Animal Kingdom | CBSE Class 11 Biology Notes

This note covers NCERT Class 11 Biology Chapter 4, Animal Kingdom: the features used as the basis of classification, the eleven phyla from Porifera to Chordata, and the classes of vertebrates. It follows the current NCERT text section by section. Every feature and every example, with its common name, is given as NCERT states it, because this chapter is examined through exact matches: which phylum has which feature, and which animal belongs where.

Why classify animals, and what is the basis of classification?

Over a million species of animals have been described till now, so the need for classification is all the more important. Classification also helps in assigning a systematic position to newly described species.

In spite of differences in structure and form, animals share fundamental features in relation to the arrangement of cells, body symmetry, the nature of the coelom, and the patterns of the digestive, circulatory or reproductive systems. These features are used as the basis of animal classification. NCERT discusses six of them.

1. Levels of organisation
2. Symmetry
3. Diploblastic and triploblastic organisation
4. Coelom
5. Segmentation
6. Notochord

Note: This list is also the answer to the exercise question on the steps you would follow to classify a specimen. Examine it in this order: level of organisation, symmetry, germ layers, body cavity, segmentation and notochord. Without such common fundamental features, a million species could not be arranged into groups, and a new species could not be given a position.

What are the levels of organisation in animals?

All members of Animalia are multicellular, but they do not all show the same pattern of organisation of cells.

Organ systems in different groups of animals show various patterns of complexity.

How do animals differ in symmetry and in germ layers?

Symmetry

Part (a) shows a radially symmetrical animal with arms spreading from a central axis, cut by several vertical planes that all pass through that axis; each plane gives two identical halves. Part (b) shows a crab seen from above with a single line down the middle of the body; only this one plane divides it into identical left and right halves.

See Fig. 4.1 in your NCERT textbook

Diploblastic and triploblastic organisation

Two circular sections. In (a), diploblastic, the outer ring is labelled ectoderm and the inner ring endoderm, with a layer labelled mesoglea between them. In (b), triploblastic, there is an outer ectoderm, an inner endoderm, and a layer of mesoderm between them.

See Fig. 4.2 in your NCERT textbook

Note: Mesoglea is not a germ layer. It is an undifferentiated layer between the ectoderm and the endoderm of a diploblastic animal. The third germ layer of triploblastic animals is the mesoderm.

What are coelom, segmentation and notochord?

The presence or absence of a cavity between the body wall and the gut wall is very important in classification.

Definition: The coelom is the body cavity that is lined by mesoderm.

Three circular sectional views, each with the gut at the centre and the body wall outside. In (a), coelomate, a cavity labelled coelom lies between the gut and the body wall and is lined all round. In (b), pseudocoelomate, a cavity labelled pseudocoelom is present, with the mesoderm seen only as separate pouches. In (c), acoelomate, there is no cavity: the space between the gut and the body wall is filled.

See Fig. 4.3 in your NCERT textbook

Segmentation

In some animals the body is externally and internally divided into segments, with a serial repetition of at least some organs. In the earthworm, the body shows this pattern, called metameric segmentation, and the phenomenon is known as metamerism.

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 that do not form this structure are called non-chordates, for example from Porifera to the echinoderms.

How is Kingdom Animalia broadly classified?

A branching chart with five columns: kingdom, levels of organisation, symmetry, body cavity or coelom, and phylum. Animalia (multicellular) divides first by level of organisation. The cellular level branch is mostly asymmetrical, acoelomata, and leads to Porifera. The tissue, organ and organ system branch divides by symmetry. Radial animals are acoelomata and lead to Coelenterata (Cnidaria) and Ctenophora. Bilateral animals divide by body cavity: without body cavity (acoelomates) leads to Platyhelminthes; with false coelom (pseudocoelomates) leads to Aschelminthes; with true coelom (coelomates) leads to Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata. A footnote says that Echinodermata exhibits radial or bilateral symmetry depending on the stage.

See Fig. 4.4 in your NCERT textbook

NCERT Table 4.2 gives the salient features of all the phyla. It is reproduced here as NCERT prints it.

Note: Segmentation is first seen in Annelida. In NCERT Table 4.2 it is marked present in only three phyla: Annelida, Arthropoda and Chordata. The digestive system first becomes complete in Aschelminthes.

What are the features of Porifera, Coelenterata and Ctenophora?

