[edit | edit source]

Work through the exercises in this Classification Worksheet to help you learn how to write scientific names and classify different animals.

Test Yourself[edit | edit source]

1a) True or False. Is this name written correctly? trichosurus Vulpecula.

1b) What do you need to change?

2. Rearrange these groups from the biggest to the smallest:

a) cars | diesel cars | motor vehicles | my diesel Toyota | transportation
b) Class | Species | Phylum | Genus | Order | Kingdom | Family

Websites[edit | edit source]

Classification[edit | edit source]

In fact much more than that. There is an elementary cell biology and classification quiz but the best thing about this website are the links to tables of characteristics of the different animal groups, for animals both with and without backbones.

Careful! You could waste all day exploring this wonderful website. Chose an animal or group of animals you want to know about and you will see not only the classification but photos and details of distribution, behaviour and conservation status etc.

Nice clear explanation of the different categories used in the classification of animals.

Glossary[edit | edit source]