Chapter 8: Kingdom Animalia
In 1969, an American ecologist named Robert Whittaker published a short paper that changed the way biology sorts living things.
For nearly two hundred years, every organism had been pushed into one of two boxes: plant or animal. Whittaker’s work took him among mosses, lichens, soil fungi and bacteria, and he kept meeting organisms that refused to fit. A mushroom has no chlorophyll, no root, no stem and no leaf, yet it was filed with plants because it does not walk away. A bacterium has no organised nucleus at all, yet it too sat in the plant box. Some pond organisms swim like animals in the dark and make their own food in the light.
Whittaker did not begin by asking what an organism looks like. He asked three plainer questions instead. How many cells is it made of? Is its nucleus enclosed in a membrane or not? And how does it get its food — does it make it, absorb it, or eat it?
Sorted by those three questions, the awkward organisms stopped being awkward. Five groups appeared instead of two, and each one held organisms that were genuinely alike. That scheme is the one you are about to learn, and the animal kingdom is the branch of it this chapter follows all the way down.
Diagram
Fig. 8.0-1 — Organisms That Did Not Fit Two Kingdoms Line drawing of four organisms arranged in a row with a question mark above each: a mushroom, a rod-shaped bacterium with a flagellum, a Euglena with chloroplasts and a flagellum, and a sponge attached to a rock; label each organism by name only.
8.1 Why Living Things Are Classified
Step outside for ten minutes and count the different kinds of living things you can see. A mango tree, grass, an ant, a crow, a stray dog, the mould on last week’s bread. Biologists have already described well over a million kinds of animals alone, and new ones are reported every year.
What is striking is not the range of sizes. A whale is enormous and a Paramecium is invisible to the naked eye, but size by itself tells you very little. What matters far more is the range of complexity — how the body of an organism is built and how many levels of organisation it has. Some organisms are a single cell doing every job. Others are billions of cells sorted into tissues and organs, each doing one job well. Fig. 8.1-1 shows a small sample of that range.
Diagram
Fig. 8.1-1 — A Sample Of The Diversity Of Living Things Line drawings of eleven organisms arranged in three rows, drawn roughly to show their relative complexity but not to scale; label mango tree, maize, fern, Spirogyra, bread mould, Amoeba, Paramecium, prawn, Catla, rabbit, deer.
No one can study a million kinds of animals one at a time. So biologists group them. Organisms that share many features are placed together, and organisms that share fewer features are placed further apart. The groups are also arranged so that simpler bodies come first and more complex bodies follow.
This is what classification means — sorting living things into groups on the basis of the features they share, so that a statement made about a group holds true for every member of it. It is worth the trouble, because once you know an animal is a mammal you already know a great deal about it without ever having met it.
The Ladder Of Categories
Grouping is done at several levels, one inside the next, like folders inside folders. The smallest and most exact group is the species; the largest is the kingdom. In between lie five more levels, and each one is wider than the one before it, as FC 8.1 sets out.
Flowchart
FC 8.1 — The Levels Of Classification, Smallest Group To Largest Species → Genus → Family → Order → Class → Phylum → Kingdom
Read that chain from left to right and each step swallows the one before it. A genus holds several species; a family holds several genera; and a kingdom holds everything down the line. Read it from right to left and each step is narrower and more particular, until at the far end a single species is left.
The rest of this chapter climbs that ladder. We begin at the bottom rung, with the species, because it is the one rung defined by something the organisms themselves do, rather than by a feature a biologist has chosen to notice.
Of the kingdoms you will meet, only one — Kingdom Animalia — is followed here in full detail, phylum by phylum. The other four are described briefly, so that you can see where animals sit among all living things.
8.2 What Is A Species
Picture the cats in your neighbourhood. One is grey, one is patched orange and white, one has a stub of a tail and one has a long one. They differ in almost every way you can see. Yet any of them can mate with any other, and the kittens will grow up and have kittens of their own.
That last point is the one that matters. Two organisms belong to the same species when they can breed together and produce young that are themselves able to breed. Every house cat in the world passes that test, so despite all their differences they are one single species — Felis catus.
Notice what the definition does not say. It says nothing about colour, size or shape. Appearance can mislead — two organisms may look alike and still be unable to breed together, and two members of one species may look quite unlike each other.
8.2.1 Breeding Within A Species
The same test applied to human beings gives a clear answer. People in different parts of the world differ in height, in skin colour, in hair and in features. All of them, without exception, belong to one single species, Homo sapiens, because children born anywhere in the world grow up able to have children of their own.
The test works just as well for plants. Every peepal tree belongs to one species and every mango tree to another, and each breeds true within its own kind.
Within a species there are often breeds — groups kept apart and bred by people for particular qualities such as size, speed, coat or yield. Horses give a good example. A Marwari horse and a Kathiawari horse look different and are bred separately, but they belong to the same species; cross them and the foal grows up perfectly able to breed. Breeds are our doing. A species is nature’s.
8.2.2 Crosses Between Two Different Species
A horse (Equus caballus) and a donkey (Equus asinus) are two different species. Each has its own breeds, and within each species breeding runs normally. But the two can also be mated with each other, and people have been doing exactly that for centuries.
The result is the mule, shown in Fig. 8.2-1. It is a hybrid — an offspring of parents belonging to two different species. A mule is born of a male donkey and a female horse, called a mare, and it is neither a horse nor a donkey. It takes the size and shape of its ears, its legs, its hooves, its tail shape and its short mane from its donkey father, and its height and shoulder from its horse mother. It is strong and sure-footed, which is why it has been bred for so long.
Diagram
Fig. 8.2-1 — The Mule And Its Two Parent Species Line drawing showing a male donkey on the left and a female horse (mare) on the right, with arrows from both meeting at a mule drawn below and between them; label donkey (male), Equus asinus, horse (female), Equus caballus, and mule, and mark the mule ‘sterile’.
And there the line stops. A mule cannot have young of its own. It is sterile — unable to produce offspring. Every mule that has ever worked in a field was bred fresh from a donkey and a horse.
Reverse the parents and you get a different animal with the same limitation. A male horse crossed with a female donkey, called a jenny, gives a hinny, which is also unable to breed. Table 8.1 sets the two side by side.
Other Crosses Between Species
Mules are not the only such cross that people have produced.
- A zebra mated with a donkey gives a zenkey, seen in Fig. 8.2-2. It has the ears and back of a donkey, and stripes on its legs and hips from the zebra. It is sterile.
- A male tiger mated with a female lion gives a tigon, which is also sterile.
- A goat mated with a sheep gives what is called a geep. Such crosses very rarely succeed, and the offspring cannot breed.
Diagram
Fig. 8.2-2 — A Young Zenkey With Its Donkey Mother Line drawing of a donkey standing with a young zenkey beside it; label donkey (mother) and zenkey, and mark the striped legs and hips of the zenkey with a leader line.
One more example is often mentioned alongside these animals — the pomato, a plant that carries potatoes below the ground and tomatoes above it, and sets no seed. It is worth being careful here. The pomato is not made by mating two species at all. It is made by joining a tomato plant onto a potato plant so that the two grow as one, or in the laboratory by fusing their cells directly.
Put the animal examples together and a rule stands out. Crossing between species succeeds only where the two species are already very closely related — a horse and a donkey, a zebra and a donkey, a lion and a tiger. Even then the young that result are almost always unable to breed. The boundary of a species is not a matter of opinion. It is drawn by whether fertile young can be produced, and it holds firm.
