Chapter 5 — Diversity in Living Organism

The Earth carries an enormous variety of living things. About 1.8 million species have been described and named, and biologists estimate that the true number is somewhere between 8 and 30 million. They range from bacteria a thousandth of a millimetre long to blue whales thirty metres long and banyan trees covering acres; they live in boiling springs, frozen ice, deep oceans, deserts, and inside other organisms. This variety is called biodiversity. India alone has about 8 percent of the world's known species, including around 45,000 plant species and 90,000 animal species, and is one of the world's twelve mega-diversity countries; the Western Ghats and the Eastern Himalaya are two of the world's biodiversity hotspots.

No one can study millions of kinds of organisms one at a time. We manage the same problem in daily life by grouping: a shopkeeper arranges goods in sections, a library sorts books by subject, a railway station lists trains by direction. Classification is the arrangement of organisms into groups and subgroups on the basis of their similarities and differences. It makes the study of living things possible and useful in several ways. It brings order to diversity so that we can find our way about. It lets us study a group and apply the knowledge to every member; what is learnt about one grasshopper is largely true of all grasshoppers. It reveals relationships: organisms placed together share characteristics because they share ancestors, so a good classification is also a family tree of life. It helps in identifying new organisms and in practical fields such as agriculture, medicine and conservation. And it gives every organism a place and a name understood everywhere.

What characteristics should be used? Not just any feature will do; grouping animals by colour or by whether they live in water would put whales with fish and butterflies with birds. Biologists use fundamental characteristics, those that decide many other features and appear early in an organism's structure and development. The first question is whether the cell is prokaryotic or eukaryotic, since this decides everything about the cell's organisation. The next is whether the organism is unicellular or multicellular, since a multicellular body can have tissues and organs. Then, whether it makes its own food (autotroph) or takes it from others (heterotroph), which decides its way of life. In plants, whether the body is differentiated into root, stem and leaf, whether it has vascular tissue, and whether it bears seeds. In animals, the level of organisation, the symmetry of the body, the number of embryonic layers, the presence of a body cavity, a notochord and a backbone. Characteristics are applied in this order, from the most fundamental to the most specific, so that the groups formed at each step contain organisms that are alike in all the deeper features.

Classification is thus not an arbitrary filing system but a summary of what we know about how organisms are built and how they are related. As knowledge grows, classification changes; the history of the subject, described next, is the history of better and better ways of asking which organisms belong together.

People have always sorted living things, into edible and poisonous, tame and wild, useful and harmful. The first scientific attempt was made by the Greek philosopher Aristotle (384 to 322 BCE), who classified animals according to whether they lived on land, in water or in the air, and by features such as the presence of red blood. His scheme grouped unrelated organisms together, such as fish and whales, and is no longer used, but it began the idea of grouping by observed characteristics.

For two thousand years all organisms were divided simply into two kingdoms, Plantae and Animalia. The Swedish naturalist Carolus Linnaeus (1707 to 1778) systematised this two-kingdom classification in his book Systema Naturae in 1735. He arranged organisms in a hierarchy of categories and, most importantly, introduced binomial nomenclature, the practice of giving every species a two-word Latin name, which is still followed today. Linnaeus is called the father of taxonomy, the science of classification. But the two-kingdom system had problems. It placed the fungi, which do not photosynthesise, among the plants because they have cell walls and do not move. It placed bacteria among the plants although they have no nucleus. It could not decide where to put Euglena, which has chloroplasts like a plant but moves and feeds like an animal. And it ignored the gulf between prokaryotes and eukaryotes, which is the deepest division in the living world.

As microscopes improved and the nature of bacteria, protozoa and fungi became clear, biologists proposed more kingdoms. In 1866 Ernst Haeckel proposed a third kingdom, Protista, for the unicellular organisms. In 1938 Herbert Copeland added a fourth kingdom, Monera, for the bacteria, separating prokaryotes from eukaryotes. In 1969 the American ecologist Robert H. Whittaker proposed the five-kingdom classification, which is the system used in this chapter. Whittaker based his kingdoms on three criteria: the complexity of cell structure (prokaryotic or eukaryotic), the complexity of body organisation (unicellular or multicellular) and the mode of nutrition (autotrophic by photosynthesis, heterotrophic by absorption, or heterotrophic by ingestion). His five kingdoms are Monera (prokaryotes), Protista (unicellular eukaryotes), Fungi (multicellular eukaryotes that absorb food), Plantae (multicellular eukaryotes that photosynthesise) and Animalia (multicellular eukaryotes that ingest food). Whittaker's scheme reflected evolution as well: Monera were the first cells, the Protista arose from them, and the three multicellular kingdoms arose separately from different protists.

Later, in 1990, Carl Woese proposed splitting the Monera into two domains, Bacteria and Archaea, on the basis of differences in their RNA and cell chemistry, giving a three-domain system (Archaea, Bacteria, Eukarya) above the kingdoms. This is used in research, but at school level the five-kingdom system remains the standard framework. The lesson of this history is that classification is a living science that improves as our knowledge of organisms deepens.

