CHAPTER 1 THE VARIETY OF ANIMAL LIFE iving things are divided into three main groups called domains Single celled organisms called bacteria and archaea each constitute their own domain All other organisms make up at third domain Eukarya which includes not only single celled algae and protozoa but also animals and other multicellular organisms Animals form the largest group within the Eukarya They range from very simple invertebrates such as sponges to highly complex mam mals such as whales monkeys and humans Animals display some key differences that distinguish them from other living things For example what is the difference between an animal such as a horse and a plant such as grass A horse moves around in the pasture eating grass It trots toward you when you offer it a lump of sugar and reacts favorably when you stroke its head The grass however is rooted to one place It does not respond behaviorally to people or to the horse in any A horse eats grass in Oxfordshire England Tim Graham Photo Library Getty Images I2
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A CLOSER LOOK AT THE ANIMAL KINGDo way More importantly plants use nutrients from the soil and chemical reactions from the sun to make their own food This process is called photosynthesis Animals cannot make their own food to survive they have to eat plants or other animals Scientists estimate that there may be more than ten million different kinds of ani mals on Earth today About 1 3 million kinds have been identified to date and new kinds are continually being discovered In the seas from the surface down to the black depths where no ray of light penetrates on moun taintops and in deserts in mud and in hot pools some form of animal life may be found Animals are extremely varied in form size and habits The smallest animals such as the crustaceans called copepods are so tiny that they can barely be seen without a microscope The largest the blue whales may be more than 100 feet 30 meters long and weigh 300 000 pounds 136 000 kilograms Some of the most familiar animals such as dogs birds frogs and fish have a back bone and a central nervous system They are called vertebrates meaning animals with spinal columns or backbones Animals with out backbones are called invertebrates and 14
THE VARIETY OF ANIMAL LIFE include arthropods worms mollusks and many other groups Vertebrates and most invertebrates have a head where sense organs are concentrated and legs wings or fins for locomotion Vertebrates and many invertebrates such as the arthropods and worms have bilateral or two sided symmetry This means that they have two mirror image sides a right side and a left side distinct upper and lower surfaces of the body and a distinct front and rear Some invertebrates such as jellyfish sea anemones and sea stars or starfish dis play radial symmetry similar to that of a wheel in which the parts of the body are arranged around a central axis Animals with radial symmetry live in marine or freshwa ter aquatic environments Some drift with the currents unable to swim in any definite direction Others become attached to a solid object by one end and float with the mouth end upright Tentacles arranged in a circle around the mouth sweep in food particles and ward off enemies How INVERTEBRATES MOVE Mollusks have soft bodies that are in most species enclosed in hard hinged shells Snails 15