8.3 Categories Higher Than Species
A species is a narrow group. Above it lie six wider ones, each formed by gathering together the groups below it that resemble one another. We now climb those six rungs one at a time, staying with familiar animals so that you can see what is being gathered at each step.
8.3.1 Genus
There are crows around most Indian homes, and there is more than one kind. The crow of the plains, the house crow, has a grey neck and breast and a fairly slim beak. The crow of the hills, the jungle crow, is glossy black all over and carries a heavier, more arched beak. Fig. 8.3-1 puts the two side by side, and Table 8.2 lists what separates them.
Diagram
Fig. 8.3-1 — Two Crows Of India Side-by-side line drawings of a house crow and a jungle crow in profile on a branch, drawn to relative size; label the house crow’s grey neck and breast and slim beak, and the jungle crow’s uniformly black plumage and heavy arched beak; give each its common and scientific name.
These two crows cannot interbreed, so they are two different species. Anyone can nevertheless tell at a glance that both are crows and neither is a parrot or a myna. Species that resemble one another this closely are placed in the same genus (plural: genera) — the group formed by species that are clearly alike. Both crows belong to the genus Corvus.
8.3.2 Family
The lion (Panthera leo) and the tiger (Panthera tigris) are two different species, and they share the genus Panthera. The domestic cat sits in a genus of its own, Felis.
The two genera are not the same, but they have a great deal in common — retractile claws, a short face, teeth built for cutting flesh, a hunter’s way of moving. Genera that share such features are gathered into a family. Here the family is Felidae, the cat family, and it holds the big cats and the house cat together.
8.3.3 Order
Now step back once more. The cat family Felidae and the dog family Canidae — dogs, foxes and jackals — are clearly different families. Yet both are built for hunting and eating flesh, with sharp cutting teeth and a short gut. Families that share features of this kind make up an order. The order that holds Felidae and Canidae is Carnivora. FC 8.2 traces the whole build-up from a single species to that order.
Flowchart
FC 8.2 — From Two Species To One Order Panthera leo and Panthera tigris → genus Panthera; the domestic cat → genus Felis → these genera together → family Felidae; the dogs, foxes and jackals → family Canidae → Felidae and Canidae together → order Carnivora
8.3.4 Class
Carnivora is only one order among many. Bats belong to another, whales to another, monkeys and human beings to yet another. Set these orders beside one another and a deeper likeness shows through: hair on the skin, and milk glands with which the mother feeds her young.
Related orders that share such features form a class. The class here is Mammalia, and every animal named in the last paragraph belongs to it.
8.3.5 Phylum
Above the class comes the phylum, the largest division within a kingdom, formed by related classes. Mammals, birds, reptiles, amphibians and fishes are five separate classes, and every one of them has, at some stage of life, a stiff rod along the back — a notochord — which in most of them is later replaced by a backbone. Those classes together make the phylum Chordata.
8.3.6 Kingdom
The kingdom is the largest division of living things. Every phylum sits inside one, and all animals — from the sponge fixed to a rock to the blue whale — sit inside Kingdom Animalia.
8.4 Drawbacks Of The Two Kingdom Classification
The oldest scheme still in use had only two kingdoms. Living things were either plants or animals, and nothing else was allowed. This is the arrangement set out by Carolus Linnaeus (1707–1778), and for a long time it served well enough: Kingdom Plantae held every plant, and Kingdom Animalia held every animal.
The trouble came from the organisms nobody had looked at closely when the scheme was drawn up. As microscopes improved, three groups turned out to sit badly in both boxes.
Bacteria Are Not Plants
A bacterium has no nucleus enclosed in a membrane. Its genetic material lies loose in the cytoplasm. An organism built this way is called prokaryotic, while an organism whose nucleus is wrapped in a membrane is eukaryotic. Every plant cell is eukaryotic, so a bacterium cannot sensibly be filed with plants. Several bacteria also carry a whip-like flagellum — a fine thread that lashes and drives the cell forward — so they swim, which plants do not.
Some Single Cells Are Both
Euglena is a single-celled organism of pond water, drawn in Fig. 8.4-1. In light it behaves like a plant: it carries chloroplasts and makes its own food. Kept in the dark, it behaves like an animal instead, taking in ready-made food from the water through a gullet at its front end, and it swims all the while with a flagellum. Calling it a plant hides half of what it does; calling it an animal hides the other half.
Diagram
Fig. 8.4-1 — Euglena, Neither Plant Nor Animal Labelled drawing of a single Euglena cell in side view; label flagellum, gullet, chloroplasts, nucleus, contractile vacuole and pellicle.
Fungi Have No Chlorophyll
A mushroom or a bread mould has no chlorophyll at all, and so cannot make its own food. It has no root, no stem and no leaf either. Almost the only plant-like thing about a fungus is that it stays where it is. That is not enough to make it a plant.
Table 8.3 gathers the three problem groups together.
The sensible response was not to argue about which box each awkward organism belonged in, but to add boxes. Bacteria, the single-celled organisms with a true nucleus, and the fungi were each given a kingdom of their own — Monera, Protista and Fungi. Together with Plantae and Animalia, these make the five kingdoms of the scheme used today.
8.5 The Five Kingdom Classification
Once the awkward organisms were given kingdoms of their own, five kingdoms remained, and they are the ones biology uses today. In order, they are Monera, Protista, Fungi, Plantae and Animalia.
Three questions decide which kingdom an organism goes into, and it is worth holding them in mind as you read the five descriptions that follow. Is the cell prokaryotic or eukaryotic? Is the body one cell or many? And how is food obtained — made, absorbed, or eaten? FC 8.3 puts the three questions in the order in which they are asked.
Flowchart
FC 8.3 — How The Five Kingdoms Are Separated All organisms → prokaryotic, unicellular, nucleus not organised → Kingdom Monera; all organisms → eukaryotic, nucleus organised → branches into: unicellular → Kingdom Protista; multicellular → branches into: with cell wall → does not photosynthesise → Kingdom Fungi; with cell wall → photosynthesises → Kingdom Plantae; without cell wall → Kingdom Animalia
8.5.1 Kingdom Monera
Monera holds the organisms whose cells are prokaryotic, and it mainly means the bacteria. Each one is a single cell, and that cell has no organised nucleus — no nuclear membrane shutting the genetic material away from the rest of the cytoplasm.
Bacteria are named partly by shape, and Fig. 8.5-1 shows the three common ones. They may live singly, in small groups or in chains.
Diagram
Fig. 8.5-1 — Three Common Shapes Of Bacteria Line drawing showing rod-shaped bacteria singly and in a chain, spherical bacteria singly and in a cluster, and a spiral bacterium; label the three shapes as rods, spheres and spirals, and show a flagellum on one rod.
8.5.2 Kingdom Protista
Protista — more accurately called Protoctista — holds organisms that are eukaryotic and mostly single-celled. The one cell has a well-defined nucleus with a nuclear membrane around it, which is what separates this kingdom from Monera.
A protist may feed either way. Chlamydomonas is green and makes its own food. Amoeba and Paramecium are not green and take in ready-made food, as does Euglena, which you have already met. A few plant-like protists are not single-celled at all: Pandorina lives as a small colony of cells that photosynthesise together. Four of them are drawn in Fig. 8.5-2.
