Animal Kingdom: Class 11 Biology · Chapter 4

Step into a garden on the evening after the first monsoon shower and start counting animals. An earthworm surfaces from the wet soil, a snail glides up a hibiscus stem, ants march in a line, a garden lizard waits on the wall and a mynah calls from the wires. A pond or a rocky shore would add dozens more kinds. Zoologists have already described well over a million animal species, and the list keeps growing: new frogs are still being named from the forests of the Western Ghats.

A collection that large is useless without a filing system. Classification gives every known animal an address, and it tells a biologist where a newly described species belongs: beside which relatives, and for what reason.

The hard part is choosing what to sort by. Size, colour, habitat and usefulness mislead easily: a whale swims but is no fish, and a bat flies but is no bird. Zoologists instead use deep features of the body plan that stay constant across very large groups. Section 4.1 takes these features one at a time; Section 4.2 uses them to arrange the animal kingdom into phyla and, for vertebrates, classes.

4.1 Basis of Classification

Animals vary enormously in outward form, yet underneath they follow only a handful of designs. A sea anemone and a jellyfish look nothing alike, and neither do a bat and a whale, but each pair shares the same arrangement of tissues, the same symmetry and the same kind of body cavity. Zoologists therefore sort animals by these hidden designs, such as how the cells are arranged, how the body is laid out and what cavity it has, and by how organ systems like the gut, the blood system and the reproductive organs are put together. Six features do most of the work:

1. how the cells are organised: loosely, into tissues, organs or organ systems (Section 4.1.1);
2. how the body is arranged around its axis: its symmetry (Section 4.1.2);
3. how many germ layers the embryo builds (Section 4.1.3);
4. what kind of body cavity, if any, it has (Section 4.1.4);
5. whether the body is truly segmented (Section 4.1.5);
6. whether a notochord forms (Section 4.1.6).

4.1.1 Levels of Organisation

Every animal is multicellular, but its cells can be organised in four increasingly elaborate ways (Fig. 4.1). Think of how work is shared as an organisation grows. In a tiny start-up everyone pitches in wherever needed; in a larger firm people with the same skill form teams; teams combine into departments; and departments link into divisions, each running one complete job.

• Cellular level. In sponges the cells are only loosely held together. Different cells take on different jobs, some driving water and others building the skeleton, so there is a little division of labour, but similar cells never band together into true tissues.
• Tissue level. In cnidarians such as Hydra and the corals, cells that do the same job sit together as a tissue, for example a sheet lining the gut.
• Organ level. Flatworms go a step further: tissues are combined into organs, each organ specialised for one task, such as the muscular pharynx that sucks in food.
• Organ-system level. In annelids, arthropods, molluscs, echinoderms and chordates (and, as Section 4.2.5 shows, roundworms) organs are grouped into systems, each in charge of one physiological function: digestion, circulation, excretion and so on.

Even among animals with organ systems, the systems are not all built alike. Two contrasts will come up again and again in the descriptions of the phyla (Fig. 4.2):

• Incomplete digestive system: the gut is a blind sac whose single opening works as both mouth and anus. Flatworms are built this way.
• Complete digestive system: the gut is a tube with two openings, a mouth where food enters and an anus where waste leaves, so food travels one way and each region can specialise.
• Open circulatory system: the heart pushes blood out into open spaces rather than a sealed network, so the cells and tissues sit directly in blood (arthropods, most molluscs).
• Closed circulatory system: blood stays inside tubes the whole time, moving through arteries, capillaries and veins of different widths (annelids, chordates).

4.1.2 Symmetry

Imagine slicing an animal with a flat blade, a plane, through the middle of its body. If the two pieces are mirror images, that plane is a plane of symmetry. Counting how many such planes exist sorts animals into three groups (Fig. 4.3).

• Asymmetrical: no slice through the centre gives matching halves. Most sponges grow this way, their shape set by the rock and the current.
• Radial symmetry (अरीय सममिति): the body is built around a central axis, like a wheel or a round cake, so any plane that contains the axis yields identical halves. Cnidarians, ctenophores and adult echinoderms are radial.
• Bilateral symmetry: only one plane, running down the middle from head to tail, gives matching left and right halves. Annelids, arthropods, chordates and most other phyla are bilateral.

