Animal Kingdom

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 4.

A million animals, six questions

More than a million animal species have been described, from a sponge on a rock to a blue whale. To give every newly found species a proper place, biologists ask the same questions of each animal and sort it by the answers. Learn the questions first, and the phyla stop feeling like a list of unrelated creatures.

The classification rests on these fundamentals: the arrangement of cells, body symmetry, the nature of the body cavity, and the pattern of the digestive, circulatory and reproductive systems. The table below turns them into six questions you can ask of any animal.

The six questions in detail

Two more differences show up in the exam. The digestive tract is incomplete when one opening serves as mouth and anus (Platyhelminthes), and complete when mouth and anus are separate. The circulation is open when blood flows into spaces and bathes the tissues directly, and closed when blood stays inside arteries, veins and capillaries.

Why it works this way. Radial symmetry suits animals that stay in place or drift and meet food from all sides, like sea anemones and jellyfish. Bilateral symmetry goes with moving in one direction, so one end becomes a head. A body cavity gives organs room to grow and move independently of the body wall, which is why complex organ systems appear in coelomates.

Pause and predict: a starfish larva is bilaterally symmetrical. Does that make the adult bilateral too?

Check your answer: No. Echinoderm larvae are bilateral, but the adults are radially symmetrical. This is a favourite NTA twist.

The non chordate phyla side by side

This table is the heart of the chapter. Every cell is a likely match-the-column pair.

Now the details that the table cannot hold, phylum by phylum.

Porifera. Sponges are mostly marine. Water enters through many small pores called ostia into a central cavity, the spongocoel, and leaves through the osculum. This canal system brings in food and oxygen and removes waste. The cavity and canals are lined with flagellated choanocytes (collar cells). The skeleton is made of spicules or spongin fibres. Sponges are hermaphrodites: the same individual makes eggs (from archaeocytes) and sperms (from choanocytes). Fertilisation is internal and development is indirect, through a larva very different from the adult. They also reproduce asexually by fragmentation.

Cnidaria. These are aquatic, mostly marine animals with tentacles bearing cnidoblasts (cnidocytes), which hold stinging capsules called nematocysts. These help in anchoring, defence and catching prey. There is one central gastrovascular cavity with a single opening, the mouth on the hypostome. Two body forms exist. The polyp is attached and cylindrical (Hydra, Adamsia), and the medusa is umbrella shaped and free swimming (Aurelia). A cnidarian that has both forms alternates between them, a process called metagenesis: polyps produce medusae asexually, and medusae produce polyps sexually, as in Obelia. Corals have a skeleton of calcium carbonate.

Ctenophora. Commonly called sea walnuts or comb jellies, these are exclusively marine. The eight rows of ciliated comb plates help them swim. Bioluminescence, the ability of a living organism to emit light, is well marked. The sexes are not separate, reproduction is only sexual, fertilisation is external and development is indirect.

Platyhelminthes. Flatworms are flattened from back to belly, and most are endoparasites of animals including humans. Parasitic forms have hooks and suckers for holding on. Flame cells look after osmoregulation and excretion. Planaria has a remarkable power of regeneration. Fertilisation is internal and development is usually indirect. Most flatworms are hermaphrodite.

Aschelminthes. Roundworms are circular in cross section. They have a complete gut with a muscular pharynx. Their sexes are separate (dioecious), and females are often longer than males. Development may be direct or indirect. Many are parasites.

Annelida. The body surface is marked into segments or metameres, and the muscles are both longitudinal and circular. Aquatic Nereis has lateral appendages called parapodia for swimming. Annelids have a closed circulatory system, nephridia for osmoregulation and excretion, and a nervous system of paired ganglia joined to a double ventral nerve cord. Nereis is dioecious, but earthworms and leeches are monoecious.

Arthropoda. This is the largest phylum of animals, and over two thirds of all named species are arthropods. The body is covered by a chitinous exoskeleton and divided into head, thorax and abdomen. Appendages are jointed (the word means "jointed feet"). Gases are exchanged through gills, book gills, book lungs or a tracheal system. Circulation is open and excretion is by malpighian tubules. The sexes are mostly separate and most arthropods lay eggs.

Mollusca. The second largest phylum. The body is soft and unsegmented, with a distinct head, a muscular foot and a visceral hump, and is usually covered by a calcareous shell. A fold of skin, the mantle, covers the hump, and the space under it is the mantle cavity, where feather like gills work in respiration and excretion. The mouth contains a file like rasping organ, the radula. Molluscs are usually dioecious and oviparous, with indirect development.