Classification arranges organisms in a series of groups of decreasing size, each group nested inside a larger one, like boxes within boxes. The groups are called taxonomic categories or taxa, and the sequence from the largest to the smallest is:

Kingdom -> Phylum (called Division in plants) -> Class -> Order -> Family -> Genus -> Species

A kingdom is the largest group and contains many phyla; a phylum contains many classes; and so on down to the species. As we go down the hierarchy the number of organisms in each group becomes smaller, but the number of characteristics they share becomes larger. All the members of the animal kingdom share only a few basic features; all the members of the species Panthera tigris share nearly everything. Going up the hierarchy is like widening the circle of relatives from brothers and sisters to cousins to a whole clan.

The species is the basic unit of classification. It is defined as a group of organisms that resemble one another closely and are able to interbreed among themselves in nature to produce fertile offspring. Lions and tigers are different species: they can be crossed in captivity, but the offspring are usually sterile and the two never interbreed in the wild. All the dogs of the world, from a Chihuahua to a Great Dane, belong to one species because they can interbreed. A genus is a group of closely related species; the lion, tiger, leopard and jaguar are four species of the genus Panthera. Take the human being as a worked example: Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, Species sapiens. The mango is Kingdom Plantae, Division Angiospermae, Class Dicotyledonae, Order Sapindales, Family Anacardiaceae, Genus Mangifera, Species indica.

Common names cause confusion. The same organism has different names in different languages, and even in one language a name may refer to different organisms; a robin in India is not the robin of Europe, and the word mango tells us nothing about which of the many species of Mangifera is meant. To solve this Linnaeus introduced binomial nomenclature, the system of naming each species with two words. The first word is the name of the genus, the second is the specific epithet that identifies the species within the genus. Together they form the scientific name, which is unique to that species and the same in every country and language. Thus Mangifera indica is the mango, Oryza sativa is rice, Panthera tigris is the tiger, Homo sapiens is the human being, Rana tigrina is the Indian bull frog and Escherichia coli is the intestinal bacterium.

There are rules for writing scientific names, and examiners check them. The names are in Latin or Latinised. The genus name begins with a capital letter, the species epithet with a small letter. When printed the name is in italics; when handwritten each word is underlined separately. The genus may be abbreviated to its initial after the first mention: E. coli. The name of the scientist who first described the species may follow, as in Mangifera indica L. for Linnaeus. Scientific names are governed by international codes, one for plants and one for animals, so that each species has one accepted name.

Kingdom Monera contains all the prokaryotes, organisms whose cells have no nuclear membrane and no membrane-bound organelles. It includes the bacteria, the blue-green algae (cyanobacteria) and the mycoplasmas. Monerans are the oldest, simplest, smallest and most numerous living things on Earth; they appeared about 3.5 billion years ago and a single gram of soil holds hundreds of millions of them.

Characteristics. The cells are prokaryotic: the genetic material is a single circular DNA molecule lying in the cytoplasm as a nucleoid, with no nucleolus and no nuclear membrane. There are no mitochondria, plastids, ER, Golgi bodies or lysosomes; respiration takes place on the plasma membrane and its infoldings, and in photosynthetic forms the pigments lie on membranes in the cytoplasm. Ribosomes are present but small (70S). Most monerans have a rigid cell wall made of peptidoglycan, a substance found nowhere else; mycoplasmas, the smallest cells known at about 0.1 micrometre, have no wall at all. The organisms are unicellular, though some form chains or colonies; there are no tissues. Some bacteria move by one or more whip-like flagella. Bacteria are classified by shape: spherical cocci, rod-shaped bacilli, comma-shaped vibrios and spiral spirilla. They reproduce mainly by binary fission, some species dividing every twenty minutes, and some form tough resting spores that survive heat, cold and drying.

Nutrition in the Monera is of every type known. Most bacteria are heterotrophic, either saprophytes that feed on dead organic matter, thereby decaying it and recycling its nutrients, or parasites that feed on living hosts and cause disease. Some are autotrophic: the cyanobacteria such as Nostac, Anabaena and Oscillatoria photosynthesise with chlorophyll and release oxygen, and it was their ancestors that first filled the atmosphere with oxygen; chemosynthetic bacteria such as the nitrifying bacteria of the soil make food using the energy of chemical reactions instead of light.

Importance. Monerans are indispensable. Decomposer bacteria break down dead plants and animals and return carbon, nitrogen and minerals to the soil. Nitrogen-fixing bacteria such as Rhizobium in the root nodules of pulses and the free-living Azotobacter, and cyanobacteria such as Anabaena in paddy fields, convert atmospheric nitrogen into compounds plants can use. Lactobacillus turns milk into curd; other bacteria are used to make vinegar, cheese, antibiotics such as streptomycin, and vitamins; E. coli in our intestine makes vitamin K. Bacteria clean sewage and are the tools of genetic engineering. On the other side, bacteria cause cholera, typhoid, tuberculosis, tetanus, diphtheria, leprosy and pneumonia in humans, anthrax in cattle, and blights and rots in crops, and they spoil food.