Symmetry suits a way of life. A radial animal, fixed to a rock or drifting, meets food and danger equally from every side. A bilateral animal moves head first, so its sense organs crowd at the front end, the beginning of a true head.

4.1.3 Diploblastic and Triploblastic Organisation

Early in development an embryo arranges its cells into sheets called germ layers, and each layer goes on to build particular tissues. How many layers form is a basic character (Fig. 4.4).

Diploblastic animals form only two: an outer ectoderm and an inner endoderm. Between the two lies the mesoglea, a jelly-like layer that never becomes an organised cell layer. Cnidarians are the standard example; ctenophores are diploblastic too.

Triploblastic animals add a third layer, the mesoderm, sandwiched between ectoderm and endoderm. Every phylum from Platyhelminthes up to Chordata is triploblastic. The mesoderm builds muscles and, as the next section shows, decides what kind of body cavity an animal ends up with.

4.1.4 Coelom

Many animals have a fluid-filled space separating the gut from the body wall. Whether this space exists, and what tissue lines it, is one of the most useful characters in all of animal classification.

Three plans are possible (Fig. 4.5):

• Coelomates have a true coelom, with mesoderm lining both the body-wall side and the gut side: annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
• Pseudocoelomates also have a cavity, but nothing lines it: their mesoderm is reduced to separate pockets dotted between the outer and inner layers. Such a cavity is a pseudocoelom, a false coelom, and it is found in Aschelminthes (roundworms).
• Acoelomates have no cavity at all; mesoderm fills the whole space between gut and body wall. Platyhelminthes (flatworms) are acoelomate.

Why does a coelom matter so much? It lets the gut and the body wall move independently, so food can be churned while the body crawls, and it gives organs room to grow and shift. In earthworms the coelomic fluid also works as a liquid skeleton that the muscles push against while burrowing.

4.1.5 Segmentation

Run a finger along an earthworm and you feel rings. They are more than skin deep. Thin internal walls, the septa, divide the inside into matching compartments, and the same organs, such as a nerve ganglion and the excretory nephridia, turn up again in compartment after compartment (Fig. 4.6).

A body built as a row of similar segments, divided on the outside and the inside, with some organs repeated in each, shows metameric segmentation; the phenomenon is called metamerism. A tapeworm's long chain of units is a useful contrast: those units, the proglottids, are budded off behind the head as reproductive packets, so the tapeworm is not truly metameric. In this chapter's sequence of phyla, true segmentation first appears in Annelida.

4.1.6 Notochord

The notochord (पृष्ठरज्जु) is a flexible rod that appears along the back, the dorsal side, of some embryos. It grows from mesoderm and stiffens the body while the rest of the embryo is still soft.

Animals that form a notochord at some stage of life are chordates; all others are non-chordates. Porifera through Echinodermata are non-chordates, and so is Hemichordata, whose look-alike rod turns out not to be a true notochord (Section 4.2.10). In vertebrates such as you, the notochord of the embryo is largely replaced by the backbone (the chordate subphyla).

4.2 Classification of Animals

Put the features of Section 4.1 together and the animal kingdom falls into a branching scheme. Fig. 4.7 reads like a key: answer one question at each fork, and every answer narrows the choice until a single phylum is left.

The first few questions decide a great deal. Level of organisation separates the sponges from everything else. Symmetry, which travels together with the number of germ layers, splits the radial, diploblastic cnidarians and ctenophores from the bilateral, triploblastic majority. The body cavity then separates flatworms, roundworms and six coelomate phyla. Among the coelomates the notochord picks out Chordata, and each remaining phylum has a hallmark of its own. Echinoderms are the awkward case: their larvae are bilateral but the adults turn radial, so the key places them with the coelomates and flags their symmetry.

4.2.1 Phylum – Porifera

The soft, yellowish natural sponge once sold for bathing was the skeleton of an animal. Sponges never move as adults and have no mouth, no gut and no nerves, yet they are animals: their many cells cooperate to feed by filtering water.

What powers a sponge is a lining of choanocytes, or collar cells, along its canals and central cavity. Each has a beating flagellum that pulls water along and a collar of fine projections that traps food. Their combined effort draws water in through countless tiny pores, the ostia, into a central cavity, the spongocoel, and pushes it out through one large opening at the top, the osculum (Fig. 4.9). That single current brings food, exchanges gases and carries wastes away.