Phylum Porifera

Members of this phylum are commonly known as sponges. They are generally marine and mostly asymmetrical animals. They are primitive multicellular animals with the cellular level of organisation.

Sponges have a water transport or canal system. The pathway of water is:

1. Water enters through minute pores (ostia) in the body wall.
2. It passes into a central cavity, the spongocoel.
3. It goes out through the osculum.
4. Function of the whole pathway: it helps in food gathering, respiratory exchange and removal of waste.

• Choanocytes or collar cells line the spongocoel and the canals.
• Digestion is intracellular.
• The body is supported by a skeleton made up of spicules or spongin fibres.
• Sexes are not separate (hermaphrodite): eggs and sperms are produced by the same individual.
• Sponges reproduce asexually by fragmentation and sexually by formation of gametes.
• Fertilisation is internal. Development is indirect, with a larval stage that is morphologically distinct from the adult.

Examples: Sycon (Scypha), Spongilla (fresh water sponge) and Euspongia (bath sponge).

Phylum Coelenterata (Cnidaria)

Coelenterates are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. The name Cnidaria is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocysts) present on the tentacles and the body. Cnidoblasts are used for anchorage, defence and the 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 extracellular and intracellular.
• Some cnidarians, such as corals, have a skeleton composed of calcium carbonate.

Cnidarians that exist in both forms show alternation of generations (metagenesis): polyps produce medusae asexually, and medusae form the polyps sexually, as in Obelia.

Examples: Physalia (Portuguese man-of-war), Adamsia (sea anemone), Pennatula (sea-pen), Gorgonia (sea-fan) and Meandrina (brain coral).

Phylum Ctenophora

Ctenophores, commonly known as sea walnuts or comb jellies, 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.
• Digestion is both extracellular and intracellular.
• Bioluminescence (the property of a living organism to emit light) is well-marked in ctenophores.
• Sexes are not separate. Reproduction takes place only by sexual means.
• Fertilisation is external, with indirect development.

Examples: Pleurobrachia and Ctenoplana.

What are the features of Platyhelminthes, Aschelminthes and Annelida?

Phylum Platyhelminthes

These animals have a dorso-ventrally flattened body and are hence called flatworms. They are mostly endoparasites found in animals, including human beings. Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with the organ level of organisation.

• Hooks and suckers are present in the parasitic forms.
• Some of them absorb nutrients from the host directly through their body surface.
• Specialised cells called flame cells help in osmoregulation and excretion.
• Sexes are not separate. Fertilisation is internal, and development is through many larval stages.
• Some members, like Planaria, possess a high regeneration capacity.

Examples: Taenia (tapeworm) and Fasciola (liver fluke).

Phylum Aschelminthes

The body of the 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 have the organ-system level of body organisation. They are bilaterally symmetrical, triploblastic and pseudocoelomate animals.

• The alimentary canal is complete, with a well-developed muscular pharynx.
• An excretory tube removes body wastes from the body cavity through the excretory pore.
• Sexes are separate (dioecious): males and females are distinct. Often females are longer than males.
• Fertilisation is internal. Development may be direct (the young ones resemble the adult) or indirect.

Examples: Ascaris (roundworm), Wuchereria (filaria worm) and Ancylostoma (hookworm).

Phylum Annelida

Annelids may be aquatic (marine and fresh water) or terrestrial, free-living and sometimes parasitic. They show the organ-system level of body organisation and bilateral symmetry. They are triploblastic, metamerically segmented and coelomate animals. Their body surface is distinctly marked out into segments or metameres, hence the phylum name Annelida (Latin annulus, little ring).

• They possess longitudinal and circular muscles, which help in locomotion.
• Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
• A closed circulatory system is present.
• Nephridia (singular nephridium) help in osmoregulation and excretion.
• The neural system consists of paired ganglia connected by lateral nerves to a double ventral nerve cord.
• Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious. Reproduction is sexual.

Examples: Nereis, Pheretima (earthworm) and Hirudinaria (blood-sucking leech).

What are the features of Arthropoda and Mollusca?

Phylum Arthropoda

This is the largest phylum of Animalia, and it includes the insects. Over two-thirds of all named species on earth are arthropods. They have the organ-system level of organisation, and are bilaterally symmetrical, triploblastic, segmented and coelomate animals.