Diagram
Fig. 8.5-2 — Four Common Protists Line drawings of Amoeba with pseudopodia, Paramecium with cilia and oral groove, Chlamydomonas with two flagella and a cup-shaped chloroplast, and a Pandorina colony of cells in a jelly sphere; label each with its name and give each a scale note.
8.5.3 Kingdom Fungi
Fungi are eukaryotic and usually multicellular, and none of them has chlorophyll. They feed by saprophytic nutrition — absorbing food in solution from dead and decaying matter around them.
The body of most fungi is not built of separate cells at all. It is a mesh of fine threads called hyphae, and inside a hypha the cytoplasm runs on continuously with many nuclei scattered through it. Bread mould, toadstool, yeast and Penicillium are shown in Fig. 8.5-3. Yeast is the exception among them: it is a single cell.
Diagram
Fig. 8.5-3 — Four Common Fungi Line drawings of bread mould with upright hyphae and round sporangia, a toadstool with cap and stalk, budding yeast cells, and a Penicillium hypha with brush-like chains of spores; label each with its name and label a hypha in the bread mould.
8.5.4 Kingdom Plantae
Plants are multicellular and eukaryotic, and every one of their cells is enclosed in a cell wall. They carry chlorophyll and are autotrophic — they make their own food by photosynthesis and so feed themselves.
The kingdom is divided into five major divisions, drawn in Fig. 8.5-4: Thallophyta (the algae), Bryophyta (mosses and liverworts), Pteridophyta (ferns), Gymnosperms (pine and its relatives) and Angiosperms (the flowering plants).
Diagram
Fig. 8.5-4 — The Five Divisions Of Kingdom Plantae Line drawings of a filamentous alga, a moss with liverwort alongside, a fern with fronds and rhizome, a pine branch bearing a cone, and a flowering plant bearing a fruit; label each drawing with its division name and its common name.
8.5.5 Kingdom Animalia
Animals are multicellular and eukaryotic, and their cells have no cell wall — only a cell membrane. They carry no chlorophyll, so they cannot make food. Every animal is heterotrophic, obtaining ready-made food by eating or sucking it.
Most animals move about, and that is what people expect an animal to do. It is not part of the definition. Sponges and corals stay fixed to one spot for life — they are sessile — and they are animals all the same.
8.5.6 Plantae And Animalia Compared
Of the five kingdoms, the two you meet daily are Plantae and Animalia, and the differences between them run right through the organism — from the single cell to the way the whole body behaves. Table 8.4 sets out the clearest of them.
8.6 Naming An Organism
Grouping organisms is only half the task. Each one also has to be named, and named in a way that means the same thing to everybody who uses it.
8.6.1 Why Common Names Are Not Enough
The names we use every day are local names, and they change from one district to the next. The pumpkin alone is called kaddu, kumhra, petha and several other things across India, and the same word may attach to quite a different fruit a few hundred kilometres away. A name that shifts with the language of the speaker cannot be used to record a fact.
A common name is often not exact enough either. There are many kinds of frog, many kinds of earthworm, and a great many kinds of grass, and ‘frog’ names none of them in particular. Even where a language does distinguish two kinds — the jungle crow and the house crow of the last few pages — it has to reach for an adjective to do it, and the adjective can mislead. A jungle crow that flies down to the plains is still a jungle crow.
So a name is needed that belongs to the organism rather than to the locality: one name for one kind, used everywhere.
8.6.2 Binomial Nomenclature
The practice adopted in science is to give every species a name of two parts. The first part is the name of the genus and begins with a capital letter; the second names the particular species within that genus and begins with a small letter. Both parts are written in Latin form and printed in italics, as Fig. 8.6-1 shows.
Diagram
Fig. 8.6-1 — How A Scientific Name Is Built Diagram showing the name Musca domestica printed large in italics, with a labelled arrow to the first word reading ‘genus name, capital first letter’ and a labelled arrow to the second word reading ‘species name, small first letter’; below it, the same name shown as handwritten with each word underlined separately.
This method of naming is called binomial nomenclature — from bi, meaning two, and nomen, meaning name — and it was introduced by Linnaeus. Because the name is fixed by rule and not by locality, it carries the same meaning in every country and in every language.
Here are the scientific names of some familiar animals.
- Human being — Homo sapiens
- Domestic cat — Felis catus
- Dog — Canis familiaris
- Indian honeybee — Apis cerana indica
- Housefly — Musca domestica
- Peacock — Pavo cristatus
- Cobra — Naja naja
- Hoopoe — Upupa epops
8.6.3 Rules For Writing Scientific Names
Two rules govern the writing of a scientific name, and both are followed strictly.
- The name is always written in the Roman script. In print it is set in italics. When you write it by hand, italics are impossible, so the two words are underlined separately instead.
- The genus name begins with a capital letter and the species name with a small letter, whatever the position of the name in a sentence.
Naming Applied To Whole Groups
The full classification of an organism names it at every rank, not just the last two. Table 8.5 does this for four animals and one plant, and reading down any one column shows the ladder of Section 8.1 filled in from top to bottom.
Read across any row instead, and you can see exactly where two organisms part company. Cat and tiger travel together as far as the family and separate only at the genus. Dog leaves them one rung earlier, at the family. Mango parts from all four at the very first rung.
8.7 The Major Groups Of Animals
More than a million species of living animals have been identified so far, and fresh ones are described every year. All of them are sorted into groups and subgroups, and the first sorting divides Kingdom Animalia into nine major phyla.
They are listed here in the order in which you will study them, which is also roughly the order of increasing complexity of the body.
- Phylum Porifera
- Phylum Cnidaria
- Phylum Platyhelminthes
- Phylum Nematoda
- Phylum Annelida
- Phylum Arthropoda
- Phylum Mollusca
- Phylum Echinodermata
- Phylum Chordata
The order is not accidental. The first phylum on the list is built at the simplest level — a mass of cells with no true tissues — and each phylum after it adds something: a tissue layer, a body cavity, segments, jointed limbs, a skeleton inside the body. FC 8.4 shows the same nine names as a single run from simplest to most complex, and Fig. 8.7-1 puts a familiar animal against each of them.
Flowchart
FC 8.4 — The Nine Phyla, Simplest Body To Most Complex Porifera → Cnidaria → Platyhelminthes → Nematoda → Annelida → Arthropoda → Mollusca → Echinodermata → Chordata
Diagram
Fig. 8.7-1 — One Representative From Each Phylum Nine small line drawings arranged in three rows in phylum order, each labelled with its phylum name and the animal’s common name: a vase-shaped sponge, a Hydra with tentacles, a flat planarian, a slender roundworm, an earthworm, a grasshopper, a snail with its shell, a five-armed starfish, and a fish.
Everything from here to the end of the chapter is a walk down this list. The first eight phyla are taken together in Section 8.9, and the ninth, which includes you, is taken separately in Section 8.11.
8.8 Invertebrata And Vertebrata
Run a finger down the middle of your back and you can feel a row of hard knobs under the skin. That row is the backbone, or vertebral column, and it divides the animal kingdom into two very unequal halves.
The first eight phyla of the list — Porifera through to Echinodermata — contain no animal with a backbone. They are grouped together as Invertebrata, the invertebrates. The ninth phylum, Chordata, contains the animals that have one, and these are the Vertebrata, or vertebrates.
This split is a convenience rather than a formal rank of classification. ‘Invertebrate’ is not a phylum; it is simply a word for the eight phyla taken together, and they differ from one another far more than any of them differs from a fish.