Examples to remember: Escherichia coli, Lactobacillus, Rhizobium, Vibrio cholerae (cholera), Mycobacterium tuberculosis (TB), Streptococcus, Bacillus, Nostoc and Anabaena (cyanobacteria), Mycoplasma.

Kingdom Protista contains the unicellular eukaryotes: organisms made of a single cell which, unlike a bacterial cell, has a true nucleus with a nuclear membrane and membrane-bound organelles such as mitochondria. Some protists form colonies, but there is no division into tissues. Most live in water, fresh or salt, or in damp soil, and some live as parasites inside other organisms. Protists are a bridge between the Monera and the three multicellular kingdoms: the plants, fungi and animals each arose from different protist ancestors, and the kingdom includes forms that resemble each of them in miniature.

Characteristics. The body is a single eukaryotic cell, often with special structures for movement: pseudopodia (temporary cytoplasmic extensions) in amoeba, cilia (numerous short hairs) in paramecium, or flagella (one or a few long whips) in euglena. Nutrition may be autotrophic, heterotrophic or both. Reproduction is usually asexual, by binary fission or multiple fission, though many protists also show forms of sexual reproduction such as conjugation in paramecium.

Protists are of three broad types according to their way of feeding.

• Plant-like protists (unicellular algae) contain chloroplasts and photosynthesise. Euglena has chloroplasts, a flagellum and a light-sensitive eyespot, and can photosynthesise in light yet feed on organic matter in the dark; it has both plant and animal features, which is why the two-kingdom system could not place it. Diatoms are algae with beautiful glassy silica shells; they are the chief producers of the oceans and their shells form deposits used in toothpaste and filters. Dinoflagellates cause the red tides of the sea. Chlamydomonas is a green flagellate.
• Animal-like protists (protozoa) are heterotrophs that ingest food. Amoeba moves and feeds by pseudopodia and lives in ponds. Paramecium, the slipper animalcule, is covered with cilia and has two nuclei and contractile vacuoles. Plasmodium, a parasite carried by the female Anopheles mosquito, causes malaria. Entamoeba histolytica causes amoebic dysentery, and Trypanosoma causes sleeping sickness. Leishmania causes kala-azar.
• Fungus-like protists include the slime moulds, which creep over rotting wood as a mass of cytoplasm and then form spore-bearing structures like a fungus.

Importance. Photosynthetic protists such as diatoms produce a large share of the oxygen in the atmosphere and are the base of every aquatic food chain; without them there would be no fish. Protozoa in the soil and water feed on bacteria and keep their numbers in check; some, living in the gut of termites and cattle, digest cellulose for their hosts. Parasitic protists cause some of the most widespread human diseases, malaria alone infecting more than 20 crore people a year. Protists are also favourite subjects for studying how a single cell performs all the activities of life.

Kingdom Fungi includes the moulds, mildews, yeasts, mushrooms, toadstools, puffballs, bracket fungi, rusts and smuts. For a long time fungi were treated as plants because they have cell walls and do not move, but they neither photosynthesise nor have cellulose walls, and they are now a kingdom of their own. About one lakh species are known.

Characteristics. Fungi are eukaryotic and, with the exception of yeasts, which are unicellular, multicellular. The body, called a mycelium, is a mass of fine, branching, thread-like filaments called hyphae. In many fungi the hyphae are divided by cross walls into cells; in others such as Rhizopus the hyphae are continuous tubes containing many nuclei, a condition called coenocytic. The cell wall is made of chitin, the same tough substance that forms the outer skeleton of insects, and not of cellulose. Fungi have no chlorophyll and cannot make food. They are heterotrophic and feed by absorption: the hyphae secrete digestive enzymes on to the food outside the body, break it down, and absorb the soluble products through their walls. Most fungi are saprophytes, feeding on dead and decaying organic matter such as fallen leaves, dung, bread or fruit; some are parasites on living plants and animals; and some live in mutual partnership with other organisms, as in lichens (a fungus with an alga) and mycorrhizae (fungal threads on plant roots that help them absorb minerals). Food is stored as glycogen, like animals, not as starch. Fungi reproduce mainly by spores, produced in vast numbers on special hyphae or in fruiting bodies and scattered by wind; a single mushroom may shed thousands of crores of spores. Yeasts multiply by budding. Many fungi also reproduce sexually by fusion of hyphae.

Examples. Rhizopus, the bread mould, grows as a white cottony mass with black pin-heads, the sporangia, on moist bread. Mucor is similar. Penicillium, the blue-green mould of citrus fruits and old bread, is the source of penicillin, the first antibiotic. Aspergillus is a common black mould. Yeast (Saccharomyces) is used to make bread rise and to brew alcohol by fermenting sugar. Agaricus is the edible mushroom; Amanita is a poisonous toadstool. Puccinia causes rust of wheat, Ustilago causes smut of sugarcane and maize, and Phytophthora caused the potato blight that led to the Irish famine.