• The body is covered by a chitinous exoskeleton.
• The body consists of head, thorax and abdomen.
• They have jointed appendages (arthros, joint; poda, appendages).
• The respiratory organs are gills, book gills, book lungs or the tracheal system.
• The circulatory system is of the open type.
• Sensory organs such as antennae, eyes (compound and simple) and statocysts or balancing organs are present.
• Excretion takes place through malpighian tubules.
• They are mostly dioecious. Fertilisation is usually internal. They are mostly oviparous. Development may be direct or indirect.

Phylum Mollusca

This 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 animals.

• The body is covered by a calcareous shell and is unsegmented, with a distinct head, muscular foot and visceral hump.
• A soft and spongy layer of skin forms a mantle over the visceral hump.
• The space between the hump and the mantle is called the mantle cavity, in which feather-like gills are present. The gills have respiratory and excretory functions.
• The anterior head region has sensory tentacles.
• The mouth contains a file-like rasping organ for feeding, called the radula.
• They are usually dioecious and oviparous, with indirect development.

Examples: Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea-hare), Dentalium (tusk shell) and Chaetopleura (chiton).

What are the features of Echinodermata and Hemichordata?

Phylum Echinodermata

These animals have an endoskeleton of calcareous ossicles, hence the name Echinodermata (spiny bodied). All are marine, with the organ-system level of organisation. They are triploblastic and coelomate animals.

• The adult echinoderms are radially symmetrical, but the larvae are bilaterally symmetrical.
• 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.
• An excretory system is absent.
• Sexes are separate. Reproduction is sexual. Fertilisation is usually external.
• Development is indirect, with a free-swimming larva.

Examples: Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber) and Ophiura (brittle star).

Phylum Hemichordata

Hemichordata was earlier considered a sub-phylum under phylum Chordata. It is now placed as a separate phylum under non-chordata. Hemichordates have a rudimentary structure in the collar region called the stomochord, a structure similar to the notochord.

The 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 animals.

• The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk.
• The circulatory system is of the open type.
• Respiration takes place through gills.
• The excretory organ is the proboscis gland.
• Sexes are separate. Fertilisation is external. Development is indirect.

Examples: Balanoglossus and Saccoglossus.

What characterises Phylum Chordata, and what are its subphyla?

Animals belonging to phylum Chordata are fundamentally characterised by the presence of three features:

• a notochord,
• a dorsal hollow nerve cord, and
• paired pharyngeal gill slits.

Chordates are bilaterally symmetrical, triploblastic and coelomate, with the organ-system level of organisation. They possess a post-anal tail and a closed circulatory system.

A side view of a simple elongated chordate body. The nerve cord runs along the dorsal side, and the notochord lies just below it. Gill slits are shown in the pharyngeal region towards the front, and the post-anal part is the portion of the body behind the anus. The four labels are: nerve cord, notochord, gill slits and post-anal part.

See Fig. 4.16 in your NCERT textbook

Phylum Chordata is divided into three subphyla.

The subphyla Urochordata and Cephalochordata are often referred to as protochordates, and are exclusively marine.

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.

Note: "All vertebrates are chordates but all chordates are not vertebrates." Vertebrates have a notochord in the embryo, so they are chordates. But in Urochordata and Cephalochordata the notochord is never replaced by a vertebral column, so these chordates are not vertebrates.

How is Vertebrata divided, and what are the features of cyclostomes and fishes?

Class Cyclostomata

All living members of the class Cyclostomata are ectoparasites on some fishes.

• They have an elongated body bearing 6 to 15 pairs of gill slits for respiration.
• They have a sucking and circular mouth without jaws.
• The body is devoid of scales and paired fins.
• The cranium and vertebral column are cartilaginous.
• Circulation is of the closed type.

The life cycle of cyclostomes involves a migration:

1. Cyclostomes are marine.
2. They migrate to fresh water for spawning.
3. After spawning, within a few days, they die.
4. Their larvae, after metamorphosis, return to the ocean.

Examples: Petromyzon (lamprey) and Myxine (hagfish).

Chondrichthyes and Osteichthyes compared

More on Chondrichthyes: the notochord is persistent throughout life. The teeth are modified placoid scales, which are backwardly directed. The jaws are very powerful, and these animals are predaceous. Some have electric organs (for example Torpedo) and some possess a poison sting (for example Trygon). Sexes are separate in both classes.

What are the features of Amphibia, Reptilia, Aves and Mammalia?