• Hallmark: the water canal system (ostia → spongocoel → osculum), lined by choanocytes.
• Body plan: cellular level, with no true tissues; mostly asymmetrical. Sponges are regarded as primitive multicellular animals.
• Feeding: filter feeding; digestion is intracellular.
• Support: a skeleton of spicules (tiny mineral needles) or spongin fibres.
• Reproduction: hermaphrodite, one sponge making both eggs and sperm. Broken-off fragments grow into new sponges (asexual); gametes give the sexual route, with internal fertilisation. Development is indirect, through a larva quite unlike the adult.
• Habitat: generally marine; Spongilla is one of the few freshwater sponges.
• Examples: Euspongia (bath sponge), Spongilla (freshwater sponge) and Sycon (also called Scypha).

4.2.2 Phylum – Coelenterata (Cnidaria)

Swimmers at Indian beaches sometimes come out of the sea with red, burning welts from a jellyfish. That sting gives the phylum its second name, Cnidaria. It comes from cnidoblasts (cnidocytes), cells that each hold a capsule, the nematocyst, packed with a coiled thread. Cnidoblasts stud the tentacles and the body, where they anchor the animal, defend it and capture prey (Fig. 4.10).

Cnidarians come in two basic body forms. A polyp is a cylinder attached at its base, mouth and tentacles facing up, like Hydra or the sea anemone Adamsia. A medusa is a free-swimming, umbrella-shaped bell with the mouth facing down, like the jellyfish Aurelia. In effect, a medusa is a polyp turned upside down and flattened.

Some cnidarians, such as Obelia, pass through both forms. Their polyps produce medusae asexually, by budding; the medusae make eggs and sperm, and the zygote grows through a larva into new polyps, which is the sexual route. This alternation between an asexual polyp generation and a sexual medusa generation is called metagenesis (Fig. 4.11).

• Body plan: tissue level, diploblastic, radially symmetrical.
• Feeding: food enters, and wastes leave, by one mouth set on a raised cone, the hypostome; it leads into the central gastro-vascular cavity. Digestion begins in the cavity (extracellular) and is completed inside the lining cells (intracellular).
• Skeleton: reef-building corals secrete calcium carbonate, $\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}{\phantom{X}}_3$, the material of the reefs around Lakshadweep and the Andaman Islands.
• Habitat: aquatic and mostly marine; some stay fixed (sessile) while others swim freely.
• Examples: the floating, colonial Physalia (Portuguese man-of-war)11A Portuguese man-of-war is not one animal but a colony of many specialised individuals.; the reef builders Meandrina (brain coral) and Gorgonia (sea fan); the feather-shaped Pennatula (sea pen); and Adamsia, a sea anemone.

4.2.3 Phylum – Ctenophora

Ctenophores, called comb jellies or sea walnuts, look like glassy little jellyfish but form a phylum of their own. Their hallmark is eight rows of comb plates running down the body, each plate a band of fused cilia. The plates beat in waves that row the animal along and, in sunlight, flash rainbow colours. Many ctenophores also glow: bioluminescence, light made by a living body, is a well-marked feature of the group.

• Body plan: tissue level, diploblastic and radial, like cnidarians, but with no stinging cells.
• Feeding: digestion is partly extracellular and partly intracellular.
• Reproduction: hermaphrodite; reproduction is only sexual, with external fertilisation and indirect development.
• Habitat: exclusively marine.
• Examples: Ctenoplana and Pleurobrachia.

4.2.4 Phylum – Platyhelminthes

Flatworms are flattened from top to bottom (dorsoventrally), which is where their common name comes from. A flat body keeps every cell close to the surface, a help to an animal that has no circulatory or breathing organs. Many are parasites of people and livestock: Taenia, the tapeworm, reaches people through undercooked infected pork, and Fasciola, the liver fluke, lives in the liver of sheep and cattle.

• Hallmark: a flat body; parasitic forms carry hooks and suckers to grip the host.
• Body plan: bilateral, triploblastic and acoelomate, at the organ level of organisation.
• Feeding: some parasites, such as the tapeworm, simply absorb the host's digested food through their body surface.
• Excretion: specialised flame cells handle excretion and osmoregulation.
• Reproduction: hermaphrodite, with internal fertilisation; development runs through many larval stages.
• Regeneration: the free-living Planaria can regrow a whole body from a small piece.
• Habitat: mostly endoparasites inside other animals, humans included.
• Examples: Fasciola (liver fluke) and Taenia (tapeworm).