Importance. Fungi and bacteria are the great decomposers; without them the dead bodies and fallen leaves of the world would never rot and the nutrients they contain would never return to the soil. Fungi give us antibiotics, bread, alcohol, cheese, soy sauce, citric acid, enzymes and vitamins, and mushrooms are a valuable protein food. Mycorrhizal fungi help trees and crops absorb phosphorus. On the other hand fungi cause ringworm, athlete's foot and dandruff in humans, spoil stored food and grain, rot timber and cloth, and cause serious crop diseases such as rusts, smuts and blights.

Kingdom Plantae includes all the multicellular, eukaryotic, autotrophic organisms that have cellulose cell walls and chlorophyll and make their food by photosynthesis. They range from a thread of Spirogyra in a pond to a banyan tree, and about four lakh species are known. Plants store their food as starch. They do not move from place to place, they grow throughout life from meristems, and they show both asexual and sexual reproduction, often alternating between a spore-producing and a gamete-producing generation.

The plant kingdom is divided into groups on the basis of three questions, applied in order.

First: is the plant body differentiated into root, stem and leaf? The simplest plants have a body that is not divided into these organs; such an undifferentiated body is called a thallus, and these plants are the Thallophyta (the algae). All other plants have at least some differentiation of the body.

Second: does the plant have special tissues for conducting water and food? The Bryophyta (mosses and liverworts) have a body differentiated into stem-like and leaf-like parts but no vascular tissue, no xylem or phloem, so water can move through them only by diffusion from cell to cell, and they remain small and confined to damp places. All higher plants have vascular tissue and are called vascular plants or Tracheophyta.

Third: does the vascular plant produce seeds? The Pteridophyta (ferns and their relatives) have vascular tissue and true roots, stems and leaves but reproduce by spores, not seeds; their sexual stage still needs water. The seed plants, or Phanerogams, produce seeds, in which an embryo is protected and provided with food, and they no longer need water for fertilisation because pollen carries the male gamete. Plants without seeds, the thallophytes, bryophytes and pteridophytes, have hidden and inconspicuous reproductive organs and are together called Cryptogams (hidden marriage).

Fourth, within the seed plants: is the seed naked or enclosed in a fruit? The Gymnosperms (naked seeds) bear their seeds exposed on the scales of cones, as in pine and cycas. The Angiosperms (enclosed seeds) bear their seeds inside a fruit formed from the ovary of a flower; these are the flowering plants, the largest and most successful group.

The five groups thus form a series of increasing complexity and increasing independence from water: Thallophyta -> Bryophyta -> Pteridophyta -> Gymnospermae -> Angiospermae. Each step added a feature that let plants live further from water: a differentiated body, then vascular tissue, then seeds, then flowers and fruits. This is also the order in which the groups appeared in the history of the Earth, from algae in the ancient seas about 1,000 million years ago to the flowering plants that spread over the land about 130 million years ago. The next four sections describe the groups in this order.

Thallophyta are the simplest plants. Their body is a thallus, not differentiated into root, stem and leaf, and it may be a single cell, a filament of cells, a flat sheet or a branching mass. They have no vascular tissue. Almost all thallophytes are algae, and most algae live in water, fresh or marine, though some grow on wet soil, tree trunks and damp walls. They photosynthesise with chlorophyll, and in many the green is masked by other pigments, giving green, brown and red algae. Their reproductive organs are single cells, without a protective jacket, and their reproduction is by fragmentation, by spores, or by fusion of gametes, all of which require water. Examples: Spirogyra, the green filamentous pond scum with spiral chloroplasts; Ulothrix, another filament; Chara, the stonewort with a stem-like body; Ulva, the sea lettuce; Cladophora; and the large brown seaweeds such as kelp and Fucus and red seaweeds such as Polysiphonia. Algae are important as the producers of aquatic food chains and the main makers of atmospheric oxygen; seaweeds are eaten and give agar, used in laboratories and food, and alginates and iodine; Spirulina is grown as a protein-rich food supplement; and algae are used to treat sewage.

Bryophyta are the amphibians of the plant kingdom: they live on land but need water for reproduction, because their male gametes must swim to the egg, so they grow only in moist, shady places such as damp walls, rocks, stream banks and the bark of trees in the rainy season. The plant body is small, a few centimetres at most, and is differentiated into stem-like and leaf-like structures, but there are no true roots, only hair-like rhizoids that anchor the plant and absorb water, and no vascular tissue; water and food move by diffusion from cell to cell, which limits the size of the plant. Bryophytes show a clear alternation of generations: the green leafy plant is the gamete-producing generation (gametophyte), and from the fertilised egg grows a stalked capsule, dependent on the leafy plant, which produces spores (the sporophyte). Examples: Riccia and Marchantia, the liverworts, with a flat green lobed thallus lying on damp soil; and Funaria and Polytrichum, the mosses, with upright stems bearing tiny leaves and a spore capsule on a slender stalk. Bryophytes help to form soil on bare rocks, hold soil against erosion, and the peat moss Sphagnum stores water and is used as a packing material and fuel.