Echinodermata. Spiny skinned, marine animals with an endoskeleton of calcareous ossicles. The digestive system is complete, with the mouth on the lower side and the anus on the upper. The water vascular system helps in locomotion, capturing and transporting food, and respiration. There is no excretory system. The sexes are separate, fertilisation is external and development is indirect, with a free swimming larva.

Hemichordata. A small group of worm like marine animals. The body has an anterior proboscis, a collar and a long trunk. The collar carries the stomochord, a rudimentary structure that looks like a notochord. Circulation is open, respiration is by gills and excretion is by the proboscis gland. The sexes are separate and fertilisation is external. Hemichordates were once put under Chordata but are now a separate non chordate phylum.

Beyond the textbook. For the exam: Aschelminthes is a phylum of roundworms, and Cnidaria is also called Coelenterata. In reality: Aschelminthes was a mixed group, and the true roundworms are now placed in their own phylum, Nematoda, so Aschelminthes is no longer used as a single natural group. Why it matters: CUET still uses Aschelminthes, so use it in the exam. If you see "Nematoda" in a newer book, it is the same worm.

Beyond the textbook. For the exam: echinoderms are non chordates and hemichordates are a separate non chordate phylum. In reality: echinoderms, hemichordates and chordates share an early developmental pattern and are grouped together as deuterostomes. A starfish is a closer cousin of yours than of a crab. Why it matters: it explains why hemichordates with their stomochord were once placed in Chordata.

Pause and predict: an exam option says "earthworm is dioecious". Which phylum rule do you use to check it?

Check your answer: Annelida. Only Nereis is dioecious. Earthworms and leeches are monoecious (both sexes in one body), so the option is wrong.

Reproduction at a glance: who is hermaphrodite?

Questions about sex organs are easy marks once you have the pattern.

Chordata: three features and a notochord

Chordates have a notochord, a dorsal hollow nerve cord, paired pharyngeal gill slits and a post anal tail, and their heart is on the ventral side. Some of these features appear only at an early stage of life in some chordates. Compare them with non chordates.

Chordata has three subphyla, separated by what happens to the notochord.

So all vertebrates are chordates, but not all chordates are vertebrates. Vertebrates also have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages (fins or limbs).

Vertebrata splits first by jaws and then by limbs.

Fishes: cartilage, bone and sucker mouths

Cartilaginous and bony fishes are cold blooded (poikilothermous), so their body temperature changes with the surroundings.

Beyond the textbook. For the exam: all living cyclostomes are ectoparasites on fishes. In reality: hagfishes are mainly scavengers that feed on dead and dying animals, and many lampreys are not parasitic as adults. Why it matters: write "ectoparasites" in the exam, but remember that it does not describe every cyclostome.

Tetrapods: from wet skin to feathers to milk

Why it works this way. Warm blooded animals burn a lot of fuel to stay warm, so they need oxygen delivered fast. A four chambered heart keeps oxygen rich and oxygen poor blood completely apart, which is why only birds and mammals (and the crocodile) have it. In a three chambered heart the two kinds of blood mix a little.

The pneumatic bones of birds are hollow and filled with air cavities, which lightens the body for flight, and air sacs joined to the lungs help with breathing.

Beyond the textbook. For the exam: Reptilia and Aves are separate classes, and the platypus (Ornithorhynchus) is the oviparous mammal. In reality: birds descended from a group of dinosaurs, so by evolutionary relationships birds sit inside the reptile lineage, and crocodiles are their closest living relatives. Echidnas also lay eggs, along with the platypus. Why it matters: the class level is a convenient grouping for the exam, but it hides how close birds and reptiles really are.

Pause and predict: an animal has moist skin without scales, a three chambered heart and a cloaca, and it breathes through gills, lungs and skin at different times. Which class is it?

Check your answer: Amphibia. Reptiles also have a cloaca and a three chambered heart, but their skin is dry and scaly, and they do not breathe through the skin.

Exam traps

60-second revision

Where this chapter connects

• Previous: Plant Kingdom completes the plant side of the diversity story, and Biological Classification gave the five kingdoms.
• Next: Structural Organisation in Animals studies the tissues and organ systems of the earthworm, cockroach and frog, three animals from this chapter.
• Class XII link: Evolution uses the animal groups here to trace the move from water to land, and Human Health and Disease returns to parasites such as Ascaris, Wuchereria and Taenia.