Animal Kingdom

Over a million animal species have been described, and new ones are added every year. Two things are true of them at once: they differ enormously in form, and yet they share a small set of fundamental features. The chapter is built on that second fact. If we sort animals by a few common characters, cell organisation, body symmetry, the nature of the coelom, the patterns of the digestive, circulatory and reproductive systems, the whole kingdom falls into eleven phyla, and any new specimen can be placed by asking the same questions in the same order. NCERT exercise 1 asks what would go wrong without such fundamental features: every species would have to be studied separately, no generalisation would be possible, and a new animal could not be related to any known group.

The first character is the level of organisation. Sponges show the cellular level: their cells are arranged as loose aggregates, with some division of labour but no tissues. Coelenterates reach the tissue level: cells performing the same function are arranged into tissues. Platyhelminthes and the higher phyla reach the organ level, where tissues are grouped into organs, each specialised for one function. Annelids, arthropods, molluscs, echinoderms and chordates show the organ system level: organs are associated to form functional systems, each concerned with one physiological function.

NCERT immediately adds that organ systems show further patterns worth noting. The digestive system may be incomplete, with a single opening serving as both mouth and anus (Platyhelminthes), or complete, with two openings, a mouth and an anus. The circulatory system may be open, in which blood pumped out of the heart bathes the cells and tissues directly (arthropods, most molluscs), or closed, in which blood is circulated through a series of vessels of varying diameters, arteries, veins and capillaries (annelids, chordates). These three pairs of words, cellular to organ-system, incomplete versus complete, open versus closed, are the most frequently asked 1-mark items from the opening pages.

Symmetry is decided by asking how many planes can divide the animal into identical halves. Sponges are mostly asymmetrical: no plane passing through the centre divides them into equal halves. In radial symmetry, any plane passing through the central axis divides the organism into two identical halves; this is seen in coelenterates, ctenophores and adult echinoderms. In bilateral symmetry, only one plane (the sagittal plane, through the mid-line) produces identical left and right halves; annelids, arthropods and all the higher phyla are bilateral. Radial symmetry suits a sessile or drifting animal that meets its environment from all sides; bilateral symmetry goes with directed movement and a head end. Echinoderms are the classic trap: the adult is radial but the larva is bilateral, which is why the phylum is kept with the bilateral, triploblastic, coelomate animals.

Germ layers are the second character on this page. Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are diploblastic. Between the two lies an undifferentiated jelly-like layer, the mesoglea, which is not a third germ layer. Coelenterates and ctenophores are diploblastic. Animals in which the developing embryo has a third germinal layer, the mesoderm, between the ectoderm and endoderm are triploblastic; Platyhelminthes onwards, up to the chordates, are triploblastic. Mesoderm is what makes muscles, a coelom and true organs possible, so the diploblastic to triploblastic step is the biggest jump in complexity in the chapter.

NCERT Figure 4.1 (radial and bilateral symmetry) and Figure 4.2 (a cross-section showing ectoderm, mesoglea and endoderm in a diploblastic animal beside ectoderm, mesoderm and endoderm in a triploblastic one) are asked as 'draw and label' items, and a 2-mark question often asks you to define both symmetries with one example each.

The coelom is the body cavity that lies between the body wall and the gut wall and is lined by mesoderm. Whether such a cavity is present, and how it is lined, is 'very important in classification', as NCERT puts it, and exercise 3 asks exactly this. Three conditions exist.

Coelomates have a true coelom, lined on both sides by mesoderm: the outer (parietal) mesoderm lines the body wall and the inner (visceral) mesoderm covers the gut. Annelids, molluscs, arthropods, echinoderms, hemichordates and chordates are coelomates. Pseudocoelomates have a body cavity that is not lined by mesoderm; instead the mesoderm is present as scattered pouches between the ectoderm and endoderm, and the cavity is called a pseudocoelom. Aschelminthes (roundworms) are the pseudocoelomates. Acoelomates have no body cavity at all; the space between the body wall and the gut is filled with mesodermal cells (parenchyma). Platyhelminthes are acoelomates.