Pteridophyta are the first vascular plants and the first to have true roots, stems and leaves. The presence of xylem and phloem lets them grow larger than mosses and live in drier places, and their leaves have a cuticle and stomata. They reproduce by spores produced in sporangia, which in ferns are clustered in brown patches called sori on the underside of the leaves. The spore grows into a tiny heart-shaped green plate, the prothallus, which bears the sex organs; the male gametes swim in a film of water to the egg, so pteridophytes still need moisture for reproduction and are most common in damp, shady forests and hill slopes. Because their reproductive organs are hidden and they form no seeds, they are cryptogams. The spore-bearing fern plant is the dominant generation, the reverse of the bryophytes. Examples: ferns such as Dryopteris, Pteris, Adiantum (the maiden-hair fern), Nephrolepis and the water fern Marsilea, which has four-lobed leaves like a clover; Equisetum, the horsetail, with jointed hollow stems; Lycopodium and Selaginella, the club mosses. In the Carboniferous period, about 300 million years ago, tree ferns and giant club mosses formed vast forests whose remains became the coal we burn today. Ferns are grown as ornamental plants, Azolla, a floating fern with nitrogen-fixing cyanobacteria in its leaves, is used as green manure in paddy fields, and some ferns are eaten.

Gymnospermae (gymnos, naked; sperma, seed) are the first seed plants. They are mostly evergreen, woody trees and shrubs with well-developed vascular tissue, true roots, stems and leaves, and often needle-like or scale-like leaves that reduce water loss and survive cold. Their great advance is the seed: the embryo is enclosed with stored food in a protective coat and can wait for good conditions, and the male gametes are carried by wind-blown pollen, so water is no longer needed for fertilisation. Gymnosperms have no flowers and no fruits; their reproductive organs are cones, male cones producing pollen and female cones bearing ovules exposed on the surface of the scales. After fertilisation the ovule becomes a seed that lies naked on the cone scale, not enclosed in an ovary. Examples: Pinus (pine) and Cedrus (deodar) of the Himalaya, Cycas, a palm-like plant of gardens with a crown of large leaves, Ginkgo, the maidenhair tree, and Thuja, the ornamental cypress; the giant redwoods of California, the tallest trees on Earth, are gymnosperms. Gymnosperms give us softwood timber for furniture and paper, resin and turpentine from pine, and the pine nuts (chilgoza) of the north-west Himalaya; deodar timber is prized for building.

Angiospermae (angeion, vessel; sperma, seed) are the flowering plants, whose seeds are enclosed inside a fruit. They are the largest, most varied and most successful group of plants, with more than 2.5 lakh species, and they include nearly every plant we eat, wear, build with or plant for shade and beauty: rice, wheat, maize, pulses, vegetables, fruit trees, cotton, teak, grasses, cacti, water lilies, orchids and the duckweed Wolffia, the smallest flowering plant, less than a millimetre across.

Their defining feature is the flower, the reproductive organ. The ovules are enclosed within the ovary at the base of the flower; pollen from the anthers is carried to the stigma by wind, insects, birds or other agents; after fertilisation the ovules become seeds and the ovary becomes the fruit that encloses and protects them and helps to disperse them. Angiosperms have the most efficient vascular tissue, with true vessels in the xylem, and a huge range of body forms, from annual herbs that complete their life in one season to trees that live for centuries. Most have broad flat leaves. Their success is due to the flower, which allows precise pollination by animals, the fruit, which protects and spreads seeds, and the short life cycle of many species.

Angiosperms are divided into two classes according to the number of cotyledons (seed leaves) in the embryo. A cotyledon is the leaf of the embryo that stores or absorbs food for the seedling; you can see them as the two halves of a gram or bean seed. This one difference goes along with a whole set of others.

Some memory aids: monocot leaves are usually long with veins running side by side like the grass blade or the banana leaf; dicot leaves have a midrib with a network of veins like the mango or peepal leaf. Pull up a grass and you see a bunch of equal fibrous roots; pull up a mustard seedling and you see one main tap root. Count the petals of a lily (six, in two rings of three) and a hibiscus (five). Split a maize grain and you find a single cotyledon fused to the endosperm; split a soaked gram and it falls into two cotyledons with the tiny embryo between them.

The angiosperms are the culmination of the series you have followed through the plant kingdom: a differentiated body (added by the bryophytes), vascular tissue (added by the pteridophytes), seeds (added by the gymnosperms), and now flowers and fruits. Each addition made plants more independent of water and better able to spread over the land, which is why the flowering plants cover most of the Earth's surface today.

Kingdom Animalia includes all the multicellular, eukaryotic, heterotrophic organisms that ingest their food, that is, take it into the body and digest it inside. Animal cells have no cell wall and no chlorophyll; most animals can move about, at least at some stage of life, and they respond quickly to stimuli through nerves and muscles. They store food as glycogen and fat. About 15 lakh species are known, three quarters of them insects.

