How Animals Are Classified

Introduction

Same and different are simple words, but what an amazing impact they have had on the world of science. Scientists call the process of sorting living things into groups taxonomy, or classification.

Classifying living things makes it possible for biologists to communicate their findings in a precise way when studying living things. It also helps us explain evolutionary processes and changes in ecological communities. All living things, including the many different life-forms that exist in this pond, possess certain characteristics which distinguish them from nonliving things. For one thing, living things are capable of reproduction and growth. They are also able to use food to provide energy.

Living things can be divided into groups that possess similar features. Since earliest times, the most fundamental way to divide living things has been between plants and animals. However, one system of classification used by many scientists today recognizes not two, but five, distinct groups of living things called kingdoms. It is this system of five-kingdom classification that we'll be discussing in this presentation.

Each of these five kingdoms, Monera, Protista, Fungi, Plantae, and Animalia, contains organisms that are grouped together because of common characteristics.

The simplest living organisms are the monerans. This group includes bacteria and primitive blue-green algae. Second in complexity are the protists, which are single-celled or simple multi-celled creatures that may be both plantlike and animal like. Third are fungi, such as mushrooms and penicillium. Fourth are the huge variety of plants that cover much of the earth. Fifth, and finally, are the multitude of animals that live in the depths of the ocean, soar through the sky, and roam the far reaches of the planet.

Each of the five kingdoms is subdivided into many smaller groups. These groups are called phyla. Each phylum contains several related groups called classes, and so on. Within each subgroup, organisms become increasingly more similar to one another than between groups.

What are Animals?

In this presentation we will focus on how members of the kingdom Animalia, the animals, are classified.

The process of classifying living things can be complicated. For instance, even distinguishing between plants and animals can be difficult. We know that plants seem to be fixed to one spot whereas animals move about, and that plants can manufacture their own food while animals must seek out food that they can eat.

However, while these two characteristics help set apart most plants from animals, there are occasional exceptions. For instance, here is an animal-eating pitcher plant. Or how about some of these tiny, microscopic creatures known as euglena? Euglena resemble plants in that they possess chloroplasts by which they can make their own food. If euglenoids are maintained in darkness, they can lose their chloroplasts and must survive by absorbing nutrients from their surroundings.

Should we classify euglenoids as plants or animals? And how should we classify the sea anemone and corals, which are plant-like animals that do not move about from place to place? These are the sorts of problems scientists face when they try to classify living things. Scientists have developed many different systems for classifying living things. Even today the guidelines for classification seem to change constantly.

The History of Taxonomy

One of the earliest attempts at animal classification was made by the Greek philosopher Aristotle 2,300 years ago. Aristotle based his system on similarities of anatomy and life history. However, the basis for today's systems of classification was developed in the 18th century by Swedish naturalist Carolus Linnaeus.

Linnaeus studied various animals, such as these sporozoans, and arranged them in groups according to structural similarities. Starting with the most generalized characteristics, Linnaeus gradually worked his way toward the specific as he grouped various organisms. In this way, all animals, from microscopic ones to the elephant, were assigned two names.

One name, such as Equus, identifies a group of animals with similar characteristics and is called genus. A second name, in the case of this horse, caballus, refers to a specific member, or species, of that group. Scientific names often have both Latin and ancient Greek origins. Many educated people of the 18th and 19th centuries could read and speak these languages, and we still use these words today for classification purposes. In this way, an animal known regionally by one name can also be recognized immediately by its scientific name. In England and the United States, this is a horse; in Germany, it's called a pferd; and in Mexico, a caballo. However, to scientists, no matter where they live, this animal is identified as Equus caballus.

Families Genera and Species

To distinguish horses from other hoofed mammals that also possess an uneven number of toes, the members of the order Perissodactyla are classified into smaller groups called families. The family Equidae contains the zebra, donkey, and horse, as well as a number of extinct animals. To separate the modern members of the family Equidae from those which existed in the distant past, horses, zebras, and donkeys are classified within the genus Equus.

There are obvious physical differences between these members of the genus Equus. These differences resulted in one final division, the last grouping called species. Because of its longer hair and mane, larger hooves, shorter ears, and overall size, the horse is placed in the species caballus. Subspecies, or varieties, of horse are determined by looking at even more characteristics of a given breed.

Kingdoms and Phyla

Let us now see how the horse came to be classified as Equus caballus, using the modern techniques of grouping and naming animals.

Scientists define animals in general as multicellular organisms composed of cells with membrane-bound nuclei. What animals don't have are cells with a rigid cell wall or chloroplasts. Such cell features are found only in plants. By this definition, the horse is clearly a member of the kingdom Animalia.

All animals are further divided into large groups known as phyla, which is plural for "phylum." In one widely used system, 22 phyla exist in the animal kingdom. Animals that do not have an internal supporting structure, such as a spinal column or a notochord, belong to one of several phyla of invertebrates. The horse belongs to the phylum Chordata because, like all members of this group, it possesses a backbone or notochord.

A notochord is a stiff rod of cartilage that supports the body. Like tadpoles, horses and other members of the phylum Chordata possess notochords only during an embryonic stage of development. By the time these animals mature, the notochord has been replaced by a bony vertebral column. A few chordates, such as sea squirts, develop notochords in their larva stages, but as adults possess neither a notochord nor a vertebral column. In addition to a supporting notochord, chordates have a spinal cord of nerves and, at some time in their life, usually at the embryonic stage, have something like gill slits, as seen in this human embryo.

Subphyla Classes and Orders

Chordates, like the horse, whose notochords are replaced by backbones made up of bony structures called vertebrae, are classified into the subphylum Vertebrata and are generally known as Vertebrates.

In turn, all vertebrates are classified into one of seven distinct classes: class Agnatha, which includes jawless or round-mouthed fish, such as the sea lamprey and hagfish; class Chondrichthyes, which includes cartilage fish, like sharks; class Osteichthyes, which includes bony fish; class Aves, which includes birds; class Amphibia, which includes frogs; class Reptilia, which includes lizards and snakes; and finally, class Mammalia, which is the group to which the horse belongs. Like all mammals, horses have mammary glands, which secrete milk for their young, and they also have bodies that are partially or totally covered with hair.

Mammals are further classified into smaller groups called orders. Hoofed mammals called ungulates, such as deer, buffalo, giraffe, and, of course, horses, are divided into two orders based on the number of toes they have. Horses possess an uneven number of toes, one of which is enclosed in a hoof. Therefore, horses are classified within the order Perissodactyla, which is from a Greek word meaning "an odd number of fingers or toes."

Review

In review, like all other living organisms, the horse can be classified by looking first at its most general characteristics and then at its more specific ones. For example, the most general characteristics of this horse, that it is a multi-cellular organism composed of cells without rigid cell walls and chloroplasts, establish it as a member of the kingdom Animalia.

Moving from the general to the more specific, we find that the size and shape of its ears and hooves and other specific features identify it as a member of the species caballus.

Using our modern systems of classification, every known plant and animal inexistence, over 2 million species in all, can be identified and classified.