The coelom matters functionally too. It provides room for organs to grow and move independently of the body wall, a fluid-filled space that works as a hydrostatic skeleton in worms, and a route for transport of materials. Because it is a mesodermal structure it appears only in triploblastic animals, so diploblastic coelenterates are neither coelomate nor acoelomate in the strict NCERT scheme; the question is asked only from Platyhelminthes upward.

NCERT Figure 4.3 shows three cross-sections, coelomate, pseudocoelomate and acoelomate, with the mesoderm (usually shaded) in different positions. In the coelomate section the shaded mesoderm lines both the outside of the gut and the inside of the body wall, with the coelom between; in the pseudocoelomate section the shaded patches touch the body wall only and the cavity has no mesodermal lining on the gut side; in the acoelomate section the shading fills the whole space. Draw this figure for any 5-mark 'basis of classification' answer.

Segmentation. In some animals the body is externally and internally divided into segments, with a serial repetition of at least some organs. NCERT's example is the earthworm, where the body shows this pattern, called metameric segmentation, and the phenomenon is known as metamerism. Each segment (metamere) carries its own pair of nephridia, its own ganglia and its own set of muscles, so the body is built of repeated units. Metamerism first appears in phylum Annelida (NCERT exercise 13, option c), and continues in Arthropoda and Chordata; the superficial rings on a tapeworm (proglottids) are not true metameres because they are budded off from the neck and do not share a repeated organ plan, which is why Platyhelminthes is the wrong option in that exercise.

Notochord. The notochord is a mesodermally derived, rod-like structure formed on the dorsal side during embryonic development in some animals. Animals with a notochord are called chordates; animals without it are non-chordates, and NCERT lists Porifera to Echinodermata (and Hemichordata) as non-chordates. In vertebrates the notochord is present only in the embryo and is later replaced by the vertebral column; in Cephalochordata (Branchiostoma) it persists for life from head to tail; in Urochordata it is confined to the larval tail. All of this returns in section 4.2.11.

With these six characters in hand, NCERT's Figure 4.5 gives the broad classification: Kingdom Animalia splits into Non-chordates (ten phyla: Porifera, Coelenterata/Cnidaria, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata) and phylum Chordata, which has three subphyla (Urochordata, Cephalochordata, Vertebrata). Vertebrata splits into Agnatha (class Cyclostomata) and Gnathostomata; Gnathostomata into Pisces (Chondrichthyes, Osteichthyes) and Tetrapoda (Amphibia, Reptilia, Aves, Mammalia). Exercise 2 ('If you are given a specimen, what steps would you follow to classify it?') expects you to apply the six characters in order and then move down this chart.

Members of Porifera are commonly known as sponges. They are generally marine and mostly asymmetrical. They are primitive multicellular animals with the cellular level of organisation. The defining feature is the water transport or canal system: water enters through minute pores, the ostia, in the body wall, passes into a central cavity called the spongocoel, and leaves through a single large opening, the osculum. This one-way current does three jobs at once: gathering food, respiratory exchange and removal of waste. The spongocoel and the canals are lined by choanocytes or collar cells, flagellated cells whose beating drives the current and whose collars trap food particles. Digestion is intracellular; there is no gut. The body is supported by a skeleton made of spicules (calcareous or siliceous) or spongin fibres.

Reproduction: sponges are hermaphrodite, meaning eggs and sperms are produced by the same individual. They reproduce asexually by fragmentation and sexually by formation of gametes. Fertilisation is internal, and development is indirect, with a larval stage that is morphologically distinct from the adult. Examples to memorise exactly: Sycon (Scypha), Spongilla (fresh water sponge) and Euspongia (bath sponge). Spongilla is the exception to 'generally marine'; Euspongia is the one with a spongin skeleton used commercially.

Why examiners like sponges: the canal system is a ready 3-mark question ('describe the water transport system of sponges and mention its functions'), 'choanocytes' is a permanent match-the-column entry (exercise 14), and 'intracellular digestion' links to exercise 4. The word 'ostia' is the plural (singular ostium); write the plural for the pores.

Coelenterates are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. The name Cnidaria comes from the cnidoblasts or cnidocytes, cells that contain the stinging capsules or nematocysts and are present on the tentacles and the body. Cnidoblasts are used for anchorage, defence and capture of prey. Cnidarians show the tissue level of organisation and are diploblastic. They have a central gastro-vascular cavity with a single opening, the mouth, on the hypostome. Digestion is both extracellular and intracellular: food is broken down first in the cavity by enzymes, and the pieces are then engulfed by cells of the lining. Some cnidarians, the corals, have a skeleton composed of calcium carbonate; coral reefs are built of it.