Animals are classified into phyla on the basis of features of their body plan, applied in order from the most fundamental.

Level of organisation. In the sponges the cells are loosely arranged and there are no true tissues (cellular level). In the coelenterates cells form tissues but there are no organs (tissue level). In all higher animals tissues form organs and organ systems (organ system level).

Symmetry. Symmetry is the way the body can be divided into matching halves. Sponges are mostly asymmetrical. Coelenterates and adult echinoderms are radially symmetrical: the body is arranged around a central axis like the spokes of a wheel and can be divided into equal halves by any plane through the axis; this suits animals that sit still or drift and meet the environment from all sides. All other animals are bilaterally symmetrical: the body has a left and a right half, a head end and a tail end, and only one plane, running down the middle, divides it into mirror images; this suits animals that move in one direction with a head that meets the world first.

Germ layers. The embryo of an animal develops from two or three layers of cells. Coelenterates are diploblastic, with an outer ectoderm and an inner endoderm and only a jelly between them. All animals from flatworms onward are triploblastic, with a third layer, the mesoderm, in between, from which muscles, bones, blood and most organs develop.

Body cavity (coelom). Triploblastic animals differ in whether there is a fluid-filled cavity between the body wall and the gut. Flatworms have none and are acoelomate; roundworms have a cavity not fully lined by mesoderm and are pseudocoelomate; annelids, arthropods, molluscs, echinoderms and chordates have a true coelom lined by mesoderm and are coelomate. The coelom cushions the organs, lets the gut move independently of the body wall, and provides space for organs to grow.

Notochord. Finally, chordates possess at some stage a stiff supporting rod along the back, the notochord, which in vertebrates is replaced by a backbone of vertebrae. All other animals lack it and are called invertebrates or non-chordates. Additional characters used are segmentation of the body (annelids, arthropods, chordates), the presence of an exoskeleton or endoskeleton, and the type of appendages.

Applying these characters gives the sequence of phyla studied in the following sections: Porifera, Coelenterata, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca, Echinodermata and Chordata, arranged in increasing complexity of body plan.

Phylum Porifera (pore bearers) are the sponges, the simplest multicellular animals. They are mostly marine, sessile (fixed to rocks), and asymmetrical or radially symmetrical, with a body organised at the cellular level only: there are no tissues or organs. The body is a sac whose wall is pierced by numerous tiny pores (ostia) through which water is drawn in by the beating flagella of special collar cells; food particles and oxygen are filtered from the water, and the water leaves through a large opening at the top, the osculum. The body is supported by a skeleton of tiny needles (spicules) of calcium carbonate or silica, or of spongin fibres, the material of the bath sponge. Sponges have great powers of regeneration. Examples: Sycon (Scypha), Spongilla (a freshwater sponge) and Euspongia (bath sponge).

Phylum Coelenterata (Cnidaria) includes Hydra, jellyfish, sea anemones and corals. They are aquatic, mostly marine, with radial symmetry, a tissue level of organisation and a diploblastic body wall of two layers with a jelly (mesoglea) between. The body is a hollow sac with a single opening, the mouth, surrounded by tentacles; the cavity inside, the coelenteron or gastrovascular cavity, serves both for digestion and for circulation. The tentacles carry stinging cells (cnidoblasts) that paralyse prey, the feature that gives the phylum its other name. Two body forms occur: the fixed cylindrical polyp (Hydra, sea anemone, coral) and the free-swimming umbrella-like medusa (jellyfish); some species alternate between them. Corals secrete limestone skeletons that build reefs, such as those of Lakshadweep and the Andamans. Examples: Hydra, Aurelia (jellyfish), Adamsia (sea anemone), Physalia (Portuguese man-of-war), corals.

Phylum Platyhelminthes (flatworms) are the first animals with bilateral symmetry, a triploblastic body and an organ level of organisation. The body is dorsoventrally flattened like a ribbon or a leaf, which is why they are called flatworms, and there is no body cavity (acoelomate). The gut, when present, has a single opening; there are no blood vessels and no respiratory organs, gases diffusing through the thin body. Most are parasites with hooks and suckers for attachment and a tough cuticle to resist the host's enzymes, and most are hermaphrodite, each animal having both male and female organs. Examples: Planaria, a free-living flatworm of ponds with remarkable regeneration; Fasciola, the liver fluke of sheep and cattle; Taenia, the tapeworm of the human intestine, acquired from undercooked pork or beef, whose ribbon-like body of segments may be several metres long; and Schistosoma, the blood fluke.

Phylum Nematoda (Aschelminthes, roundworms) have a cylindrical, unsegmented body tapering at both ends, bilateral symmetry, a triploblastic wall and a pseudocoelom, a body cavity not lined by mesoderm. They are the first animals with a complete gut having both a mouth and an anus. The body is covered by a tough cuticle, sexes are separate, and there is no circulatory or respiratory system. Roundworms are among the most numerous animals on Earth; free-living species swarm in soil and water, and parasitic species infect plants and animals. Examples: Ascaris, the common roundworm of the human intestine, spread by contaminated food and soil, especially in children; Wuchereria, the filarial worm carried by Culex mosquitoes, which blocks the lymph vessels and causes elephantiasis, the enormous swelling of the legs; Ancylostoma, the hookworm that enters through bare feet and causes anaemia; and Enterobius, the pinworm.