4.2.5 Phylum – Aschelminthes

Cut a roundworm across and the slice is a circle, which explains the common name. Three human parasites are the standard examples: Ascaris, the large intestinal roundworm; Wuchereria, the filarial worm, whose larvae mosquitoes spread and which causes filariasis; and Ancylostoma, the hookworm. You will meet the diseases they cause in Class 12 (Human Health and Disease).

• Hallmark: a body circular in cross-section, with a pseudocoelom.
• Body plan: organ-system level; bilateral and triploblastic.
• Feeding: a complete gut, from mouth through a well-developed muscular pharynx to anus.
• Excretion: an excretory tube drains wastes from the body cavity to the outside through an excretory pore.
• Reproduction: sexes separate (dioecious), the female often longer than the male; internal fertilisation; development direct (young like small adults) or indirect (through larvae).
• Habitat: free-living in soil and water, or parasitic in plants and animals.

4.2.6 Phylum – Annelida

The name comes from the Latin annulus, a little ring, after the rings you met on the earthworm in Section 4.1.5. Those rings are true metameres, repeated inside as well as outside.

• Hallmark: true metameric segmentation; the segments, or metameres, show on the surface and repeat inside.
• Body plan: organ-system level; bilateral, triploblastic and coelomate.
• Movement: longitudinal and circular muscles work together for locomotion; the marine Nereis also has paddle-like side flaps, parapodia, for swimming.
• Transport and excretion: a closed circulatory system; nephridia (singular nephridium) handle excretion and osmoregulation.
• Nerves: a double ventral nerve cord carrying paired ganglia (singular ganglion), with lateral nerves branching to the body.
• Reproduction: sexual. Nereis has separate sexes (dioecious), whereas earthworms and leeches are monoecious, each individual carrying both sexes.
• Habitat: the sea, fresh water and land; mostly free-living, a few parasitic.
• Examples: Pheretima, the earthworm of Indian soils; the swimming marine worm Nereis; and Hirudinaria, a leech that feeds on blood.

4.2.7 Phylum – Arthropoda

Count the animal species that scientists have named and more than two in every three turn out to be arthropods, insects above all. That makes Arthropoda by far the largest phylum: for every named vertebrate, more than ten kinds of insect have been named.

Two features give the phylum its success and its name. A chitinous exoskeleton protects the body and cuts water loss, which opened dry land to arthropods. And the body carries jointed appendages: the name Arthropoda joins the Greek arthron, a joint, and pous (podos), a foot.

• Body plan: organ-system level; bilateral, triploblastic, segmented and coelomate; the body is in three regions, head, thorax and abdomen.
• Breathing: gills (prawns), book gills (king crab), book lungs (scorpions, spiders) or a tracheal system (insects).
• Circulation: open.
• Senses: antennae; compound and simple eyes; statocysts, which are organs of balance.
• Excretion: by Malpighian tubules.
• Reproduction: mostly dioecious; fertilisation usually internal; mostly oviparous (egg-laying); development direct, or indirect through larvae as in butterflies.

4.2.8 Phylum – Mollusca

Mollusca is the second-largest animal phylum. A garden snail, a pearl oyster, a cuttlefish and an octopus look very different, yet all are built on one plan (Fig. 4.12).

Each part of that plan has a job. The muscular foot underneath moves the animal, creeping in snails and digging in tusk shells. The visceral hump on top houses most of the organs. The head at the front carries sensory tentacles and the mouth, and inside the mouth sits the radula, a ribbon of tiny teeth that scrapes food like a file. Draped over the hump is a soft, spongy fold of skin, the mantle, which usually lays down a calcareous shell. Between mantle and hump lies the mantle cavity, where feather-like gills serve both breathing and excretion.