Cnidarians exhibit two basic body forms. The polyp is a sessile, cylindrical form, as in Hydra and Adamsia; the medusa is an umbrella-shaped, free-swimming form, as in Aurelia (the jelly fish). Cnidarians that exist in both forms show alternation of generation, called metagenesis: the polyps produce medusae asexually and the medusae form polyps sexually. NCERT's example is Obelia. Notice the exact direction: polyp gives medusa by asexual budding, medusa gives polyp through gametes. Both generations are diploid; metagenesis is not the same as the alternation of a haploid gametophyte and diploid sporophyte in plants, and an Assertion–Reason item tests exactly that difference.

Examples, with the common names NCERT attaches: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan), Meandrina (brain coral), and from the figures Aurelia (medusa) and Hydra. A 3-mark 'salient features of Coelenterata' answer should carry cnidoblasts, tissue level, diploblastic, gastro-vascular cavity with one opening, both kinds of digestion, polyp and medusa, and two examples.

Ctenophores are commonly known as sea walnuts or comb jellies. They are exclusively marine, radially symmetrical, diploblastic organisms with the tissue level of organisation. The body bears eight external rows of ciliated comb plates, which help in locomotion; the comb plates are the 'ctenes' that give the phylum its name and the entry 'comb plates → Ctenophora' in NCERT exercise 14. Digestion is both extracellular and intracellular, as in cnidarians. Bioluminescence, the property of a living organism to emit light, is well-marked in ctenophores; this is the fact NCERT chooses to mention, and it is the MCQ the examiner chooses to set.

Reproduction: sexes are not separate (they are hermaphrodite); reproduction takes place only by sexual means, fertilisation is external, and development is indirect. Examples: Pleurobrachia and Ctenoplana.

The phylum is small in NCERT but rich in contrasts with Cnidaria, and a 2-mark 'distinguish between Coelenterata and Ctenophora' is common. Ctenophores have no cnidoblasts, move with comb plates rather than by muscular contraction or medusa pulsing, show no polyp-medusa alternation, and are exclusively marine while some cnidarians (Hydra) are fresh-water. Both are radial, diploblastic and at the tissue level with a gastro-vascular cavity. If a question asks for a diploblastic animal that is not a cnidarian, Pleurobrachia is the answer.

Platyhelminthes have a dorso-ventrally flattened body, hence the name flatworms. They are mostly endoparasites found in animals including human beings. They are bilaterally symmetrical, triploblastic and acoelomate with the organ level of organisation. Three features follow from the parasitic habit, and together they answer NCERT exercise 6: hooks and suckers are present in the parasitic forms for attachment to the host; some of them absorb nutrients from the host directly through their body surface, so a gut may be reduced or absent (the tapeworm has none); and the body is flat so that every cell is close enough to the surface for exchange without a circulatory system. The digestive system, where present, is incomplete (single opening).

Excretion and osmoregulation are performed by specialised cells called flame cells, the first entry in the excretory-organ sequence that the examiner loves to test (flame cells, nephridia, Malpighian tubules, proboscis gland, kidneys). Flatworms are hermaphrodite; fertilisation is internal and development is through many larval stages. Planaria possesses a high regeneration capacity, a fact that appears as a 1-mark item. Examples: Taenia (tapeworm) and Fasciola (liver fluke).

For a 3-mark salient-features answer include: flat body, endoparasitic habit, bilateral, triploblastic, acoelomate, organ level, hooks and suckers, flame cells, hermaphrodite with internal fertilisation and larval stages, and two examples. Exercise 15 (animals parasitic on humans) draws on this phylum (Taenia solium, Fasciola hepatica) together with Aschelminthes (Ascaris, Wuchereria, Ancylostoma), Annelida (Hirudinaria, an ectoparasite) and Arthropoda (mosquitoes, lice, bed bugs as ectoparasites and vectors).

