Animal Kingdom

There are millions of animal species, so scientists sort them the way a librarian sorts books: first by big, obvious features, then by finer ones. The question is always the same: what is the body plan of this animal? Is it one cell or many? Is the body the same on both sides? How many layers of cells build the embryo? Does it have a body cavity? Does it have a notochord? Answer these few questions and any animal finds its place in the Animal Kingdom (Kingdom Animalia).

Animals are multicellular, eukaryotic, heterotrophic organisms. Their cells have no cell wall and no chlorophyll, and most of them can move at some stage of life. Single-celled animal-like organisms such as Amoeba, Paramecium and Euglena belong to Kingdom Protista in the five-kingdom system (R.H. Whittaker, 1969), although older books still call them Protozoa.

Now the main criteria. Read them as a ladder: each rung asks one question, and the answer decides which group the animal goes to.

Classification also has a fixed ladder of ranks. Every animal carries its address through these ranks, and the last two ranks give its scientific name (binomial nomenclature, introduced by Carolus Linnaeus, the Father of Taxonomy). Aristotle is called the Father of Biology and Zoology.

Two more everyday splits come up in questions. Vertebrates have a backbone, invertebrates do not. Warm-blooded (homeothermic or endothermic) animals, birds and mammals, keep a steady body temperature; cold-blooded (poikilothermic or ectothermic) animals, fish, amphibians and reptiles, take on the temperature of their surroundings.

Non-chordates are the animals without a notochord, and they make up the great majority of the animal kingdom. Walk through them like a staircase of complexity: from a sponge that is little more than a colony of cells, up to the starfish with a body plan close to our own. For each phylum, fix three things: the signature feature, the body plan, and two or three examples.

Here is the master table. If you can recall the 'signature feature' column, you can answer most direct questions.

A few worm and parasite facts are asked again and again. Filaria (Wuchereria bancrofti) is carried by the Culex mosquito and blocks lymph vessels, causing elephantiasis. Hookworm (Ancylostoma duodenale) lives in the intestine and causes anaemia. Taenia solium, the pork tapeworm, has a head (scolex) with hooks and suckers. Leech sucks blood and releases the anticoagulant hirudin. Earthworm, the 'farmer's friend', loosens and fertilises the soil, and is used in vermicomposting.

The arthropods deserve their own look because they are the largest phylum. Insects have a head, thorax and abdomen, 3 pairs of legs and usually 2 pairs of wings. Silk comes from the cocoon of Bombyx mori (mulberry silkworm); wild silks include Tasar (Antheraea mylitta), Muga (Antheraea assamensis) and Eri (Philosamia ricini). Lac comes from the lac insect (Laccifer lacca); honey from Apis. Anopheles carries malaria; Aedes carries dengue and chikungunya; Culex carries filaria. Limulus, the king crab, is called a living fossil.

A chordate is an animal that, at some point in its life, shows four hall-marks: a notochord (a flexible rod for support), a dorsal hollow nerve cord (in non-chordates the nerve cord is solid and on the belly side), pharyngeal gill slits, and a post-anal tail. In most fish and tetrapods the notochord is replaced by a backbone in the adult, which is why vertebrates are the stars of this phylum.

The five classes follow the story of evolution: life in water (fish), the first steps onto land (amphibians), full conquest of dry land (reptiles), and two warm-blooded lines (birds and mammals). Notice how the heart improves along the way: 2, 3, 3, 4, 4 chambers.

Fish split into two kinds. Chondrichthyes (cartilaginous fish: shark, dogfish Scoliodon, ray, skate, sawfish) have a skeleton of cartilage, placoid scales, a mouth on the underside, exposed gill slits and no air bladder. Osteichthyes (bony fish: Rohu, Catla, Hilsa, Hippocampus or sea horse, flying fish Exocoetus) have a bony skeleton, a gill cover (operculum) and an air bladder that helps them float. Sharks must keep swimming to stay afloat because they lack a swim bladder.

Amphibians are the 'double-life' animals: the young (tadpole) breathes with gills in water, the adult breathes with lungs and skin on land. The skin is moist and has no scales, there is a tympanum (eardrum) on the surface, the heart is three-chambered, and fertilisation is external. Examples: Rana tigrina (frog), Bufo (toad), Hyla (tree frog), Salamandra, and Ichthyophis, a limbless worm-like amphibian (caecilian), which is the answer in the apodan-type PYQ. Reptiles are the first true land-dwellers: dry scaly skin that stops water loss, shelled leathery eggs laid on land, lungs only. Examples: turtle (Chelone), tortoise (Testudo), garden lizard (Calotes), chameleon, wall lizard (Hemidactylus), crocodile, cobra (Naja), krait (Bangarus), viper (Vipera).

Birds (Aves) are built for flight: feathers, forelimbs changed into wings, a toothless beak, light pneumatic (hollow) bones, a four-chambered heart, lungs connected to air sacs, and hard-shelled eggs. They are warm-blooded, have no sweat glands, and have an oil (preen) gland at the base of the tail. Not all birds fly: ostrich (Struthio), emu, kiwi and penguin (Aptenodytes) are flightless. Mammals (Mammalia) are the group of milk-feeders: mammary glands, hair, external ears (pinna), a diaphragm, a four-chambered heart, heterodont teeth in sockets and, in nearly all, seven neck vertebrae (even in giraffe and whale).

This is the 'general awareness' side of zoology. The examiner does not want a lecture on classification here; he wants a quick, reliable fact: which is the largest, which is the fastest, what is the national animal, what do we call the baby of a swan. These facts are best learnt as small tables, so read each table once slowly and then once quickly.

India-specific animal facts are asked even more often than world records. The national animal is the tiger (Panthera tigris) and Project Tiger began in 1973. The national bird is the peacock (Pavo cristatus). The national aquatic animal is the Ganges river dolphin (Platanista gangetica). The elephant is the national heritage animal. The Asiatic lion lives wild only in Gir, Gujarat. The one-horned rhinoceros is the pride of Kaziranga, Assam. Cheetah was declared extinct in India in 1952 and was brought back from Namibia to Kuno National Park, Madhya Pradesh, in September 2022.

Snake facts come up often. India's 'Big Four' venomous snakes are the Indian cobra (Naja naja), common krait (Bungarus caeruleus), Russell's viper (Daboia russelii) and saw-scaled viper (Echis carinatus). Cobra and krait venom attacks the nerves (neurotoxic); viper venom attacks blood and tissues (haemotoxic). Antivenom is made by injecting a small dose of venom into a horse and collecting the antibodies from its blood.

A few more 'did you know' facts the paper likes. The octopus has three hearts and blue blood, because its blood pigment is copper-based haemocyanin and not iron-based haemoglobin. The starfish has no brain and no blood; it uses seawater in its water vascular system. The camel's hump stores fat, not water. Elephant tusks are enlarged incisor teeth. A chameleon changes colour using special skin cells called chromatophores. Bats and dolphins find their way by echolocation, using sound echoes. Hibernation is winter sleep (bear, frog); aestivation is summer sleep (snail, some frogs).