ANIMALS (Part 3 of 4)

Intelligence

Most animal activities that appear to indicate intelligence are simply instinctive. The most intelligent animals are the apes and monkeys. Dogs and elephants have been trained to serve humans in many ways. Horses, seals, porpoises, lions, and tigers are often taught to perform in circuses and aquariums. Talking birds, such as parrots, parakeets, and mynahs, learn to imitate sounds, but they do not have the capacity to think or to understand what they are saying.

Relationship to Human

Humans require the presence of other animals in a variety of ways. The domestication of animals has been important to the development of civilization. By pollinating flowers, bees help in the cultivation of orchard fruits, alfalfa, clover, and many vegetables. The earthworm, by churning up the soil, improves the growth of plants.

Birds eat insect pests, weed seeds, and rodents. Certain bats eat so many mosquitoes and other insects that some communities erect shelters for them to encourage their help.

Hyenas, vultures, and carrion beetles keep country regions clean by devouring dead animals.

Countless animal products are used by humans: pearls (from the oyster), shellac and lacquer (from the lac insect), glue, and fertilizers are only a few examples. Important drugs are produced from the blood and glands of animals. Serums and antivenins for snakebite are made from the blood of horses. Experiments performed on such animals as rats, mice, guinea pigs, and monkeys have been responsible for great advances in medical knowledge and the conquest of human disease.

Dangerous animals include the parasites in the human body and in domesticated animals that cause serious diseases. Fleas, lice, rats, and mosquitoes are also carriers of such serious conditions as malaria and encephalitis. Insect pests cause billions of dollars' worth of damage every year.

THE BASIC FORMS OF ANIMAL LIFE AND HOW THEY DIFFER

More than a million different kinds of animals inhabit the Earth. No one knows exactly how many kinds there are, for many new ones are discovered and named every year.

Beginnings of Animal Life

The first organisms in the history of the Earth must have been one-celled bits of protoplasm floating in shallow seas and ponds. Here they remained for millions of years.

They developed from one cell to many cells, becoming more and more complex. In time some animals moved into fresh water. Others began to live on land. In these surroundings they changed still more, until today there is a bewildering variety of forms.

The creatures that developed a backbone and an internal skeleton are called vertebrates. They include all the familiar animals the mammals, reptiles, birds, fish, and amphibians. Animals without backbones are called invertebrates. They include insects, sponges, corals, jellyfish, clams, lobsters, and starfish.

The vertebrates make up only about 5 percent of all animal species. Invertebrates compose the remaining 95 percent. There are some 4,000 species of mammals. Insects number about one million species.

How Animals Are Classified

Phylum (from Greek, meaning tribe), a major division in biological classification.

To study the many forms of animal life in a systematic way, scientists have divided the animal kingdom into groups. These groups are based upon the structure of the animal's body. The largest divisions are phyla (singular, phylum). The word phylum means "race" or "tribe." The phyla are groups of animals with fundamentally different body plans.

Order, in biological classification, a group of related families.

Genus (plural, genera), a group of related species of plants or animals.

Each phylum is divided into classes, the classes into orders, and the orders into families.

Families are subdivided into genera (singular, genus), and each genus is divided into species. All members of the same species are closely related. They are capable of interbreeding and producing fertile offspring. Animals of different species do not normally interbreed. Every animal has a scientific name, or binomial (having two names), consisting of the genus and species.

How Classification Shows Relationships

Classification shows relationships between animals in an increasingly specific order, from remotely related members of the same phylum to closely related species within a genus.

House cats (Felis catus) and bobcats (Felis rufus) belong to the same genus (Felis) and family (Felidae) but to different species.

Dogs and cats do not appear to be related. Both, however, have backbones and are meat-eating mammals. Hence they belong to phylum Chordata (having a spinal cord), class Mammalia (mammals), and order Carnivora (flesh eaters); because of differences between them, however, they belong to separate families (dog, Canidae; cat, Felidae).

Whales and sharks both appear to be kinds of fish. Both are strong, streamlined swimmers of the sea. However, the whale is a mammal. It has lungs and is warm-blooded, gives birth to live young, and nurses its offspring with milk. Whales therefore belong to the class Mammalia. The shark, on the other hand, is a primitive kind of fish with a skeleton of cartilage instead of bone. Sharks, whales, and true fishes all have a backbone. Thus, they are placed in the same phylum (Chordata) and subphylum (Vertebrata). Sharks and fishes, however, are also in different classes, the sharks being in the Chondrichthyes and the true fishes in the Osteichthyes.