The body of aschelminthes is circular in cross-section, hence the name roundworms. They may be free-living, aquatic and terrestrial, or parasitic in plants and animals. Roundworms show the organ-system level of body organisation. They are bilaterally symmetrical, triploblastic and pseudocoelomate: the body cavity is not lined by mesoderm. The alimentary canal is complete, with a well-developed muscular pharynx, so Aschelminthes is the first phylum in NCERT order with a complete digestive tract (mouth and anus). An excretory tube removes body wastes from the body cavity through the excretory pore.

Sexes are separate (dioecious), and females are often longer than males: this sexual dimorphism is why NCERT figures Ascaris as separate male and female worms, the male smaller with a curved posterior end. Fertilisation is internal, and development may be direct (the young resemble the adult) or indirect. Examples: Ascaris (round worm), Wuchereria (filaria worm, the cause of elephantiasis) and Ancylostoma (hookworm). All three are human parasites and belong in the exercise 15 list.

Compare with the flatworms to score the 2-mark differentiation: flatworms are flat, acoelomate, hermaphrodite, with an incomplete gut and flame cells; roundworms are cylindrical, pseudocoelomate, dioecious, with a complete gut and an excretory tube. Note the phylum name too: the rationalised NCERT still calls the phylum Aschelminthes (the older name Nematoda is not used in the text, though some papers use it).

Annelids may be aquatic (marine and fresh water) or terrestrial, free-living and sometimes parasitic. They exhibit the organ-system level of organisation and are bilaterally symmetrical, triploblastic, metamerically segmented and coelomate. Their body surface is distinctly marked out into segments or metameres (Latin annulus, little ring), and this is the first phylum to show true metamerism and the first with a true coelom. They possess longitudinal and circular muscles which help in locomotion, working against the coelomic fluid as a hydrostatic skeleton. Aquatic annelids like Nereis possess lateral appendages, the parapodia, which help in swimming (exercise 14: parapodia → Annelida).

A closed circulatory system is present, the first in NCERT order. Nephridia help in osmoregulation and excretion (the second entry in the excretory-organ series, after flame cells). The neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord, which is the non-chordate plan that Table 4.1 contrasts with the single, dorsal, hollow nerve cord of chordates.

Reproduction: Nereis is dioecious (sexes separate), but earthworms and leeches are monoecious (hermaphrodite). Reproduction is sexual. Examples: Nereis, Pheretima (earthworm) and Hirudinaria (blood-sucking leech). Hirudinaria is an ectoparasite and is the 'parasitic' annelid NCERT has in mind. The 3-mark salient-features question here usually asks you to underline what appears first: metameric segmentation, true coelom and closed circulation.

This is the largest phylum of Animalia: it includes insects, and over two-thirds of all named species on earth are arthropods. They have the organ-system level of organisation, are bilaterally symmetrical, triploblastic, segmented and coelomate. The body is covered by a chitinous exoskeleton and consists of head, thorax and abdomen. They have jointed appendages (arthros, joint; poda, appendages), the character that names the phylum. Respiratory organs vary with habitat: gills (prawn), book gills (Limulus), book lungs (scorpion, spider) or a tracheal system (insects). The circulatory system is open. Sensory organs include antennae, eyes (compound and simple) and statocysts or balance organs. Excretion takes place through Malpighian tubules. They are mostly dioecious, fertilisation is usually internal, they are mostly oviparous, and development may be direct or indirect (indirect development in insects involves larval stages and metamorphosis).

NCERT groups the examples by importance, and the grouping itself is asked. Economically important insects: Apis (honey bee), Bombyx (silkworm), Laccifer (lac insect). Vectors: Anopheles, Culex and Aedes (mosquitoes). Gregarious pest: Locusta (locust). Living fossil: Limulus (king crab). Figures also show a butterfly, a scorpion and a prawn.

Exercise 7 asks why arthropods form the largest group; the answer is a list of the reasons for their success: a tough chitinous exoskeleton that protects and prevents water loss, jointed appendages for varied locomotion and feeding, a small body size, wings and the power of flight in insects, a wide range of respiratory organs that allow every habitat, a high reproductive rate with internal fertilisation and protected eggs, well-developed sense organs, and metamorphosis that lets larva and adult use different food. Give at least four points for 3 marks.