Phylum Annelida (ringed worms) have a long, cylindrical body divided into many similar ring-like segments (metameres), visible as grooves on the outside and repeated in the internal organs. They are bilaterally symmetrical, triploblastic and are the first animals with a true coelom lined by mesoderm, in which the gut hangs free. They have a closed circulatory system with blood in vessels, a nervous system with a brain and a ventral nerve cord, excretory organs called nephridia in each segment, and a complete gut. Many bear bristles called setae or chaetae that grip the ground. Examples: Pheretima, the earthworm, which aerates and fertilises the soil and is called the farmer's friend; Hirudinaria, the leech, a blood-sucking parasite of ponds with suckers at both ends; and Nereis, a marine sandworm with paddle-like appendages.

Phylum Arthropoda (jointed feet) is the largest phylum in the animal kingdom, containing more than two thirds of all known species. Arthropods have a segmented body, usually divided into head, thorax and abdomen, with pairs of jointed appendages that serve as legs, antennae, mouthparts or wings. The body is covered by a tough exoskeleton of chitin that protects it and gives attachment to muscles; because it cannot stretch, the animal must moult (shed the skeleton) to grow. The coelom is reduced, and the body cavity is a blood-filled space called the haemocoel; the circulatory system is open, the blood bathing the organs directly. Respiration is by gills (in aquatic forms), by a network of air tubes called tracheae (insects) or by book lungs (spiders). Sexes are separate. Arthropods are divided into classes: Insecta (three pairs of legs, usually two pairs of wings, one pair of antennae: cockroach, housefly, mosquito, butterfly, honey bee, ant, grasshopper); Crustacea (aquatic, two pairs of antennae, gills: prawn, crab, lobster); Arachnida (four pairs of legs, no antennae: spider, scorpion, tick, mite); and Myriapoda (many legs: centipede with one pair per segment, millipede with two). Arthropods pollinate crops, give silk, honey, lac and wax, and are food; they also carry malaria, dengue, filaria, plague and typhus and destroy crops and stored grain.

Phylum Mollusca (soft-bodied) is the second largest phylum. The body is soft, unsegmented and bilaterally symmetrical, divided into a head, a muscular foot for creeping or swimming, and a visceral mass containing the organs, which is covered by a fold of skin, the mantle, that secretes a hard calcareous shell in most species. Respiration is by gills (ctenidia) in the mantle cavity, and the circulatory system is open. Many have a rasping tongue, the radula. Examples: Pila, the apple snail of ponds, with a coiled shell; Unio, the freshwater mussel, with a shell of two hinged valves; the garden snail and slug; Sepia (cuttlefish), Loligo (squid) and Octopus, fast-swimming marine molluscs with tentacles and a reduced or absent shell; and the pearl oyster, which forms pearls inside its shell.

Phylum Echinodermata (spiny-skinned) are exclusively marine animals whose adults show radial symmetry, usually in five parts, though the larva is bilateral. They have an internal skeleton of calcareous plates (an endoskeleton) that often bears spines, and a unique water vascular system, a network of water-filled canals connected to hundreds of tube feet used for locomotion, feeding and respiration. They have a true coelom, a complete gut, no head and no brain, and great powers of regeneration; a starfish can regrow a lost arm. Examples: Asterias (starfish), Echinus (sea urchin), Holothuria (sea cucumber), Antedon (feather star) and the brittle star. Echinoderms are the closest invertebrate relatives of the chordates.

Phylum Chordata contains the animals with which we are most familiar, fish, frogs, snakes, birds, cattle and ourselves, and is defined by four features that every chordate possesses at some stage of its life, even if only as an embryo:

• A notochord, a flexible rod of cells running along the back beneath the nerve cord, which supports the body; in vertebrates it is present in the embryo and is later replaced by the vertebral column.
• A dorsal hollow nerve cord, a tube of nervous tissue above the notochord that becomes the brain and spinal cord; in all other animals the nerve cord is solid and ventral.
• Pharyngeal gill slits, paired openings in the wall of the pharynx behind the mouth, used for feeding or breathing; they persist as gills in fishes and appear briefly in the embryos of mammals.
• A post-anal tail, an extension of the body beyond the anus, at least in the embryo.

Chordates are bilaterally symmetrical, triploblastic, coelomate and segmented, with a closed circulatory system and a ventral heart, and an internal skeleton (endoskeleton). Chordata is divided into three subphyla. Two, the protochordates, are small marine animals that have a notochord but no backbone: Urochordata (tunicates or sea squirts such as Herdmania, in which only the larva has a notochord and tail) and Cephalochordata (the lancelet Amphioxus or Branchiostoma, a small fish-like animal of sandy shores in which the notochord runs the whole length of the body throughout life). Protochordates are important as living examples of what the ancestors of the vertebrates were like.