Classification also suggests which kinds of animals may have descended from other types. All multi-celled animals, for example, are supposed to be descendants of one-celled animals. This does not mean descent from one living kind of animal to another, however.

All living animals are believed to have descended from common ancestors that were less specialized than they. These relationships may be shown on a treelike diagram called a phylogenetic tree. The word phylogenetic comes from two Greek words meaning "race history."

Animals Without Backbones Invertebrates

The simplest animal-like organisms consist of a single cell a bit of protoplasm containing one nucleus. These organisms are called protozoans, which means "first animals" in Greek. These creatures are sometimes considered animals, but most classification schemes place them in a separate kingdom known as the Protista. Protozoans are very adaptable.

They live in salt and fresh water, in moist earth, and as parasites in other animals.

Aside from the protozoans, all members of the kingdom Animalia have many cells and are referred to as metazoans. The simplest multi-celled animals make up the phylum Porifera ("pore bearers"). The most familiar kinds are the sponges. They are called pore bearers because they are covered with millions of tiny holes. Water flows through the holes, and from the water the sponges take in oxygen and the tiny water borne organisms that constitute their food and filter out wastes. Sponges have no mouth or digestive cavity, no nervous system, and no circulatory system. Several types of cells are present, but each generally functions as a unit without forming tissues, as in more complex metazoans.

(Tissues are groups of similar cells bound together to perform a common function.)

Pouch like Animals

Coelenterata, phylum of animals including coral, hydra, jellyfish, and sea anemone.

The next, less primitive structural pattern in invertebrates is a hollow gut. The representative phylum is Coelenterata, a term stemming from the Greek words koilos (hollow) and enteron (intestine). Among the coelenterates are the corals, hydras, jellyfishes, and sea anemones. The body is composed of two tissue layers. The inner layer, or endoderm, lines the central digestive cavity. The outer layer, or ectoderm, protects the animal externally.

The coelenterates have a mouth like opening the only opening into the gut that takes in food and ejects waste material. Food-gathering organs such as tentacles and protective structures such as stinging cells surround the mouth. There is a primitive nervous system.

(Coelenterates are also sometimes called cnidarians.)

Bilateral Animals with Heads

All the animals described above are headless creatures. They are either irregular masses or animals with shapes like a globe, a cylinder, a bowl, or a wheel. The latter are said to have spherical or radial symmetry; that is, they have similar body parts regularly arranged around a centre or a central axis, respectively. Some drift around in ocean currents, unable to swim efficiently in any particular direction. Some of them in their adult stages the corals and sponges, for example fasten themselves to fixed objects and do not move at all.

A flatworm called Dugesia, or planaria, is interesting because it shows two very important improvements in body structure. It belongs to the phylum Platyhelminthes (flatworms). It is the most primitive animal that has a definite head bearing sense organs. The mouth is on the underside of the triangular head. The body is differentiated into a front end and a rear end, a top and a bottom. It has bilateral symmetry: each half of the body is a mirror image of the other half. Most of the higher animals, including humans, are built on this pattern of body structure.

Platyhelminths are also the most primitive, living animals to have three cell layers. Between the ectoderm and the endoderm, which first appeared in the jellyfish and their relatives, is a middle layer: the mesoderm. Two-layered animals are small and fragile. The third layer gives solidity to the body and permits the animal to grow to a large size.

Muscles and other complex organs develop from this layer.

Segmented Worms

Segmented worms have a more developed digestive system than Dugesia, which takes in food and ejects waste material through the same opening in the head. Segmented worms have a digestive tube with two openings a mouth and an anus through which wastes are expelled. The phylum Annelida (meaning ringed, or segmented) has a digestive system built on the same plan as the vertebrates. Earthworms and leeches are familiar annelids.

The Soft-Bodied Animals

The phylum Mollusca (from the Latin word for "soft") includes the clam, oyster, chiton, snail, octopus, and squid. Molluscs have soft, fleshy bodies not divided into segments. The main part of the body is enclosed in a fold of tissue called the mantle. They have bilateral symmetry. Many of them are covered by a shell. They have a solid, protective structure outside the body, called an exoskeleton.