The backbone is the difference people notice first, but it is not the only one, and the rest are more interesting. Fig. 8.8-1 cuts an insect and a fish across the body and places the two sections side by side.
Diagram
Fig. 8.8-1 — A Typical Invertebrate And Vertebrate Compared In Cross Section Two transverse sections drawn side by side, an insect on the left and a fish on the right, each with the back at the top and the underside at the bottom; in the insect label the solid nerve cord low down, the heart high up and the hard outer covering; in the fish label the hollow nerve cord high up inside the vertebral column, the heart low down and the internal skeleton.
In an animal, the side towards the back is called the dorsal side and the side towards the belly the ventral side. Read Fig. 8.8-1 with that in mind and a pattern appears: what lies high in one animal lies low in the other. The nerve cord of a vertebrate runs along the back and is hollow; the nerve cord of an invertebrate runs along the underside and is solid. The heart of a vertebrate lies towards the belly; the heart of an invertebrate, where there is one, lies towards the back. Table 8.6 sets out these differences together with the rest.
8.9 The Invertebrate Phyla — Porifera To Echinodermata
The eight phyla in this section have nothing in common except the absence of a backbone. Read them in order and watch for what each one adds to the body plan of the one before it.
8.9.1 Phylum Porifera
Porifera are the simplest of all multicellular animals. The name means ‘pore-bearing’, and it describes the body exactly. Two of them are drawn in Fig. 8.9-1.
Diagram
Fig. 8.9-1 — Two Sponges Line drawings side by side of Sycon, a vase-shaped sponge attached to a rock, and a bath sponge; on Sycon label the pores or ostia in the body wall, the central cavity, the osculum at the top, a spicule in the wall and the substratum, and show arrows for water entering through the pores and leaving through the osculum.
The body is little more than a hollow tube fixed to a rock. There is no single mouth. Instead the body wall is pierced by many small pores, and water carrying food particles and oxygen is drawn in through them. Food is captured from that water inside the body. One large opening at the top, the osculum, lets the used water out again.
A sponge could not do this if it were solid, and could not hold its shape without support. The support comes from a skeleton of microscopic spicules — tiny needles of hard material — or, in some sponges, of elastic spongin fibres. It is the springy spongin skeleton, washed clean of the living cells, that was once sold as a bath sponge.
Almost every sponge lives in the sea; only one or two kinds are found in fresh water. Sycon and the bath sponge are the examples to remember.
8.9.2 Phylum Cnidaria
Cnidarians are sac-like animals, and Fig. 8.9-2 shows three of them. The body wall here is not a loose collection of cells but two definite layers of tissue, an outer epidermis and an inner gastrodermis. This is the first phylum built at the tissue level.
Diagram
Fig. 8.9-2 — Three Cnidarians Line drawings of a sea anemone, a jellyfish and a piece of branching coral; label the tentacles, mouth and basal disc of the sea anemone, the tentacles and umbrella-shaped body of the jellyfish, and the hard skeleton and small polyps of the coral.
The two layers enclose a single cavity called the coelenteron, in which the food is digested. That cavity opens to the outside at one end only, through a mouth. Whatever cannot be digested must leave the way it came in.
Around the mouth stands a ring of tentacles, armed with stinging cells, with which the animal catches small organisms and pushes them inside. Because the tentacles are arranged evenly all round the mouth, food can be seized from any direction — which suits an animal that cannot chase anything.
Cnidarians live in water, nearly all of them in the sea. Hydra, jellyfish, sea anemone and corals belong here.
The Next Three Phyla — The Worms
Platyhelminthes, Nematoda and Annelida are often lumped together in ordinary speech as ‘worms’. The word is not a scientific group at all. It simply means a crawling or creeping animal with a fairly soft body, and the three phyla differ from one another in ways that matter a great deal.
8.9.3 Phylum Platyhelminthes
These are the flatworms — small, soft and flattened from top to bottom, and usually without segments. Two are shown in Fig. 8.9-3.
Diagram
Fig. 8.9-3 — Two Flatworms Line drawings of a planarian in surface view with its eye spots and broad flat body labelled, and the front end of a tapeworm with its scolex and suckers labelled; mark the flattened shape in each.
A flatworm has no body cavity between its gut and its body wall — it is acoelomate. Its alimentary canal has a single opening, the mouth, which serves as an exit as well.
Most flatworms live on or inside other animals as parasites; the liver fluke and the tapeworm are examples. A few, such as the planarian, live freely in the sea or in fresh water.
8.9.4 Phylum Nematoda
The roundworms are long, cylindrical and unsegmented, and they taper towards both ends. Fig. 8.9-4 shows two of them.
Diagram
Fig. 8.9-4 — Two Roundworms Line drawings of an Ascaris worm showing its smooth cylindrical unsegmented body with mouth and anus labelled at the two ends, and an eelworm drawn much magnified with a note that it is greatly enlarged.
Between the gut and the body wall there is a fluid-filled space, but it is not a true body cavity; it is called a false body cavity, and animals with it are pseudocoelomate. The alimentary canal is an improvement on the flatworm’s: it opens at both ends, at a mouth and at an anus, so food travels one way through the body.
Most roundworms are parasites — the hookworm, Ascaris and the filaria worm live in human beings, and eelworms attack the roots of potato plants — but some live freely in soil.
8.9.5 Phylum Annelida
The body of an annelid is cylindrical and divided along its length into a series of ring-like segments, which is what the name records. Three annelids are drawn in Fig. 8.9-5.
Diagram
Fig. 8.9-5 — Three Annelids Line drawings of an earthworm, a leech and Nereis, drawn to show the ring-like segments along each body; label the mouth, clitellum, body segments and anus of the earthworm, the posterior sucker of the leech, and the prostomium and parapodia of Nereis.
An annelid has a well-developed digestive system with the alimentary canal open at both ends, at a mouth and an anus. It also has, for the first time in this list, a true body cavity — the coelom — a fluid-filled space between the gut and the body wall, lined on both sides.
That cavity matters more than it sounds. Because the gut is slung inside a fluid-filled space instead of being packed against the body wall, it can move and churn food independently of the animal’s own crawling. Excretion in annelids is carried out by fine coiled tubes called nephridia.
Table 8.7 sets the three worm phyla against one another.
8.9.5.1 The Earthworm
Every earthworm lives in a burrow in damp soil, and it makes that burrow by eating its way through the ground. Soil is swallowed in large quantities and passed out at the other end of the body as castings — the small coiled heaps you find on wet ground after rain. What the worm digests out of that soil is the organic matter in it: humus and broken bits of leaves.
Earthworms avoid light and are nocturnal, coming up to the surface at night to feed and to mate. Dry soil does not suit them, and the reason is a matter of life and death: an earthworm has no lungs and breathes through its skin, which works only while the skin stays moist.
The common Indian earthworm is Pheretima posthuma, drawn in Fig. 8.9-6. Its long cylindrical body is divided into a series of 80 to 100 ring-like segments.
Diagram
Fig. 8.9-6 — External Features Of Pheretima Posthuma Whole-animal drawing of an earthworm in side view with the front end to the left; label the mouth at the anterior end, the ring-like segments, the clitellum as a thick band across segments 14 to 16, the lighter ventral side and the anus at the posterior end; use straight, non-crossing lead lines.
Look for a head and you will not find one. There are no legs, no eyes and no tail. The mouth is a simple opening at the front end, and there are no jaws. The anus lies at the extreme hind end.