The third subphylum, Vertebrata, contains the great majority of chordates. In vertebrates the notochord of the embryo is replaced by a vertebral column or backbone, a chain of bony or cartilaginous vertebrae that protects the spinal cord, and the brain is enclosed in a skull (cranium). Vertebrates have a well-developed head with paired eyes and other sense organs, an internal skeleton of bone or cartilage with usually two pairs of limbs, a muscular heart with two, three or four chambers, red blood with haemoglobin in cells, paired kidneys, and a high degree of coordination through a large brain. They are the largest and most complex animals.

Vertebrata is classified into five classes on the basis of the skeleton, the covering of the body, the method of breathing, the number of heart chambers, whether the body temperature is constant (warm-blooded, homoiothermic) or varies with the surroundings (cold-blooded, poikilothermic), and the way in which the young are produced: Pisces (fishes), Amphibia, Reptilia, Aves (birds) and Mammalia. Together with the jawless fishes (Cyclostomata: lampreys and hagfishes) these make up the vertebrates. The five classes, described in the next section, also form a series in the history of life: fishes appeared first, about 500 million years ago; amphibians moved on to land about 370 million years ago; reptiles conquered the dry land; and birds and mammals arose from reptilian ancestors.

Class Pisces (fishes). Fishes are aquatic, cold-blooded vertebrates with a streamlined body covered by scales and a slimy skin, fins for swimming and balance, and gills for breathing oxygen dissolved in water. The heart has two chambers, one auricle and one ventricle, and pumps only deoxygenated blood. Most lay eggs in water (oviparous) and fertilisation is usually external. Fishes are of two kinds. Cartilaginous fishes (Chondrichthyes) have a skeleton of cartilage, a mouth on the underside, exposed gill slits and rough tooth-like scales; they are marine: sharks (Scoliodon), rays and skates (Torpedo, the electric ray). Bony fishes (Osteichthyes) have a bony skeleton, gills covered by a flap (operculum), overlapping scales and a swim bladder for buoyancy; examples are rohu, catla, mrigal, hilsa, seer fish, tuna, the flying fish and the sea horse.

Class Amphibia. Amphibians (amphi, both; bios, life) live both in water and on land; they were the first vertebrates to come on to land, but they must return to water to breed. The skin is moist, soft and without scales, richly supplied with blood, and is itself an organ of respiration; adults also breathe with lungs, and the larva (tadpole) breathes with gills. The heart has three chambers, two auricles and one ventricle, so oxygenated and deoxygenated blood partly mix. They are cold-blooded, lay jelly-covered eggs in water with external fertilisation, and undergo metamorphosis from an aquatic tadpole to a four-legged adult. Examples: frog (Rana), toad (Bufo), the tree frog (Hyla) and salamander.

Class Reptilia. Reptiles are the first truly terrestrial vertebrates. They crawl (repere, to creep) on short limbs or, as snakes, on the belly. The body is covered by dry, scaly skin or horny plates that prevent water loss. They breathe entirely by lungs. The heart has three chambers with the ventricle partly divided, except in crocodiles, whose heart has four chambers. They are cold-blooded and bask in the sun to warm up. Fertilisation is internal and they lay leathery-shelled eggs on land which do not dry out, so they are free of the need for water to breed. Examples: house lizard (Hemidactylus), garden lizard (Calotes), chameleon, snakes such as cobra (Naja), krait and python, turtles and tortoises, crocodile and gharial. The dinosaurs were reptiles.

Class Aves (birds). Birds are warm-blooded vertebrates whose body is covered with feathers and whose forelimbs are modified into wings for flight. The body is streamlined and light: the bones are hollow, teeth are replaced by a horny beak, and air sacs connected to the lungs extend into the body. The heart has four chambers, completely separating oxygenated and deoxygenated blood, which allows the high metabolism that flight and a constant body temperature (about 40 degrees) demand. Fertilisation is internal and they lay hard-shelled eggs which the parents incubate. Examples: crow, pigeon, sparrow, parrot, peacock (the national bird), eagle, duck, hen, and the flightless ostrich, emu and penguin.

Class Mammalia. Mammals are warm-blooded vertebrates with hair on the body and mammary glands that produce milk to feed the young; the name comes from mamma, the breast. They have a four-chambered heart, lungs, a muscular diaphragm separating chest from abdomen, external ears (pinnae), teeth of different kinds, sweat and oil glands in the skin, and the largest brain in proportion to body size. Fertilisation is internal, and most give birth to live young (viviparous), nourished before birth through a placenta. There are exceptions: the egg-laying mammals of Australia, the platypus and echidna; and the pouched mammals such as the kangaroo whose young are born very immature and complete development in a pouch. Examples: human, monkey, cat, dog, cow, elephant, tiger (the national animal), rat, bat (the only flying mammal), whale and dolphin (aquatic mammals that breathe air).