Class Amphibia

As the name indicates (Greek amphi, dual; bios, life), amphibians can live in aquatic as well as terrestrial habitats. Most of them have two pairs of limbs. The body is divisible into head and trunk, and a tail may be present in some. The eyes have eyelids. The alimentary canal, urinary and reproductive tracts open into a common chamber called the cloaca, which opens to the exterior. Sexes are separate.

Examples: Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander) and Ichthyophis (limbless amphibia).

Class Reptilia

The class name refers to their creeping or crawling mode of locomotion (Latin repere or reptum, to creep or crawl). They are mostly terrestrial animals. Limbs, when present, are two pairs. Snakes and lizards shed their scales as skin cast. Sexes are separate.

Examples: Chelone (turtle), Testudo (tortoise), Chameleon (tree lizard), Calotes (garden lizard), Crocodilus (crocodile), Alligator (alligator) and Hemidactylus (wall lizard). Poisonous snakes: Naja (cobra), Bangarus (krait) and Vipera (viper).

Class Aves

The characteristic features of Aves (birds) are the presence of feathers, and most of them can fly, except flightless birds such as the ostrich. 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 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.
• Sexes are separate.

Examples: Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin) and Neophron (vulture).

Class Mammalia

Mammals are found in a variety of habitats: polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some have adapted to fly or to live in water. The most unique mammalian characteristic is the presence of milk-producing glands (mammary glands) by which the young ones are nourished.

• They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying.
• Different types of teeth are present in the jaw.
• Sexes are separate.

How do you answer the NCERT exercise questions?

How useful is the study of the body cavity and coelom in classification?

The presence or absence of a cavity between the body wall and the gut wall separates animals into three groups: acoelomates (for example platyhelminthes), pseudocoelomates (for example aschelminthes) and coelomates (for example annelids to chordates). In Figure 4.4 it is the feature that divides the bilateral animals into phyla.

Distinguish between intracellular and extracellular digestion.

In intracellular digestion the food is digested inside the cells, as in sponges. In extracellular digestion the food is digested outside the cells, in a cavity such as the gastro-vascular cavity or the alimentary canal. Coelenterates and ctenophores show both.

What is the difference between direct and indirect development?

In direct development the young ones resemble the adult. In indirect development there is a larval stage that is morphologically distinct from the adult.

What peculiar features are found in parasitic platyhelminthes?

Hooks and suckers are present, and some absorb nutrients from the host directly through their body surface. The body is dorso-ventrally flattened.

Why is the air bladder important in Pisces?

The air bladder regulates buoyancy in Osteichthyes. Chondrichthyes do not have one, so they have to swim constantly to avoid sinking.

What modifications help birds to fly?

The forelimbs are modified into wings, the body bears feathers, the long bones are hollow with air cavities (pneumatic), and air sacs connected to the lungs supplement respiration.

The multiple-choice and matching questions

Glossary

• Coelom — The body cavity, between the body wall and the gut wall, that is lined by mesoderm.
• Pseudocoelom — A body cavity not lined by mesoderm, in which the mesoderm is present as scattered pouches between ectoderm and endoderm.
• Metamerism — External and internal division of the body into segments with a serial repetition of at least some organs.
• Notochord — A mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.
• Choanocytes — The collar cells that line the spongocoel and the canals of sponges.
• Cnidoblasts — Cells on the tentacles and body of cnidarians that contain stinging capsules, used for anchorage, defence and capture of prey.
• Metagenesis — Alternation of generations in cnidarians: polyps produce medusae asexually and medusae form polyps sexually, as in Obelia.
• Flame cells — Specialised cells of flatworms that help in osmoregulation and excretion.
• Nephridia — The organs of annelids that help in osmoregulation and excretion.
• Malpighian tubules — The structures through which excretion takes place in arthropods.
• Radula — The file-like rasping organ for feeding found in the mouth of molluscs.
• Water vascular system — The most distinctive feature of echinoderms, helping in locomotion, capture and transport of food, and respiration.
• Stomochord — A rudimentary structure in the collar region of hemichordates, similar to the notochord.
• Protochordates — The subphyla Urochordata and Cephalochordata taken together; they are exclusively marine.
• Poikilothermous — Cold-blooded; lacking the capacity to regulate body temperature, as in fishes, amphibians and reptiles.
• Homoiothermous — Warm-blooded; able to maintain a constant body temperature, as in birds and mammals.