In a mature worm a short band of thick glandular skin, the clitellum, surrounds the body like a belt across segments 14 to 16 — about one-third of the way back from the front end. The clitellum serves in reproduction, and it is the surest way of telling the front of a worm from the back. The lighter-coloured ventral side near it bears several small openings, and these too are concerned with reproduction.
8.9.5.2 Economic Importance Of The Earthworm
An animal that eats soil and lives underground turns out to be one of the farmer’s most useful allies.
- As it burrows it loosens the soil and lets air into it. Roots respire, and they respire better in aerated soil, so plants grow well and send their roots deeper.
- Earthworms move continually between the surface and the layers below, carrying the top soil down and the lower soil up, and this mixing raises the fertility of the whole.
- Their excreta is rich in nitrogenous matter, which plants need in order to grow.
- Earthworms are reared deliberately in vermiculture to convert farm and kitchen waste into a high-quality manure.
- Many people use earthworms as bait when catching fish.
8.9.6 Phylum Arthropoda
Arthropoda contains more species than any other phylum in the animal kingdom — crayfish, crabs, millipedes, centipedes, insects, scorpions and spiders all belong to it.
The name means ‘jointed legs’, and that is the first thing to notice. The limbs are jointed, and there is one pair on some or on all of the body segments. A jointed limb can bend at fixed points, which is what makes walking, gripping and swimming possible for an animal encased in armour.
The armour is the second thing to notice. An arthropod has an exoskeleton — a hard covering on the outside of the body — made of a tough substance called chitin. It protects the animal and gives its muscles something to pull against, but it cannot stretch. An animal sealed inside a rigid case cannot grow inside it.
The way out is to shed it. During growth in early life the arthropod casts off the whole exoskeleton and grows a new and larger one, and the casting off together with the regrowing is called moulting.
8.9.6.1 The Four Classes Of Arthropods
The phylum is further divided into four classes, drawn in Fig. 8.9-7.
Diagram
Fig. 8.9-7 — The Four Classes Of Arthropods Four labelled panels of line drawings — a crayfish and a crab for Crustacea, a centipede and a millipede for Myriapoda, a grasshopper and a beetle for Insecta, and a spider and a scorpion for Arachnida; note under each panel the number of pairs of legs.
Crustacea. Crayfish and crabs are examples. Crustaceans are mainly aquatic and have a hard chitinous exoskeleton. The body is divided into head, thorax and abdomen, but the head and thorax are fused together. They carry many appendages, used for feeding, walking, running and swimming.
Myriapoda. Centipedes and millipedes belong here. Their bodies have many segments, and each segment carries one or two pairs of limbs. The trunk is not divided into thorax and abdomen.
Insecta. Silverfish, housefly, mosquito, beetle, grasshopper, locusts and cockroach are all insects. Three features mark the class:
- The body is divided into head, thorax and abdomen.
- The thorax carries three pairs of jointed legs.
- There are usually two pairs of wings, sometimes a single pair, and sometimes none at all.
Arachnida. Spiders, scorpions, mites and ticks belong here. The body is in two parts: a front part in which head and thorax are fused together, and a hind part which is the abdomen. Arachnids have four pairs of jointed legs, and they lack both wings and antennae.
8.9.7 Phylum Mollusca
Molluscs have a soft, unsegmented body, which in most of them is protected by a hard shell of calcareous material — the shells you pick up on a beach. Fig. 8.9-8 shows several.
Diagram
Fig. 8.9-8 — Some Common Molluscs Line drawings of a mussel, a chiton, a snail with its coiled shell, an octopus, a slug, Sepia the cuttlefish and Loligo the squid, each labelled; on the snail label the shell and the muscular foot.
Beneath the shell lies a broad muscular foot. The animal presses and stretches it against the ground to creep along, and in some molluscs the foot serves as a weapon as well. Aquatic molluscs breathe by means of gills.
Snail, slug, oyster, mussel, clam, squid and octopus are the examples to know. The octopus and the squid show how far the plan can be pushed: the shell is reduced or lost altogether, and the foot is drawn out into arms.
8.9.8 Phylum Echinodermata
Echinoderms are unsegmented marine animals with a spiny surface over an exoskeleton — the name means ‘spiny-skinned’. Four of them appear in Fig. 8.9-9.
Diagram
Fig. 8.9-9 — Four Echinoderms Line drawings of a starfish, a brittle-star, a sea urchin and a sea cucumber; on the starfish label the five arms, the spiny upper surface and the tube feet on the underside of one arm.
They have no head. Instead, similar parts — usually five of them — are arranged regularly around a central point, so that the body looks the same from any side. A body built this way is described as radially symmetrical, a term Section 8.10 takes up in full.
Movement is managed by hundreds of small tube feet on the underside, which grip and release the sea floor in turn and draw the animal slowly along. Starfish, brittle-star, sea urchin and sea cucumber all belong here.
8.10 Body Symmetry And Body Directions
Two sets of words have been used loosely in the last few pages — words for the shape of an animal’s body, and words for its directions. Both are settled properly here, because the descriptions of the vertebrate classes ahead depend on them.
8.10.1 Body Symmetry
Imagine trying to cut an animal into two equal halves. Whether you can, and in how many ways, tells you something real about how the body is built. Fig. 8.10-1 tries this on four animals.
Diagram
Fig. 8.10-1 — Three Kinds Of Body Symmetry Four line drawings, each crossed by straight double-headed arrows showing possible planes of cutting: an Amoeba with several arrows through it, none giving equal halves; a starfish with arrows along several radii, each giving equal halves; a housefly in top view and a human figure in front view, each with a single arrow down the middle line; label the three panels asymmetry, radial symmetry and bilateral symmetry.
Amoeba. However you cut it, the two halves never match, because the shape itself keeps changing. Amoeba is asymmetrical — it shows no symmetry at all.
Starfish. Any line through the centre, along any arm, divides the body into two similar halves. The starfish is radially symmetrical, which is what you would expect of an animal with five similar arms set round a central point.
Housefly and human being. Here only one cut works: a single plane down the middle of the body, separating a left half from a matching right half. This is bilateral symmetry. Almost all vertebrates and a great many invertebrates are built this way. Table 8.8 sets the three side by side.
8.10.2 Body Directions
Describing an animal accurately needs a fixed set of directions, and five words cover them. They are shown on the rabbit in Fig. 8.10-2.
Diagram
Fig. 8.10-2 — Body Directions Side view of a rabbit facing left; label anterior with an arrow towards the head, posterior with an arrow towards the hind end, dorsal with an arrow upwards away from the ground, ventral with an arrow downwards towards the ground, and mark the lateral surfaces as the right and left sides.
- Anterior — towards the front, the head end.
- Posterior — towards the hind end.
- Dorsal — the back, or the upper side of the body.
- Ventral — the underside, or belly side.
- Lateral — the sides, right or left.
These words are fixed to the animal and not to the room it stands in. The dorsal surface of a crawling lizard faces the sky, and the dorsal surface of a swimming fish may face sideways as it turns, but in both animals it is the same surface of the same body.
8.11 Phylum Chordata
The last of the nine phyla is the one you belong to. Every chordate has, at some stage of its life, a notochord — a firm, flexible rod running along the mid-dorsal axis of the body, just under the nerve cord.
In a few primitive chordates that rod stays for life. In all the rest it is replaced during development by a backbone, the vertebral column, built of separate pieces. This includes you: a human embryo has a notochord long before it has a single vertebra.
