Animals

Animals are a group of multicelled, living organisms that take in food. Most animals reproduce sexually (with sperm fertilizing an egg), can move about, and are able to respond to their surroundings. Of the separate divisions of living organisms, the animal kingdom forms the largest in terms of the number of species.

Animals range in size from barely visible one-celled animals to the 100-foot (3.48 meter) blue whale. Different creatures may slither, burrow, climb, run, swim, and fly—yet they are all considered part of the Animalia kingdom. The Animalia kingdom is one of the five major divisions of all living organisms. The other kingdoms are Monera (bacteria with no nucleus); Protista (one-celled organisms with a nucleus); Fungi (multicelled organisms that take in food); and Plantae (multicelled plants that make their own food).

CHARACTERISTICS OF ANIMALS

There are six major characteristics of all animals, whether they are worms or whales. First, animals cannot produce their own food (as plants do) and must therefore rely on eating other living things. Second, animals cannot use protein, fats, and carbohydrates directly and so must first digest or break them down into smaller molecules. Third, because of their need to find food and a mate, as well as escape from an enemy, animals have developed a way to move from place to place. Fourth, animals are multicellular, having many cells that are highly specialized. Fifth, animal cells are eukaryotic, meaning that each has a nucleus surrounded by a membrane. Sixth, animals are able to respond quickly and in the correct manner to changes in their environment.

VERTEBRATES AND INVERTEBRATES

To classify the different types of animals or to group them by their similarities, biologists have divided animals into vertebrates (those with a backbone) and invertebrates (those without backbones). Although vertebrates, such human beings, whales, elephants, and dolphins, are the biggest and brainiest of the animals, about 97 percent of the entire animal kingdom is made up of invertebrates like worms, sponges, clams, and insects. The next thing a person who classifies animals considers is the physical arrangement of an animal's body parts. Some animals (like humans) have what is called "bilateral symmetry." This means that if an imaginary line were drawn from top to bottom of an animal, each half of its body would be a mirror image of the other. Those with "radial symmetry," like sea anemones, have their body parts arranged in a circle around a central point. Others like a sponge, have no definite shape at all and are called "asymmetrical."

IDENTIFYING BY FEATURES

Among the major divisions of better-known animals, it is possible to group several according to some of their easily identifiable features. One of these groups includes animals with exoskeletons or strong and light skeletons on the outside of their bodies. Examples of animals with exoskeletons include horseshoe crabs, oysters, and snails as well as insects like spiders and ticks. Animals with a backbone include fishes with gills, amphibians who live on land but need to breed in water, and cold-blooded (animals whose body temperature changes with the environment) reptiles like lizards, snakes, and crocodiles. Birds evolved from reptiles as their scales changed into feathers. They lay eggs and can fly. Mammals are warm-blooded and give birth to live young. The strangest group of animals may be the echinoderms or "spiky skin" animals like starfish. Their structure and shape are completely different from other animals, including five identical parts and a skeleton of plates inside their bodies. Although echinoderms have no head, brain, or blood, they are still members of the animal kingdom. The animal kingdom, of which human beings are one part, includes a wide variety of different but related life forms.

[See alsoArthropods; Echinoderms; Mollusks; ]