Mollusca is the second largest animal phylum. Molluscs are terrestrial or aquatic (marine or fresh water), with the organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate. The body is covered by a calcareous shell and is unsegmented, with a distinct head, a muscular foot and a visceral hump. A soft and spongy layer of skin forms a mantle over the visceral hump, and the space between the hump and the mantle is the mantle cavity, in which feather-like gills are present. The gills have both respiratory and excretory functions. The anterior head region has sensory tentacles. The mouth contains a file-like rasping organ for feeding, the radula (exercise 14: radula → Mollusca).

Reproduction: they are usually dioecious and oviparous, with indirect development. Examples, all of which have appeared as MCQ options: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea-hare), Dentalium (tusk shell), Chaetopleura (chiton).

Two contrasts are worth holding in mind. Annelida is segmented; Mollusca is unsegmented, though both are coelomate. Arthropoda has an exoskeleton of chitin; Mollusca has a shell of calcium carbonate secreted by the mantle. NCERT does not give the circulatory system for molluscs, but if a question expects it, most molluscs have an open circulatory system (cephalopods such as Octopus are the exception with a closed one). For a 3-mark answer write the six NCERT points: shell, unsegmented body with head-foot-visceral hump, mantle and mantle cavity, gills with two functions, tentacles, radula, and add reproduction and two examples.

Echinoderms have an endoskeleton of calcareous ossicles, and hence the name (echinos, spiny; derma, skin), spiny bodied. Note the word endoskeleton: the plates lie under the skin, unlike the arthropod exoskeleton. All are marine, with the organ-system level of organisation. The adult echinoderms are radially symmetrical but the larvae are bilaterally symmetrical. They are triploblastic and coelomate. The digestive system is complete, with the mouth on the lower (ventral) side and the anus on the upper (dorsal) side.

The most distinctive feature is the water vascular system, which helps in locomotion, capture and transport of food, and respiration. Sea water enters through the madreporite, passes through canals and ends in the tube feet, which extend and retract hydraulically. This answers exercise 8 (water vascular system is the characteristic of echinoderms), and the three functions are three marking points. An excretory system is absent, so in the excretory-organ series the entry for Echinodermata is 'none'. Sexes are separate, reproduction is sexual, fertilisation is usually external, and development is indirect with a free-swimming larva.

Examples with common names: Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber), Ophiura (brittle star). The 'sea lily' name for Antedon is set as a trap in odd-one-out MCQs because it sounds like a plant. The Assertion–Reason favourite: echinoderms are grouped with bilateral animals although the adult is radial, because the larva is bilateral and the adult radial symmetry is secondary.

Hemichordata was earlier considered a sub-phylum under phylum Chordata, but it is now placed as a separate phylum under non-chordata. The reason, which the Assertion–Reason item asks, is that the structure once taken for a notochord (the stomochord or buccal diverticulum in the proboscis) is not homologous to the true notochord. This phylum consists of a small group of worm-like marine animals with the organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate. The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk. These three regions are the labels expected on the diagram of Balanoglossus.

The circulatory system is open. Respiration takes place through gills (pharyngeal gill slits open in the trunk, which is one reason they were once placed with chordates). The excretory organ is the proboscis gland, the fourth entry in the excretory-organ series. Sexes are separate, fertilisation is external, and development is indirect (with a tornaria larva, though NCERT names only 'indirect development'). Examples: Balanoglossus and Saccoglossus.

The examiner's 2-mark question is 'Why is Hemichordata no longer placed under Chordata?' or 'Name the three body regions and the excretory organ of Balanoglossus'. Both are answered above. If a question lists non-chordate phyla, remember to include Hemichordata as the tenth, after Echinodermata.

Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. These three are 'the fundamental characters' and every chordate shows them at some stage of life. Chordates are bilaterally symmetrical, triploblastic, coelomate, with the organ-system level of organisation. They possess a post-anal tail and a closed circulatory system. NCERT's comparison table sets chordates against non-chordates on five rows, and a 3-mark comparison is a standard question: notochord present versus absent; central nervous system dorsal, hollow and single versus ventral, solid and double; pharynx perforated by gill slits versus not; heart ventral versus dorsal (if present); post-anal tail present versus absent.

