Animal

I

Introduction

Animal, multicellular organism that obtains energy by eating food. With over 2 million known species, and many more awaiting identification, animals are the most diverse forms of life on earth. They range in size from 30-m (100-ft) long whales to microscopic organisms only 0.05 mm (0.002 in) long. They live in a vast range of habitats, from deserts and Arctic tundra to the deep-sea floor. Animals are the only living things that have evolved nervous systems and sense organs that monitor their surroundings. They are also the only forms of life that show flexible patterns of behavior that can be shaped by past experience. The study of animals is known as zoology.

Animals are multicellular organisms, a characteristic they share with plants and many fungi. But they differ from plants and fungi in several important ways. Foremost among these is the way they obtain energy. Plants obtain energy directly from sunlight through the process of photosynthesis, and they use this energy to build up organic matter from simple raw materials. Animals, on the other hand, eat other living things or their dead remains. They then digest this food to release the energy that it contains. Fungi also take in food, but instead of digesting it internally as animals do, they digest it before they absorb it.

Most animals start life as a single fertilized cell, which divides many times to produce the thousands or millions of cells needed to form a functioning body. During this process, groups of cells develop different characteristics and arrange themselves in tissues that carry out specialized functions. Epithelial tissue covers the body’s inner and outer surfaces, while connective tissue binds it together and provides support. Nervous tissue conducts the signals that coordinate the body (see Nervous System), and muscle tissue–which makes up over two-thirds of the body mass of some animals–contracts to make the body move. This mobility, coupled with rapid responses to opportunities and hazards, is one feature that distinguishes animals from other forms of life.

Some kinds of animal movement, such as the slow progress of a limpet as it creeps across rocks, are so slow that they are almost imperceptible. Others, such as the attacking dive of a peregrine falcon or the leap of a flea, are so fast that they are difficult or even impossible to follow. Many single-celled organisms can move, but in absolute terms, animals are by far the fastest-moving living things on earth.

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Animal life spans vary from less than 3 weeks in some insects to over a century in giant tortoises. Some animals, such as sponges, mollusks, fish, and snakes, show indeterminate growth, which means that they continue to grow throughout life. Most, however, reach a pre-defined size at maturity, at which point their physical growth stops.

II

Types of Animals

Like all living things, animals show similarities and differences that enable them to be classified into groups. Birds, for example, are the only animals that have feathers, while mammals are the only ones that have fur. The scientific classification of animals began in the late 18th century. At this time, animals were classified almost entirely by external features, mainly because these are easy to observe. But external features can sometimes be misleading. For example, in the past, comparison of physical features led to whales being classified as fish and some snakes being classified as worms.

Presently, animals are classified according to a broader range of characteristics, including their internal anatomy, patterns of development, and genetic makeup. These features provide a much more reliable guide to an animal’s place in the living world. They also help to show how different species are linked through evolution. Scientists divide the animal kingdom into approximately 30 groups, each called a phylum (plural phyla).

A

Vertebrates and Invertebrates

One phylum of animals, the chordates, has been more intensively studied than has any other, because it comprises nearly all the world’s largest and most familiar animals as well as humans. This phylum includes mammals, birds, reptiles, amphibians, and fish together with a collection of lesser-known organisms, such as sea squirts and their relatives (see Tunicates). The feature uniting these animals is that at some stage in their lives, all have a flexible supporting rod, called a notochord, running the length of their bodies. In the great majority of chordates, the notochord is replaced by a series of interlocking bones called vertebrae during early development. These bones form the backbone, and they give these animals their name—the vertebrates.

Vertebrates total about 40,000 species. Thanks to their highly developed nervous systems and internal skeletons, they have become very successful on land, sea, and air. Yet vertebrates account for only about 2 percent of animal species. The remaining 98 percent, collectively called invertebrates, are far more numerous and diverse and include an immense variety of animals from sponges, worms, and jellyfish to mollusks and insects. The only feature these diverse creatures share in common is the lack of a backbone.

Some invertebrate phyla contain relatively few species. An extreme example is the phylum Placozoa, which contains just one species. Measuring less than 0.5 mm (0.02 in) across, this unique animal was first discovered in 1883 in a saltwater aquarium in Austria. Its flat body consists of just two layers of cells, making it the simplest known member of the animal kingdom, although not the smallest. Another minor phylum, the loriciferans, was classified in 1983 with the chance discovery of a tiny organism dredged up in marine gravel. Several other species of loriciferans have since been identified, but little is known about how they live.

At the other end of the spectrum, some invertebrate phyla contain immense numbers of species. These major phyla include the annelids (segmented worms), with 12,000 known species; the nematodes (roundworms), also with 12,000 known species; and the mollusks, including bivalves, snails, and octopuses, with at least 100,000 species. The arthropods, with about 1 million known species, include the insects, spiders, and crustaceans. These figures include only species that have been described and named, which are only a portion of those that actually exist. Some biologists estimate that the total number of nematode species may be as high as a quarter of a million, while the total number of arthropods could exceed 10 million.

Compared to vertebrates, most invertebrates are animals of modest dimensions. Giant squids, which are the largest invertebrates, can exceed 18 m (60 ft) in length, but the great majority of invertebrate animals are less than 2.5 cm (1 in) long. Their small size enables them to exploit food sources and infiltrate habitats that larger animals cannot use, but it also leaves them exposed to changing environmental conditions. This is not often a problem in the sea, but it can create difficulties on land. Land-dwelling invertebrates have to cope with the constant threat of drying out, and most of them quickly become inactive in low temperatures.

B

Cold-blooded and Warm-blooded Animals

On land, some invertebrates manage to overcome the problem of cold by using muscles to warm themselves. For example, many large moths and bumblebees use a special form of shivering to raise their body temperature to 35°C (95°F) before they take off, which allows them to fly in cool weather. Bees also maintain warm conditions in their nests, which speeds up the development of their young. But in invertebrates as a whole, temperature regulation is very unusual. In vertebrates, on the other hand, it has developed to a high degree.

Vertebrates are customarily divided into cold-blooded and warm-blooded animals, but these labels are not very precise. Biologists normally use the terms ectoderm and endoderm to describe temperature regulation more accurately. An ectoderm is an animal whose temperature is dictated by its surroundings, while an endoderm is one that keeps its body at a constant warm temperature by generating internal heat.

Reptiles, amphibians, and fish are ectoderms. Although they do not maintain a constant warm temperature, some of these animals do manage to raise their body temperature far above that of their surroundings. They do this by behavioral means, such as basking in direct sunshine when the surrounding air is cool. Mammals and birds are endoderms. These animals generate heat through their metabolic processes, and they retain it by having insulating layers of fat, fur, or feathers. Because their bodies are always warm, they can remain active in some of the coldest conditions on earth.

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