Common errors and misconceptions

• Misconception: Any cavity inside the body is a coelom. Correct: A coelom is a body cavity lined by mesoderm. A cavity not lined by mesoderm is a pseudocoelom.
• Misconception: Platyhelminthes are pseudocoelomates. Correct: Platyhelminthes are acoelomates. Aschelminthes are the pseudocoelomates.
• Misconception: Echinoderms are radially symmetrical at every stage. Correct: The adults are radially symmetrical, but the larvae are bilaterally symmetrical.
• Misconception: Arthropods have a closed circulatory system. Correct: The circulatory system of arthropods is of the open type. Annelids have a closed circulatory system.
• Misconception: Hemichordates are chordates because they have a notochord. Correct: They have a stomochord, which is only similar to a notochord, and Hemichordata is now a separate phylum under non-chordata.
• Misconception: All chordates are vertebrates. Correct: All vertebrates are chordates, but Urochordata and Cephalochordata are chordates that are not vertebrates.
• Misconception: Cartilaginous fishes have an operculum and an air bladder. Correct: Chondrichthyes have neither. The operculum and the air bladder are features of Osteichthyes.
• Misconception: All reptiles have a three-chambered heart. Correct: The heart is usually three-chambered, but it is four-chambered in crocodiles.
• Misconception: All mammals are viviparous. Correct: Mammals are viviparous with few exceptions. Ornithorhynchus (platypus) is oviparous.
• Misconception: Ctenophores have cnidoblasts like coelenterates. Correct: The distinctive feature of ctenophores is eight rows of ciliated comb plates. Cnidoblasts belong to the coelenterates.

Exam-style questions with model answers

Q1. Name the phylum in which the water vascular system is found, and give one function of the system. [1 mark]

1. Echinodermata. The water vascular system helps in locomotion (also in the capture and transport of food, and in respiration).

Q2. Distinguish between open and closed circulatory systems. [2 marks]

1. In the open type, the blood is pumped out of the heart and the cells and tissues are directly bathed in it, as in arthropods.
2. In the closed type, the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries), as in annelids and chordates.

Q3. What is metagenesis? Name an animal that shows it. [2 marks]

1. Metagenesis is the alternation of generations seen in cnidarians that exist in both body forms: polyps produce medusae asexually, and medusae form the polyps sexually.
2. It is seen in Obelia.

Q4. Distinguish between coelomates, pseudocoelomates and acoelomates, with one example of each. [3 marks]

1. Coelomates possess a coelom, a body cavity lined by mesoderm, for example annelids (also molluscs, arthropods, echinoderms, hemichordates and chordates).
2. Pseudocoelomates have a body cavity not lined by mesoderm; the mesoderm is present as scattered pouches between the ectoderm and endoderm, for example aschelminthes.
3. Acoelomates have no body cavity, for example platyhelminthes.

Q5. Give three differences between chordates and non-chordates. [3 marks]

1. A notochord is present in chordates and absent in non-chordates.
2. The central nervous system is dorsal, hollow and single in chordates; it is ventral, solid and double in non-chordates.
3. The pharynx is perforated by gill slits in chordates; gill slits are absent in non-chordates. (Also acceptable: the heart is ventral in chordates and dorsal, if present, in non-chordates; a post-anal tail is present in chordates and absent in non-chordates.)

Q6. "All vertebrates are chordates but all chordates are not vertebrates." Justify the statement. [3 marks]

1. Chordates are characterised by a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
2. Members of subphylum Vertebrata possess a notochord during the embryonic period, which is replaced by a cartilaginous or bony vertebral column in the adult. So all vertebrates are chordates.
3. In Urochordata the notochord is present only in the larval tail, and in Cephalochordata it persists throughout life. Neither has a vertebral column, so these chordates are not vertebrates.

Q7. List three reasons why arthropods are able to form the largest phylum of the animal kingdom. [3 marks]

1. The body is covered by a chitinous exoskeleton.
2. They have jointed appendages, and a body divided into head, thorax and abdomen.
3. They have a range of respiratory organs (gills, book gills, book lungs or a tracheal system) and well-developed sensory organs such as antennae and compound and simple eyes, which suits them to many habitats. NCERT notes that over two-thirds of all named species are arthropods.