Chordates that go on to build a backbone are the vertebrates. In them the vertebral column forms the main axis of an internal skeleton, which may be made of bone or of cartilage. A vertebrate body has a head, a trunk and two pairs of appendages, and gill slits appear at some time in its life, even in animals that never breathe with gills.
The vertebrates are divided into five classes: Pisces, Amphibia, Reptilia, Aves and Mammalia. Fig. 8.11-1 shows one animal from each, and FC 8.5 gives the questions that separate them.
Diagram
Fig. 8.11-1 — Representatives Of The Five Classes Of Vertebrates Five line drawings labelled with their class names: a fish with tail fin, anal fin, pelvic fin and pectoral fin labelled; a frog with eye, ear drum, fore limb and hind limb labelled; a lizard with dry scaly skin and tail labelled; a crow with beak, neck and tail feathers labelled; and a rat with ear pinna, tail and hairy skin labelled.
Flowchart
FC 8.5 — Separating The Five Classes Of Vertebrates Does the animal have hair and mammary glands? → yes → Mammalia; no → does it have feathers? → yes → Aves; no → does it have dry horny scales and breathe by lungs from birth? → yes → Reptilia; no → does it have moist non-scaly skin, with a larva that breathes by gills? → yes → Amphibia; no → fins, scales and gills throughout life → Pisces
8.11.1 Class Pisces
The class Pisces includes the fishes, and every part of a fish is fitted to life in water. It has fins and no limbs, its body is covered with scales, and it breathes by means of gills, which take dissolved oxygen from the water flowing over them. The heart has two chambers.
A fish is cold-blooded, or poikilothermal, which means its body temperature rises and falls with the temperature of the water around it. Fertilisation in most fishes is external: eggs and sperm are shed into the water.
Fishes fall into two main types, shown in Fig. 8.11-2.
- Cartilaginous fishes, whose skeleton is made of cartilage. Sharks, dogfish and skates are examples. Their gill slits open separately on the side of the body, uncovered.
- Bony fishes, whose skeleton is made of bone. Carps, roaches, herrings and trouts are examples. Their gill slits are concealed under a bony flap, the gill cover or operculum.
Diagram
Fig. 8.11-2 — A Cartilaginous Fish And A Bony Fish Two side-view line drawings of the head region: a shark on the left with five separate gill slits exposed on the side of the head, and a bony fish on the right with a single bony flap covering the gill region; label the gill slits, the operculum and the eye in each.
8.11.2 Class Amphibia
An amphibian lives a divided life, and the class is named for it. The adult spends much of its time on land, but the egg and the larva are aquatic. Eggs are laid in water and fertilised there, outside the body. The larva that hatches breathes by gills; the adult that it becomes breathes by lungs.
The skin is smooth, non-scaly and moist or slimy, and the eardrum, or tympanum, lies exposed on the surface of the head. Amphibians have two pairs of pentadactyl limbs — limbs ending in five fingers or toes. The heart has three chambers, and they are cold-blooded.
Frog, toad, salamander and newt belong to this class.
8.11.3 Class Reptilia
Reptiles are completely adapted to life on land, and two features carry that adaptation. Their eggs have a leathery shell, so they can be laid on dry ground without drying out. And they breathe by lungs right from the moment they hatch — no reptile has a gill-breathing larval stage.
The skin is rough and covered with horny scales, which greatly reduce water loss. The heart has three chambers, but the ventricle is partially divided, so oxygen-rich and oxygen-poor blood mix less than in an amphibian. The eardrum lies at the bottom of a small tubular depression rather than on the surface. Reptiles are cold-blooded.
Lizard, snake, tortoise, turtle, crocodile, alligator and the gharial all belong here. Fig. 8.11-3 shows four of them.
Diagram
Fig. 8.11-3 — Four Reptiles Line drawings of a gharial with its long narrow snout, a lizard with rough scaly skin, a cobra with expanded hood, and a turtle with its shell; label each with its common name and mark the horny scales on the lizard.
8.11.4 Class Aves
All birds belong to this class. They breathe by lungs, and their exoskeleton is a covering of feathers. The forelimbs are modified into wings, while the legs are scaly. The heart has four chambers, so oxygen-rich and oxygen-poor blood are kept completely apart.
Birds are warm-blooded, or homeothermal: they hold an almost constant, relatively high body temperature whatever the temperature of the air around them. They lay eggs with hard calcareous shells.
The body is streamlined and the skeleton light, both of which make flight possible. Not every bird flies. The ostrich is the largest and heaviest living bird and cannot fly at all, but its powerful legs make it the fastest of all birds on land. Pigeon, sparrow, crow, duck, owl, penguin, ostrich and emu are all members of this class.
8.11.5 Class Mammalia
Mammals are the most highly developed animals for life on land, though the class has spread far beyond it: whales and porpoises live in water and bats fly in air.
Two features give the class its character. Mammals have hair on the skin, and the mother feeds her young on milk secreted by mammary glands. Most give birth to living young — they are viviparous — with two exceptions, the Australian spiny ant-eater shown in Fig. 8.11-4 and the duck-billed platypus, both of which lay eggs.
Diagram
Fig. 8.11-4 — The Spiny Ant-Eater, An Egg-Laying Mammal Line drawing of the Australian spiny ant-eater in side view on the ground; label the spines and the eye, and caption it as one of the two mammals that lay eggs.
Mammals are warm-blooded and breathe by lungs, and the heart has four chambers. A sheet of muscle, the diaphragm, separates the thorax from the abdomen inside the body. As a rule a mammal has a pair of external ears, the pinnae, and the testes are carried outside the body cavity in a sac called the scrotum.
Cat, dog, cow, sheep, rat, bat, seal, monkey, apes and human beings are all mammals.
The Five Classes Together
Table 8.9 gathers the five classes so that a single feature can be followed across all of them. Read down a column to describe one class; read across a row to watch one feature change as the body becomes better fitted to life on land.
8.12 Summary
- Living things are enormously varied, and they differ far more in the complexity of their bodies than in their size. Classification sorts them into groups on the basis of shared features, so that a statement made about a group holds for all its members.
- The groups form a ladder, from the smallest and most exact to the largest: species, genus, family, order, class, phylum, kingdom.
- A species is a group of organisms of one kind whose members can interbreed among themselves to produce fertile young.
- Breeds are groups within a species and always produce fertile young. A cross between two closely related species may produce a living animal, such as a mule, a hinny, a zenkey or a tigon, but the offspring is sterile.
- Similar species make a genus; genera with common features make a family; related families make an order; related orders make a class; related classes make a phylum; and phyla are grouped into kingdoms.
- The old two-kingdom scheme of Plantae and Animalia broke down over bacteria, which are prokaryotic, over single-celled organisms such as Euglena, which are part plant-like and part animal-like, and over fungi, which have no chlorophyll.
- The five kingdoms in use today are Monera, Protista, Fungi, Plantae and Animalia. They are separated by three questions: prokaryotic or eukaryotic, one cell or many, and how food is obtained.
- Monera holds prokaryotic single cells, chiefly bacteria. Protista holds eukaryotic, mostly single-celled organisms. Fungi are eukaryotic, without chlorophyll, and feed saprophytically through a mesh of hyphae.
- Plantae are multicellular, have cell walls and chlorophyll, and are autotrophic; the kingdom has five divisions — Thallophyta, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms. Animalia are multicellular, lack a cell wall and chlorophyll, and are heterotrophic.