• Body plan: organ-system level; bilateral, triploblastic, coelomate and unsegmented.
• Shell: usually external and calcareous; reduced and internal in cuttlefish and squids, and absent in Octopus.
• Reproduction: the sexes are usually separate; eggs are laid, and the young pass through a larval stage (indirect development).
• Habitat: land, fresh water and the sea.
• Examples: snails and their kin, Pila (apple snail) and Aplysia (sea hare); a two-shelled bivalve, Pinctada (pearl oyster); the fast-swimming cephalopods Octopus (devil fish), Sepia (cuttlefish) and Loligo (squid); and two odd shapes, Dentalium (tusk shell) and Chaetopleura (chiton).

Pila is the large snail of Indian ponds and paddy fields, and pearls once made the Gulf of Mannar coast famous. Notice that a cuttlefish and a devil fish are not fish at all: common names can mislead, which is exactly why scientific names exist.

4.2.9 Phylum – Echinodermata

Turn a starfish over and you see hundreds of tiny, soft tube feet gripping the rock. They are the working ends of its water vascular system, a network of water-filled canals unique to echinoderms. The system moves the animal, catches food and passes it along, and helps in breathing.

The group's name means spiny skin. The spines and knobs come from an endoskeleton of calcareous ossicles, small plates of calcium carbonate lying just under the skin.

• Body plan: organ-system level; triploblastic and coelomate. Symmetry changes with age: the free-swimming larva is bilateral, the adult radial.
• Feeding: a complete gut, with the mouth on the lower (ventral) surface and the anus on the upper (dorsal) surface.
• Excretion: there is no excretory system.
• Reproduction: sexes separate; reproduction sexual; fertilisation usually external; development indirect, through a free-swimming larva.
• Habitat: every echinoderm lives in the sea.
• Examples: two with arms, Asterias (starfish) and Ophiura (brittle star); the ball-shaped Echinus (sea urchin); the sausage-shaped Cucumaria (sea cucumber); and Antedon (sea lily), which looks like a feathery plant. A starfish is no fish and a sea lily is no plant.

4.2.10 Phylum – Hemichordata

Acorn worms such as Balanoglossus and Saccoglossus make up Hemichordata, a small phylum of worm-shaped animals living on the sea floor. The cylindrical body has three parts: a proboscis at the front, a short collar and a long trunk.

In the collar lies a short, rudimentary rod, the stomochord. Because it resembles a notochord, hemichordates were once treated as a subphylum of Chordata. Closer study showed it is not a true notochord, so Hemichordata now stands as a separate phylum among the non-chordates.

• Body plan: organ-system level; bilateral, triploblastic and coelomate.
• Transport, breathing and excretion: an open circulatory system; gills for breathing; a proboscis gland for excretion.
• Reproduction: sexes separate, external fertilisation and indirect development.
• Habitat: worm-like burrowers of the sea floor.

4.2.11 Phylum – Chordata

Your own phylum is Chordata. Four characters, all present at least in the embryo, define it (Fig. 4.13):

1. a notochord along the back;
2. a dorsal, hollow nerve cord lying above the notochord;
3. paired pharyngeal gill slits, openings through the wall of the throat (pharynx);
4. a post-anal tail, a part of the body that continues behind the anus.

Chordates are also bilateral, triploblastic, coelomate and organised at the organ-system level, with a closed circulatory system. In humans the gill region and the tail show up clearly only in the early embryo and are later remodelled, so the embryo is often the place to look for chordate characters.

Subphyla: protochordates and vertebrates

What happens to the notochord splits Chordata into three subphyla (Fig. 4.14):

• Urochordata (Tunicata): the notochord exists only in the tail of the swimming larva and disappears when the larva settles as a sac-like adult. Examples: Ascidia, Salpa and Doliolum.
• Cephalochordata: the notochord runs from head to tail and stays for the animal's whole life. Example: Branchiostoma (Amphioxus, the lancelet).
• Vertebrata: the notochord forms in the embryo and is replaced in the adult by a vertebral column of cartilage or bone.

Urochordates and cephalochordates together are the protochordates, and both live only in the sea. Since every vertebrate passes through a notochord stage, the vertebrates sit wholly inside Chordata; but Chordata also holds the protochordates, which never build a backbone. On top of the chordate characters, vertebrates share three more: a muscular, ventral heart, which may be two-, three- or four-chambered; a pair of kidneys that remove wastes and balance water and salts; and paired appendages, fins or limbs.