Q8. Compare the classes Chondrichthyes and Osteichthyes on five points. [5 marks]

1. Endoskeleton: cartilaginous in Chondrichthyes; bony in Osteichthyes.
2. Mouth: located ventrally in Chondrichthyes; mostly terminal in Osteichthyes.
3. Gills: gill slits are separate and without operculum in Chondrichthyes; four pairs of gills are covered by an operculum on each side in Osteichthyes.
4. Scales and air bladder: Chondrichthyes have placoid scales and no air bladder, so they swim constantly to avoid sinking; Osteichthyes have cycloid or ctenoid scales and an air bladder that regulates buoyancy.
5. Reproduction: fertilisation is internal and many are viviparous in Chondrichthyes; fertilisation is usually external and most are oviparous in Osteichthyes. Examples: Scoliodon and Labeo.

Q9. Describe the salient features of phylum Porifera, including the canal system, with examples. [5 marks]

1. Poriferans, the sponges, are generally marine and mostly asymmetrical. They are primitive multicellular animals with the cellular level of organisation.
2. They have a water transport or canal system: water enters through minute pores (ostia) into a central cavity, the spongocoel, and goes out through the osculum. This helps in food gathering, respiratory exchange and removal of waste.
3. Choanocytes or collar cells line the spongocoel and the canals. Digestion is intracellular. The skeleton is made of spicules or spongin fibres.
4. Sexes are not separate. Reproduction is asexual by fragmentation and sexual by formation of gametes. Fertilisation is internal, and development is indirect with a larval stage.
5. Examples: Sycon (Scypha), Spongilla (fresh water sponge) and Euspongia (bath sponge).

Key takeaways

• Animals are classified on levels of organisation, symmetry, germ layers, coelom, segmentation and notochord, the fundamental features common to various individuals.
• Sponges have the cellular level of organisation, coelenterates and ctenophores the tissue level, platyhelminthes the organ level, and aschelminthes onwards the organ-system level.
• Platyhelminthes are acoelomates, aschelminthes are pseudocoelomates, and annelids, arthropods, molluscs, echinoderms, hemichordates and chordates are coelomates.
• Learn the features that mark each phylum: choanocytes in Porifera, cnidoblasts in Coelenterata, comb plates in Ctenophora, a flat body with suckers and flame cells in Platyhelminthes, and ring-like segments with nephridia in Annelida.
• Arthropoda is the largest phylum, with a chitinous exoskeleton and jointed appendages; Mollusca is the second largest, with a mantle and a radula.
• Echinoderms have a water vascular system and are radial as adults but bilateral as larvae; hemichordates have a stomochord and are non-chordates.
• Chordates have a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits; the subphyla are Urochordata, Cephalochordata and Vertebrata.
• Vertebrata is divided into Agnatha, with the class Cyclostomata, and Gnathostomata, with the super classes Pisces and Tetrapoda.
• Fishes have a two-chambered heart, amphibians and most reptiles a three-chambered heart, and crocodiles, birds and mammals a four-chambered heart.
• Fishes, amphibians and reptiles are cold-blooded; birds and mammals are warm-blooded, and mammals are marked by mammary glands and hair.

Test yourself

Which animals show the cellular level of organisation?

Sponges, the members of phylum Porifera, show the cellular level of organisation, with cells arranged as loose cell aggregates.

What is the undifferentiated layer between the ectoderm and endoderm of diploblastic animals called?

The undifferentiated layer present between the ectoderm and the endoderm in diploblastic animals is called the mesoglea.

Which phylum is pseudocoelomate?

Aschelminthes, the roundworms, are pseudocoelomate: their body cavity is not lined by mesoderm.

What are the two basic body forms of cnidarians?

The two basic body forms are the polyp, which is sessile and cylindrical like Hydra, and the medusa, which is umbrella-shaped and free-swimming like Aurelia.

Which arthropod does NCERT call a living fossil?

NCERT gives Limulus, the king crab, as the example of a living fossil among the arthropods.

What is the excretory organ of hemichordates?

The excretory organ of hemichordates is the proboscis gland, and respiration in them takes place through gills.

In which subphylum is the notochord present only in the larval tail?

In subphylum Urochordata, also called Tunicata, the notochord is present only in the larval tail.

How many pairs of gill slits do cyclostomes have?

Cyclostomes have an elongated body bearing 6 to 15 pairs of gill slits for respiration.

Which cartilaginous fish has electric organs, and which has a poison sting?

Among the cartilaginous fishes, Torpedo has electric organs and Trygon, the sting ray, possesses a poison sting.

Name an oviparous mammal.

Ornithorhynchus, the platypus, is an oviparous mammal; most other mammals are viviparous.