The Animal Kingdom
Today, over a million types of animal have been discovered on the Earth, and many more will undoubtedly be discovered as people continue to explore the forests, the seas and the many other habitats on our planet.
Animals form one of the two great kingdoms of the living world; the other form is plant life. There is immense variety of animal life, ranging from the microscopic protozoan, to the gigantic blue whale, 100 feet (30 m) long, and over 100 tons in total weight!
One of the biggest differences between plant and animal life is in the method of feeding, and obtaining energy. Unlike green plants, animals cannot make their own food. They have to take in ready-made food in the form of other animal or plant matter. Such food has to be found, and therefore, most animals are able to move around freely to accommodate their needs. In addition, animals have nervous systems to control their movements, and sensory organs to help them to find the suitable food they need.
These features generally distinguish larger animals from plant life, but there are still a number of microscopic organisms that defy a firm classification. These include free-swimming creatures with sensitive eyespots, which sometimes feed like plants. They do this by taking in water and carbon dioxide and combining them to form sugars. They can also take in food like animals do, as well. Zoologists classify these creatures as “animals,” yet some botanists consider them to be algae. Still others consider them to be a part of a separate kingdom known as the Protista. However we classify these difficult organisms, we can be fairly certain it was through creatures like these that both the animal and plant kingdoms arose some two billion years ago!
Zoologists have divided the animal kingdom into about 30 major groups called phyla. The members of each phylum share the same basic structure and organization, although they may look very different. Fish, birds, and humans, for example, all belong to one phylum known as the Chordata, because all of these living species have backbones. Yet, their external appearances are totally different.
The phyla are divided up into a number of classes, whose members have much more in common. All the birds, for example, are warm-blooded, feathered, egg-laying creatures, and all are placed in the class Aves.
Classes are divided into orders, and those members have even more in common with each other. For example, the Falconiformes order contains hawks and eagles, and their relatives; all day-flying birds of prey with sharp talons and hooked beaks. Within each order, there are usually a number of families, each of which contains very closely related animals. In the animal kingdom, a family name always ends in the letters -i-d-a-e (-idae).
Within each family, there are one or more genera (singular: genus), whose members are even more closely related and often very similar. For example, the Buzzard and the Rough-legged Buzzard are very much alike, and both are members of the genus Buteo. Each distinct animal species has a scientific name made up of the name of its genus and a specific name. For example, the Rough-legged Buzzard is known as Buteo lagopus, while the Buzzard is Buteo buteo. These scientific names, which are usually printed in italics, are understood by zoologists all over the world.
The members of each animal species contain the blueprint for that species, within the cells of their bodies. Because they usually mate only with their own kind, they automatically produce more of the same kind when they breed. Animals aren’t normally attracted to other species for mating, mainly because they don’t give the right “signals” to one another. In addition, mating between different species is often times physically impossible, even if they were to meet. Closely related species can occasionally mate in captivity, and the process is successful, the animals will produce offspring known as hybrids. The best known example is the Mule, resulting from the mating of a Male Ass or Donkey and a female horse. However, hybrids are generally sterile, therefore unable to produce further offspring. That way, the species do not become mixed up.
With or Without Backbones:
One commonly used method of classifying animals into the animal kingdom is to divide it into animal species with backbones (vertebrates) and animals without backbones (invertebrates). This is quite a useful method, especially for study and teaching purposes, but it is a very unequal division. There are invertebrate animals in all the phyla, but the vertebrates belong to just one part of one phylum, the Chordata.
The vertebrates include the largest, more popular animals like cats, dogs, horses, cows, and birds. Yet they also include most fish, and nearly all amphibians, and reptiles. The invertebrates group include worms, slugs, snails, insects, spiders, and many others. But don’t think that all the vertebrates are large and all the invertebrates small! There are larges invertebrates such as Giant Squid, which are a part of the genus Architeuthis. Their bodies are around 16 feet (5 m) long, with tentacles three times this length, and they can weigh up to two tons! Compare these figures with those for the smallest vertebrate, the Dwarf Goby. This tiny little fish from the Philippines measures half an inch (13 mm) long. The smallest mammal, which is the Etruscan Shrew, has a body that is only 2 inches (5 cm) long. It is a good deal smaller than many insects.