At a waterhole in the African savanna, giraffes, zebras, birds, and other thirsty animals gather to drink. These creatures are a tiny sample of the enormous variety of animals found worldwide.
From microscopic bacteria to massive blue whales, planet Earth is populated by a spectacular variety of life. But, despite their obvious differences, all living things share certain common features. They all obtain energy, grow, respond to their surroundings, and reproduce— things that nonliving objects, such as rocks, cannot do. Scientists divide life-forms into five distinct groups called kingdoms. Each has its own features, as you can see here.
These are the tiniest, most abundant, and most widespread life-forms. Bacteria consist of single cells that, despite being simpler than those in other organisms, work in the same basic ways.
Some take in food from their surroundings, while others make their own, using sunlight or other sources of energy.
Like bacteria, most protists also consist of single cells, but they are larger and just as complex as the cells that form animals and plants. Protists generally live in water or damp places.
They are divided into animal-like protozoa, which take in food from their surroundings, and plantlike algae, which make food by photosynthesis.
Mushrooms, toadstools, molds, and yeasts are just some of the organisms that make up the fungi. Some resemble plants, but they live in a very different way.
Fungi feed by releasing digestive chemicals called enzymes that break down dead or living matter, then absorb the simple nutrients that are released.
Despite their diversity, all animals share certain key features. They are all multicellular (made from many cells), and get their food by eating other living things.
All animals move at least part of their body, and many move around actively to find their food, using one or more senses to detect it.
From grasses to giant trees, all plants require water, sunlight, and soil in which to grow their roots. Most plants do not move actively or feed on other organisms.
Instead, they use a process called photosynthesis, trapping sunlight energy to turn carbon dioxide and water into food.
The animal kingdom contains an extraordinary variety of different species, which can be arranged into 34 groups called phyla. Just one of these phyla, the Chordata, includes all vertebrates (animals with backbones, such as fish, ferrets, and frogs).
The other more than 30 phyla contain 97 percent of known animal species, and are known collectively as invertebrates (animals without backbones), even though they are distantly related and share few common features. Here are some of the animal kingdom’s main phyla.
Fish, amphibians, reptiles, birds, and mammals are called vertebrates because they have a vertebral column, or backbone. This is the part of an internal skeleton that supports the skull and to which the limbs are attached.
Aside from the skeleton, several other body systems interact to produce a working vertebrate. Most are described here, using the rabbit as an example.
Most animals have a skeleton—a framework that shapes bodies, protects internal organs, and provides anchorage for muscles. Vertebrates have an internal skeleton that is usually made of bone and consists of a backbone attached to a skull and two pairs of limbs.
Insects, crustaceans, and other arthropods have a hard external skeleton or exoskeleton. Worms and some echinoderms have a hydrostatic skeleton, which consists of a fluid-filled cavity controlled by muscles.
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