Vertebrates are sorted first by jaws and then by what they move with:

• Division Agnatha (no jaws): class Cyclostomata.
• Division Gnathostomata (jaws), superclass Pisces (fins): classes Chondrichthyes and Osteichthyes.
• Division Gnathostomata, superclass Tetrapoda (limbs): classes Amphibia, Reptilia, Aves and Mammalia.

4.2.11.1 Class – Cyclostomata

A lamprey's life swings between sea and river. The adults live in the sea, swim up into fresh water to spawn and die within a few days of spawning; their larvae undergo metamorphosis and then make their way back to the ocean. Lampreys and hagfishes are the only living jawless vertebrates.

• Hallmark: no jaws; instead, a circular sucking mouth.
• Skeleton and body: long and eel-like; the cranium and vertebral column are cartilaginous; there are no paired fins.
• Skin: no scales.
• Breathing and circulation: 6 to 15 pairs of gill slits; a closed circulation.
• Lifestyle: all living members are described as ectoparasites on certain fishes; a lamprey clamps on and rasps through the host's skin.22Strictly, this is the lamprey's way of life; hagfishes mostly scavenge dead or dying fish and never enter fresh water. Exam answers follow the class description given here.
• Examples: the lamprey, Petromyzon, and the hagfish, Myxine.

4.2.11.2 Class – Chondrichthyes

Stroke a shark from tail to head and its skin feels like sandpaper. The roughness comes from minute placoid scales set in the tough skin, and a shark's backward-pointing teeth are modified placoid scales too, so teeth and skin are versions of the same structure. Sharks, rays and sawfishes make up this class of cartilaginous fishes.

They also have a buoyancy problem. With no air bladder, a cartilaginous fish is denser than sea water and must keep swimming, or it slowly sinks.

• Skeleton and body: an endoskeleton of cartilage; the notochord persists throughout life. Streamlined, with the mouth on the underside (ventral) and very powerful jaws: these are predators.
• Gills: each gill slit opens separately; there is no gill cover (operculum).
• Heart and temperature: a two-chambered heart, one auricle and one ventricle. Like all fishes they are poikilothermous (असमतापी), or cold-blooded: unable to regulate their body temperature, which follows the water.
• Reproduction: sexes separate; in males the pelvic fins bear claspers; fertilisation internal; many species viviparous, giving birth to live young.
• Special weapons: electric organs in Torpedo (electric ray) and a poison sting in Trygon (sting ray).
• Habitat: marine.
• Examples: Carcharodon (great white shark), Scoliodon (dogfish), Pristis (sawfish) and Trygon (sting ray).

4.2.11.3 Class – Osteichthyes

Most fish on an Indian dinner plate are bony fishes: rohu and katla from ponds, magur from swamps, pomfret and hilsa from the sea. Unlike sharks, a bony fish can hang still in the water, because a gas-filled air bladder inside its body adjusts its buoyancy.

• Skeleton and body: an endoskeleton of bone; a streamlined body; the mouth mostly at the front tip (terminal).
• Gills: four pairs, covered on each side of the head by a bony flap, the operculum.
• Skin: cycloid or ctenoid scales.
• Heart and temperature: two chambers, one auricle and one ventricle; cold-blooded.
• Reproduction: sexes separate; fertilisation usually external; mostly oviparous; development direct.
• Habitat: both the sea and fresh water.
• Examples: in rivers and ponds, Labeo (rohu), Catla (katla) and Clarias (magur); in the sea, Exocoetus, the flying fish, and Hippocampus, the sea horse; in home aquariums, Betta (fighting fish) and Pterophyllum (angel fish).

4.2.11.4 Class – Amphibia

The name says it: Greek amphi, both or dual, and bios, life. A frog begins as a gilled tadpole in a pond and ends as an air-breathing adult that hops on land, and that double life shapes the whole class.

• Skin: moist and without scales; it doubles as a breathing surface.
• Breathing and heart: gills, lungs and the skin are all used for breathing; the heart has three chambers, two auricles and one ventricle; cold-blooded.
• Body: divided into head and trunk; most have two pairs of limbs, and some, like salamanders, keep a tail.
• Senses: eyelids on the eyes; a tympanum, a drum-like membrane on the side of the head, serves as the ear.
• Cloaca: the gut, the urinary tract and the reproductive tract all open into one chamber, the cloaca, which opens to the outside.
• Reproduction: sexes separate; fertilisation external; eggs laid; development indirect, through a larva (the tadpole).
• Examples: the frog Rana and the toad Bufo, the climbing tree frog Hyla, the tailed Salamandra (salamander) and the legless Ichthyophis.