Chordata is divided into three subphyla: Urochordata (Tunicata), Cephalochordata and Vertebrata. The first two are together called protochordates and are exclusively marine. In Urochordata the notochord is present only in the larval tail, and is lost in the sessile adult; in Cephalochordata it extends from head to tail region and is persistent throughout life. Examples: Urochordata, Ascidia, Salpa, Doliolum; Cephalochordata, Branchiostoma (Amphioxus or Lancelet).

Members of subphylum Vertebrata possess a notochord during the embryonic period; it is replaced by a cartilaginous or bony vertebral column in the adult. Thus all vertebrates are chordates but all chordates are not vertebrates (exercise 9): the protochordates keep the notochord and never build a vertebral column, so they are chordates without being vertebrates. Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages which may be fins or limbs. Vertebrata is divided into Agnatha (jawless; class Cyclostomata) and Gnathostomata (jawed), and Gnathostomata into superclass Pisces (bear fins: Chondrichthyes, Osteichthyes) and superclass Tetrapoda (bear limbs: Amphibia, Reptilia, Aves, Mammalia). Draw this chart for the 5-mark chordate classification question; it is the answer's skeleton.

Cyclostomata. All living members are ectoparasites on some fishes. They have an elongated body bearing 6 to 15 pairs of gill slits for respiration. Cyclostomes have a sucking and circular mouth without jaws (cyclo, circular; stoma, mouth), and they belong to Agnatha for that reason. The body is devoid of scales and paired fins. The cranium and vertebral column are cartilaginous. Circulation is of closed type. Cyclostomes are marine but migrate to fresh water for spawning; after spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean. Examples: Petromyzon (lamprey) and Myxine (hagfish). Exercise 14 puts 'gill slits' against Cyclostomata and Chondrichthyes, the two groups whose gill slits are visible externally because there is no operculum.

Chondrichthyes. These are marine animals with a streamlined body and a cartilaginous endoskeleton. The mouth is located ventrally. The notochord is persistent throughout life. Gill slits are separate and without an operculum (gill cover). The skin is tough, containing minute placoid scales. Teeth are modified placoid scales which are backwardly directed; the jaws are very powerful, and these animals are predaceous. Because of the absence of an air bladder, they have to swim constantly to avoid sinking: that sentence is the answer to exercise 10 from the negative side and to the most common Assertion–Reason from this class. The heart is two-chambered (one auricle and one ventricle). Some of them have electric organs (Torpedo) and some possess a poison sting (Trygon). They are cold-blooded (poikilothermous), that is, they lack the capacity to regulate body temperature. Sexes are separate; in males the pelvic fins bear claspers. Fertilisation is internal, and many of them are viviparous. Examples: Scoliodon (dog fish), Pristis (saw fish), Carcharodon (great white shark), Trygon (sting ray).

Osteichthyes includes both marine and fresh water fishes with a bony endoskeleton. The body is streamlined, and the mouth is mostly terminal (compare the ventral mouth of sharks). They have four pairs of gills which are covered by an operculum on each side (exercise 14: operculum → Osteichthyes). The skin is covered with cycloid or ctenoid scales. The air bladder is present and regulates buoyancy, so bony fishes can hang at any depth without swimming; this is the positive side of exercise 10 ('how important is the presence of air bladder in Pisces'). The heart is two-chambered (one auricle and one ventricle), as in cartilaginous fishes; the two-chambered heart is a feature of all fishes. They are cold-blooded. Sexes are separate, fertilisation is usually external, they are mostly oviparous, and development is direct.

NCERT groups the examples by habitat and the grouping is asked. Marine: Exocoetus (flying fish), Hippocampus (sea horse). Fresh water: Labeo (rohu), Catla (katla), Clarias (magur). Aquarium: Betta (fighting fish), Pterophyllum (angel fish).

The 3-mark comparison between Chondrichthyes and Osteichthyes is one of the two or three most predictable questions in the chapter. Give it as a table with these rows: endoskeleton (cartilaginous versus bony), habitat (marine only versus marine and fresh water), mouth (ventral versus terminal), gill slits (separate, no operculum versus covered by operculum), scales (placoid versus cycloid or ctenoid), air bladder (absent versus present), fertilisation (internal versus external), reproduction (many viviparous versus mostly oviparous), and an example each (Scoliodon versus Labeo). Both classes share a two-chambered heart, cold-bloodedness and a streamlined body, so do not list those as differences.

