Principles of zoology

There are almost 1 million different species of animals living in the world today, and new ones are constantly being discovered. They range in size and complexity from microscopic creatures made up of only a few cells to the giant blue whale, weighing up to 180 tons. Faced with the task of bringing order to the living world, zoologists consider two key factors: definition (is it an animal?) and classification (what place does it occupy in the animal kingdom?).

What is an animal?

Apart from inanimate objects, humans share the world with living organisms, including plants, animals, and single-celled organisms called monerans and protists. The difference between inanimate objects and living things is not always apparent. For example, one of the characteristics of living things is that they grow. Yet crystals, which are inanimate, also grow. But only living things share in common the fundamental life process called metabolism. Metabolism is the sum of the chemical processes by which cells produce the substances and energy that an organism needs to live.

Almost all the world’s energy is provided initially by the sun. Plants trap a proportion of this energy and use it, through photosynthesis, to produce the sugars and starches that form a major part of their tissues. During this process they absorb minerals and water from the soil and carbon dioxide from the atmosphere. They release oxygen, which is a waste product of their metabolism.

Animals, however, are more complex they cannot use such simple chemicals to produce the substances from which they are made. Instead, they must eat and digest food, and reorganize the products of digestion to form their own tissues. Animals require oxygen to release the potential energy of their food, and they emit carbon dioxide as a waste product.

Thus, plants and animals are complementary, each using the other’s wastes, which are constantly recycled. This great continuous energy flow is aided by a variety of decomposers, especially bacteria and fungi. Decomposers break down dead material and waste into a form that plants can use again.
Animals differ from plants chiefly by their quick responses to stimuli and their ability to move about easily. The cells of plants and animals differ greatly, too. Only plants contain the pigment chlorophyll and have a substance called cellulose in their cell walls.

The distinction between a plant and an animal is often difficult to make among the simplest organisms, however. In the past, scientists classified the simple, single-celled organisms called protozoans as animals because they hunt and feed. But some of these organisms also contain chlorophyll, which they use to make their own food. As a result, today most scientists classify protozoans in a kingdom separate from plants and animals.

Animal species

Zoologists generally classify animals according to a system devised by the 18th-century botanist, Carolus Linnaeus. In this system, animals are divided into ever-smaller groups that have more and more features in common. The largest groups are the kingdoms. All animals belong to the kingdom Animalia. The next largest group is a phylum. Each phylum is divided into classes, classes into orders, and orders into families. Families are then broken down into genera (singular, genus} and genera into species, the basic unit of biological classification.

Animals of the same species are similar in structure and function and can breed with one another freely in the wild. Animals of different species, though they may resemble one another, generally cannot produce offspring together. The words “freely” and “in the wild” are important qualifications because people have successfully bred different species of animals together. A cross between a horse and a donkey, for example, produces a mule. In these kinds of crosses, however, the resulting offspring, called a hybrid, is usually completely sterile or lacks the vigor necessary to compete with other animals for mates.

However, scientists agree that within species, there is room for genetic diversity.They also agree that species change gradually, adapting to environmental variations as successful genetic strains outbreed the less successful ones.

Rotifers are named after their wheellike corona, which beats in a circular motion to achieve movement. These aquatic animals occur in abundance—about 1,500 species are known to exist—and are among the smallest multicellular organisms.


Scientists need an internationally accepted classification system for animals for two main reasons: first, to be able to identify a particular species throughout the world, and second, to group related kinds into larger groups that reflect evolutionary relationships. Common names are usually too imprecise for scientific use. The system of scientific classification for animals includes the categories species, genus, family, order, class, subphylum, and phylum. Using these categories, it is possible to see the relationship between animals of different species at each level.
The lion, for example, is designated by the scientific name Panthera leo, which indicates that it is the species leo belonging to the genus Panthera. This genus also includes the jaguar P. onca)and the tiger (P. tigris). These animals all belong to the family Felidae (the catlike animals), which belongs to the order of flesh-eating animals called Carnivora. This order includes other animals with similar tooth arrangement, skull form, and other features.
As a member of the order Carnivora, the lion does not much resemble the anteater, for instance, which belongs to the order Edentata toothless”). But both have mammalian features and so are designated members of the class Mammalia. All mammals have a backbone and are therefore placed in the subphylum Verte-brata, along with fishes, amphibians, reptiles, and birds. Vertebrates and other animals with a notochord, which is a stiff internal supporting rod (in vertebrates, present only in the unborn and larval states), are placed in the phylum Chordata.