Chapter 7 looks at the frog in detail.

4.2.11.5 Class – Reptilia

Reptiles take their name from the Latin repere, to creep or crawl. Their trademark is dry, cornified skin covered by epidermal scales or larger horny plates called scutes. Snakes and lizards shed the outer layer from time to time as a skin cast, the papery slough you sometimes find in a garden.

• Ears: no external ear opening; a tympanum serves as the ear.
• Limbs: two pairs when present; snakes have none.
• Heart and temperature: three heart chambers in most, four in crocodiles; reptiles are poikilotherms.
• Reproduction: sexes separate; fertilisation internal; oviparous; development direct.
• Habitat: mostly on land.
• Examples: lizards such as Calotes (garden lizard), Hemidactylus (wall lizard) and Chameleon (tree lizard); shelled reptiles, Testudo (tortoise) on land and Chelone (turtle) in the sea; the crocodilians Crocodilus and Alligator; and three poisonous snakes, Naja (cobra), Bungarus (krait) and Vipera (viper).33Some printings spell the krait's genus Bangarus; the accepted spelling is Bungarus.

4.2.11.6 Class – Aves

Feathers make a bird: no other living animal has them. Most birds fly, and much of a bird's body is shaped by that demand; the ostrich is one of the flightless exceptions.

• Built for flight: forelimbs modified into wings; a fully ossified skeleton whose long bones are pneumatic, hollow with air cavities, keeping it light; air sacs connected to the lungs supplement breathing.
• Beak and gut: a beak instead of teeth; two extra chambers in the gut, the crop, which stores food, and the gizzard, which grinds it.
• Skin: dry, with no glands except an oil gland near the root of the tail, used in preening; the hind limbs usually carry scales and are adapted for walking, swimming or clasping branches.
• Heart and temperature: a completely four-chambered heart; birds are homoiothermous (समतापी), or warm-blooded, keeping a steady body temperature whatever the weather.
• Reproduction: sexes separate; fertilisation internal; oviparous; development direct.
• Examples: the flightless Struthio (ostrich) and Aptenodytes (penguin), and Pavo (peacock), Psittacula (parrot), Neophron (vulture), Columba (pigeon) and Corvus (crow).

4.2.11.7 Class – Mammalia

Mammals are found from the snowfields of Ladakh to the Thar desert, from rainforests to the darkest caves. Bats have taken to the air, and dolphins and whales spend their whole lives in water.

Their most distinctive feature is mammary glands, which make milk to nourish the young; the word mammal comes from them.

• Skin and ears: hair on the skin; external ear flaps, the pinnae.
• Teeth: different kinds of teeth in the jaws.
• Limbs: two pairs, reshaped for every kind of movement: hooves for running in a horse, grasping hands in a monkey, digging claws in a pangolin, flippers in a dolphin and wings in a bat.
• Heart, lungs and temperature: a four-chambered heart; breathing by lungs; homoiothermous.
• Reproduction: sexes separate; fertilisation internal; viviparous with few exceptions (the platypus, Ornithorhynchus, lays eggs); development direct.
• Examples: the egg-laying Ornithorhynchus (platypus); in the air, Pteropus (flying fox); in the sea, Delphinus (common dolphin) and Balaenoptera (blue whale, the largest animal known); with a pouch, Macropus (kangaroo); and on land, Elephas (elephant), Panthera tigris (tiger), Panthera leo (lion), Camelus (camel), Equus (horse), Macaca (monkey), Canis (dog), Felis (cat) and Rattus (rat).

Comparing the vertebrate classes

Two trends run through the classes. The heart gains chambers (Fig. 4.15), and the body becomes steadily less tied to water and to the temperature outside. The table below lines up the jawed classes feature by feature.

The phyla at a glance

Table 4.2 gathers the main features of all eleven phyla in one place for revision. Read each column from top to bottom and watch the trends: organisation rises from cells to organ systems, symmetry settles into bilateral, and the coelom and closed circulation appear.