Amphibia. As the name suggests (amphi, dual; bios, life), amphibians can live in aquatic as well as terrestrial habitats. Most have two pairs of limbs. The body is divisible into head and trunk; a tail may be present in some (Salamandra). The skin is moist and without scales, and it takes part in respiration. The eyes have eyelids. A tympanum represents the ear; there is no external ear. The alimentary canal, urinary and reproductive tracts open into a common chamber, the cloaca, which opens to the exterior. Respiration is by gills, lungs and through the skin. The heart is three-chambered (two auricles and one ventricle). They are cold-blooded. Sexes are separate, fertilisation is external, they are oviparous, and development is indirect (tadpole larva). Examples: Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), Ichthyophis (limbless amphibia). Ichthyophis is the MCQ favourite: limbless, but an amphibian, not a snake.

Reptilia. The class name refers to their creeping or crawling mode of locomotion (repere, to creep). They are mostly terrestrial, and the body is covered by dry and cornified skin with epidermal scales or scutes. They do not have external ear openings; the tympanum represents the ear. Limbs, when present, are two pairs. The heart is usually three-chambered, but four-chambered in crocodiles, the fact behind the most repeated reptile MCQ. Reptiles are poikilotherms. Snakes and lizards shed their scales as skin cast. Sexes are separate, fertilisation is internal, they are oviparous, and development is direct. Examples: Chelone (turtle), Testudo (tortoise), Chameleon (tree lizard), Calotes (garden lizard), Crocodilus (crocodile), Alligator, Hemidactylus (wall lizard), and the poisonous snakes Naja (cobra), Bangarus (krait), Vipera (viper).

The 2-mark differentiation between the two classes runs on skin (moist, scaleless versus dry, scaly), fertilisation (external versus internal), development (indirect versus direct) and respiration (skin and gills used versus lungs only). Both share a tympanum for an ear, a three-chambered heart (except crocodiles) and cold-bloodedness.

Aves. The characteristic features of birds are the presence of feathers and, in most, the ability to fly, though flightless birds such as the ostrich exist. They possess a beak. The forelimbs are modified into wings. The hind limbs generally have scales and are modified for walking, swimming or clasping tree branches. The skin is dry without glands except the oil gland at the base of the tail. The endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic). The digestive tract has additional chambers, the crop and the gizzard. The heart is completely four-chambered. Birds are warm-blooded (homoiothermous): they maintain a constant body temperature. Respiration is by lungs, and air sacs connected to the lungs supplement respiration. Sexes are separate, fertilisation is internal, they are oviparous, and development is direct. Examples: Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin), Neophron (vulture). Exercise 11 (modifications for flight) is answered by picking the flight-related items out of this list: feathers, wings, pneumatic bones, air sacs, warm-bloodedness, a beak and no teeth (weight saving), and a streamlined body.

Mammalia. Mammals are found in a variety of habitats: polar ice caps, deserts, mountains, forests, grasslands and dark caves; some fly (bats) and some live in water (whales, dolphins). The most unique mammalian character is the presence of milk-producing glands (mammary glands) by which the young are nourished. They have two pairs of limbs adapted for walking, running, climbing, burrowing, swimming or flying. The skin is unique in possessing hair. External ears or pinnae are present (the only vertebrate class with them). Different types of teeth are present in the jaw (heterodont). The heart is four-chambered. They are homoiothermous. Respiration is by lungs. Sexes are separate, fertilisation is internal, they are viviparous with few exceptions, and development is direct. The exceptions are the oviparous mammals: Ornithorhynchus (platypus). Viviparous examples: Macropus (kangaroo), Pteropus (flying fox), Camelus (camel), Macaca (monkey), Rattus (rat), Canis (dog), Felis (cat), Elephas (elephant), Equus (horse), Delphinus (common dolphin), Balaenoptera (blue whale), Panthera tigris (tiger), Panthera leo (lion).

Exercise 12 asks whether an oviparous and a viviparous mother could produce equal numbers of young. No: the oviparous mother lays many eggs because the eggs are exposed to predators and the environment, so few survive; the viviparous mother nourishes a few young inside her body and gives them parental care, so survival is high and numbers are small.