- Common names vary with locality, so every species is given a two-part scientific name. This is binomial nomenclature, introduced by Linnaeus: the genus name takes a capital letter, the species name a small one, and both are printed in italics or underlined separately when written by hand.
- Kingdom Animalia is divided into nine major phyla, arranged from the simplest body plan to the most complex: Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca, Echinodermata and Chordata.
- The first eight phyla are grouped as invertebrates, without a backbone. Chordata contains the vertebrates. In vertebrates the nerve cord is dorsal and hollow, the heart ventral, and haemoglobin is held in red blood cells; in invertebrates the nerve cord is ventral and solid, the heart dorsal, and haemoglobin, when present, is dissolved in the blood.
- Porifera are the simplest multicellular animals: a hollow body drawing water in through many pores and out through an osculum, supported by spicules or spongin fibres.
- Cnidaria have a two-layered body wall enclosing a single cavity, the coelenteron, with one mouth and a ring of tentacles bearing stinging cells.
- Platyhelminthes are flat, acoelomate, and have an alimentary canal with one opening. Nematoda are cylindrical, pseudocoelomate, with a canal open at both ends. Annelida are segmented, with a true coelom, an alimentary canal open at both ends, and nephridia for excretion.
- The earthworm Pheretima posthuma has 80 to 100 segments, no head, eyes or limbs, and a clitellum across segments 14 to 16. It breathes through moist skin, is nocturnal, and is valuable in agriculture because it aerates and mixes the soil, enriches it with nitrogenous excreta, and is used in vermiculture.
- Arthropoda is the largest phylum. Its animals have jointed limbs and a chitinous exoskeleton which must be moulted for growth. Its four classes are Crustacea, Myriapoda, Insecta and Arachnida; insects have three pairs of legs and arachnids four.
- Mollusca have a soft unsegmented body, usually a calcareous shell, and a muscular foot. Echinodermata are spiny-skinned marine animals, radially symmetrical, with parts usually in fives and locomotion by tube feet.
- A body may be asymmetrical, as in Amoeba; radially symmetrical, as in a starfish; or bilaterally symmetrical, as in a housefly or a human being. Directions on an animal are given as anterior, posterior, dorsal, ventral and lateral.
- All chordates have a notochord at some stage, replaced in most by a vertebral column. Vertebrates have an internal skeleton of bone or cartilage, a head and trunk, two pairs of appendages, and gill slits at some time in life.
- The five vertebrate classes are Pisces, Amphibia, Reptilia, Aves and Mammalia. Across them the breathing organ changes from gills to lungs, the heart from two chambers to four, and the animal from cold-blooded to warm-blooded, as life on land and in air becomes fully possible.
Flowchart
FC 8.6 — The Chapter In One Chain Living organisms → five kingdoms: Monera, Protista, Fungi, Plantae, Animalia → Kingdom Animalia → nine phyla → invertebrates: Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca, Echinodermata; and vertebrates: Phylum Chordata → five classes: Pisces → Amphibia → Reptilia → Aves → Mammalia
8.13 Key Terms
- Acoelomate — having no body cavity between the gut and the body wall, as in a flatworm.
- Animalia — the kingdom of multicellular, eukaryotic, heterotrophic organisms whose cells have no cell wall.
- Anterior — towards the front or head end of an animal.
- Asymmetrical — showing no symmetry; no cut divides the body into two similar halves.
- Autotrophic — able to make one’s own food, as green plants do by photosynthesis.
- Bilateral symmetry — a body plan in which one plane down the middle divides the animal into matching left and right halves.
- Binomial nomenclature — the practice of naming each species by two words, the genus name followed by the species name.
- Bony fish — a fish whose skeleton is made of bone and whose gill slits are covered by an operculum.
- Breed — a group within a species with features fixed by selective breeding.
- Cartilaginous fish — a fish whose skeleton is made of cartilage and whose gill slits are exposed.
- Castings — the soil that an earthworm has passed through its body and deposited at the surface.
- Chitin — the tough material of which an arthropod’s exoskeleton is made.
- Class — the group formed by related orders.
- Classification — the sorting of living things into groups on the basis of shared features.
- Clitellum — the thick glandular band across segments 14 to 16 of a mature earthworm, concerned with reproduction.
- Coelenteron — the single digestive cavity of a cnidarian, opening to the outside by a mouth.
- Coelom — a true body cavity between the gut and the body wall, lined by a membrane on both surfaces.
- Cold-blooded — having a body temperature that rises and falls with the temperature of the surroundings.
- Diaphragm — the sheet of muscle that separates the thorax from the abdomen in a mammal.
- Eukaryotic — having genetic material enclosed within a nuclear membrane.
- Exoskeleton — a hard protective covering on the outside of the body.
- Family — the group formed by genera that share common features.
- Feathers — the light horny outgrowths that form the exoskeleton of a bird.
- Fins — the flattened outgrowths with which a fish steers and balances in water.
- Flagellum — a fine whip-like thread whose lashing drives a cell along.
- Foot — the broad muscular structure with which a mollusc creeps.
- Fungi — the kingdom of eukaryotic, chlorophyll-free organisms that absorb food saprophytically, usually through hyphae.
- Geep — the rare and sterile offspring of a goat and a sheep.
- Genus — the group formed by species that closely resemble one another.
- Gills — the respiratory organs that take dissolved oxygen from water.
- Hair — the outgrowths of the skin characteristic of mammals.
- Heterotrophic — unable to make one’s own food, and so dependent on ready-made food.
- Hinny — the sterile offspring of a male horse and a female donkey.
- House crow — the grey-necked crow of the Indian plains, Corvus splendens.
- Hybrid — an offspring of parents belonging to two different species.
- Hyphae — the fine threads, with continuous cytoplasm and many nuclei, that make up the body of most fungi.
- Invertebrata — animals without a backbone, covering the first eight animal phyla.
- Jungle crow — the wholly black, heavy-billed crow of hilly country, Corvus macrorhynchos.
- Kingdom — the largest division of living things.
- Lateral — relating to the right or left side of an animal.
- Mammary glands — the glands with which a female mammal secretes milk for her young.
- Monera — the kingdom of prokaryotic, single-celled organisms, chiefly bacteria.
- Moulting — the casting off of the old exoskeleton and the growing of a new, larger one.
- Mule — the sterile offspring of a male donkey and a female horse.
- Nephridia — the fine coiled tubes through which an annelid excretes.
- Nocturnal — active at night rather than by day.
- Notochord — the rod-like support along the mid-dorsal axis of a chordate, present at least in the embryo.
- Operculum — the bony flap covering the gill slits of a bony fish.
- Order — the group formed by related families.
- Pentadactyl — having limbs that end in five fingers or toes.
- Phylum — the largest division within a kingdom, formed by related classes.
- Plantae — the kingdom of multicellular, eukaryotic, autotrophic organisms with cell walls and chlorophyll.
- Pomato — a plant made by joining potato and tomato tissue, bearing potatoes below ground and tomatoes above, and producing no seeds.
- Posterior — towards the hind end of an animal.
- Prokaryotic — having genetic material lying free in the cytoplasm, with no nuclear membrane.
- Protista — the kingdom of eukaryotic, mostly single-celled organisms.
- Pseudocoelomate — having a fluid-filled false body cavity that is not lined on both surfaces.
- Radial symmetry — a body plan in which similar parts are set round a central point, so that any radial cut gives two similar halves.
- Saprophytic — feeding by absorbing food in solution from dead and decaying matter.
- Sessile — fixed in one place and unable to move about.
- Species — a group of organisms of one kind whose members interbreed to produce fertile young.
- Spicules — the microscopic hard needles that support the body of many sponges.
- Spongin — the elastic fibrous material forming the skeleton of some sponges.
- Sterile — unable to produce young.
- Tentacles — the slender arms round a cnidarian’s mouth, bearing stinging cells, used to catch food.
- Tigon — the sterile offspring of a male tiger and a female lion.
- Tube feet — the many small suckered structures with which an echinoderm moves.
- Vermiculture — the rearing of earthworms to convert waste into high-quality manure.
- Vertebral column — the backbone, forming the main axis of a vertebrate’s internal skeleton.
- Vertebrata — animals that possess a backbone; the vertebrate members of the phylum Chordata.
- Viviparous — giving birth to living young rather than laying eggs.
- Warm-blooded — holding a nearly constant body temperature whatever the temperature of the surroundings.
- Zenkey — the sterile offspring of a zebra and a donkey.
8.14 Points To Ponder
- A mule is strong, healthy and long-lived, and yet biology refuses to call it a species. If a mule cannot pass anything on to a next generation, what exactly does a species preserve that an individual does not?
- The five-kingdom scheme was built on how a cell is organised, how many cells there are, and how food is obtained. Suppose you had to sort organisms by a single question instead. Which one question would separate living things most usefully, and what would it fail to separate?
- A sponge has no mouth, no gut, no nerves and no muscles, and it never moves. What, then, does it do that a plant does not — and what does that tell you about where the line between the kingdoms is really drawn?
- An arthropod must shed its whole exoskeleton in order to grow, and for a while afterwards it is soft and defenceless. A vertebrate never faces this problem. What does each way of building a skeleton cost the animal, and what does it buy?
- Animals with radial symmetry have no head; animals with bilateral symmetry almost always do. Why should the direction in which an animal usually travels have anything to do with where its sense organs are placed?
- Across the five vertebrate classes the heart moves from two chambers to four, and the animal from cold-blooded to warm-blooded. Are these two changes connected? Consider what a warm body must be supplied with, and how quickly.
8.15 Objective Questions
Multiple Choice Questions
- The kingdom that contains only prokaryotic organisms is: (1 mark)
- Protista
- Monera
- Fungi
- Plantae
- The phylum whose animals have an alimentary canal with only one opening is: (1 mark)
- Nematoda
- Annelida
- Platyhelminthes
- Mollusca
- In a vertebrate, the nerve cord is: (1 mark)
- ventral and solid
- ventral and hollow
- dorsal and solid
- dorsal and hollow
- The class of arthropods whose members have four pairs of legs and no antennae is: (1 mark)
- Insecta
- Arachnida
- Crustacea
- Myriapoda
- A three-chambered heart in which the ventricle is partially divided is found in: (1 mark)
- Pisces
- Amphibia
- Reptilia
- Aves
Assertion And Reason
- In each of the following, choose:
- Both assertion and reason are true, and the reason explains the assertion.
- Both assertion and reason are true, but the reason does not explain the assertion.
- The assertion is true but the reason is false.
- The assertion is false but the reason is true.
- Assertion: A mule is not regarded as a species. Reason: A mule is unable to produce young of its own. (1 mark)
- Assertion: An arthropod must moult as it grows. Reason: The chitinous exoskeleton is rigid and cannot enlarge with the body. (1 mark)
- Assertion: A fungus is placed outside Kingdom Plantae. Reason: A fungus remains fixed in one place throughout its life. (1 mark)
Match The Following
- Match each animal in Column A with its phylum in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Sea anemone | i. Echinodermata |
| b. Sycon | ii. Annelida |
| c. Sea cucumber | iii. Cnidaria |
| d. Nereis | iv. Mollusca |
| e. Cuttlefish | v. Porifera |
Fill In The Blanks
The true body cavity of an annelid is called the ______. (1 mark)
The single large opening through which water leaves the body of a sponge is the ______. (1 mark)
Excretion in an earthworm is carried out by fine coiled tubes called ______. (1 mark)
True Or False
All members of one species can interbreed to produce fertile young. (1 mark)
Yeast is a single-celled member of Kingdom Fungi. (1 mark)
In an invertebrate, haemoglobin is always carried inside red blood cells. (1 mark)
Name The Following
The scientific name of the common Indian earthworm. (1 mark)
The rod-like structure present along the mid-dorsal axis of every chordate embryo. (1 mark)
The two mammals that lay eggs. (1 mark)
The category that is formed when related families are grouped together. (1 mark)
The type of nutrition in which food in solution is absorbed from dead and decaying matter. (1 mark)
Odd One Out, And Give A Reason
Liver fluke, tapeworm, earthworm, planarian (1 mark)
Shark, dogfish, skate, trout (1 mark)
Starfish, sea urchin, sea anemone, brittle-star (1 mark)
8.16 Check Your Concepts
Define a species, and state the one word in your definition that a mule fails to satisfy. (2 marks)
State two features of an arthropod that no annelid possesses. (2 marks)
Give a reason: an earthworm dies if it is kept on dry soil. (2 marks)
Give a reason: a bacterium cannot be placed in Kingdom Plantae. (2 marks)
Name the five kingdoms, and state for each one whether its members are prokaryotic or eukaryotic. (3 marks)
Differentiate between a flatworm and a roundworm, stating the basis of difference in each case. (3 marks)
Explain, with one example, what is meant by radial symmetry, and state why such an animal has no head. (3 marks)
Why is the division of animals into invertebrates and vertebrates described as a convenience rather than a formal rank of classification? (3 marks)
State the two rules for writing a scientific name, and explain why such names are preferred to common names in science. (4 marks)
Describe four ways in which the earthworm is useful in agriculture. (4 marks)
What would happen if an arthropod were unable to moult? Explain your answer in terms of the structure of its exoskeleton. (4 marks)
Arrange the seven categories of classification from the largest to the smallest, and show how a tiger is placed at each one. (5 marks)
Compare the five classes of vertebrates with respect to body covering, respiratory organ, number of heart chambers, body temperature and the way the young are produced. (5 marks)
Trace the path of water through the body of a sponge, from the surrounding sea to its exit, naming the structures it passes and stating what the sponge takes from it. (5 marks)
8.17 Figure-Based Questions
Study Fig. 8.9-6. Name the band marked across segments 14 to 16, state the function it serves, and state how it helps you tell the anterior end of the worm from the posterior end. (3 marks)
Study Fig. 8.9-1. Name the phylum to which the animal shown belongs, name the openings through which water enters the body, and state the function of the spicules. (3 marks)
Study Fig. 8.10-1. Name the kind of symmetry shown by each of the four organisms drawn, and state which of them possesses a head end. (4 marks)
Study Fig. 8.11-2. Identify the two kinds of fish shown, name the structure that covers the gill slits in one of them, and state one further difference between their skeletons. (3 marks)
Study Fig. 8.5-2. Name any two organisms drawn there, name the kingdom to which they belong, and state the single feature of the cell that places them in that kingdom rather than in Monera. (3 marks)
Study Fig. 8.11-1. Name the class to which each of the five animals belongs, and state for each one the organ by which the adult breathes. (5 marks)
Draw a neat labelled diagram of the external features of Pheretima posthuma in side view, labelling the mouth, the segments, the clitellum and the anus, and keeping all lead lines straight and non-crossing. (5 marks)