Animal Kingdom
Welcome to HSLC Guru! This complete English-medium guide to Class 11 Biology Chapter 4 — Animal Kingdom — has been prepared strictly according to the ASSEB syllabus. The chapter introduces the fascinating world of animals and the scientific basis on which biologists classify them into different groups. From the simplest sponges to the most complex mammals, you will learn how animals are organised, how their bodies are built, and how they have evolved into a stunning variety of forms. Each topic is explained in clear, simple English so that every learner can master the concepts confidently.
Summary
The animal kingdom is incredibly diverse, containing more than a million known species. To make sense of this diversity, scientists classify animals on the basis of certain fundamental features that are common to all of them. These include levels of organisation (cellular, tissue, organ, and organ-system), symmetry (asymmetrical, radial, and bilateral), the number of embryonic germ layers (diploblastic with two layers and triploblastic with three layers), the presence or absence of a body cavity called coelom (acoelomate, pseudocoelomate, and coelomate), the pattern of segmentation, and the presence or absence of a notochord. These criteria help group animals into clearly defined phyla and classes.
The non-chordate phyla include Porifera (sponges with pores and canal system), Coelenterata or Cnidaria (Hydra, jellyfish with cnidoblasts), Ctenophora (comb jellies, marine animals with eight rows of comb plates), Platyhelminthes (flatworms like Planaria, Liver fluke, Tapeworm), Aschelminthes (roundworms such as Ascaris), Annelida (segmented worms like earthworm, leech, Nereis), Arthropoda (the largest phylum including insects, spiders, crabs), Mollusca (snails, octopus, with soft bodies often covered by shells), Echinodermata (starfish, sea urchin with water vascular system) and Hemichordata (Balanoglossus with stomochord). Each phylum shows characteristic features of body plan, symmetry, coelom, and habitat.
Phylum Chordata is distinguished by the presence of a notochord, dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail at some stage of life. It is divided into three subphyla — Urochordata (Herdmania, notochord only in larval tail), Cephalochordata (Branchiostoma, notochord throughout life), and Vertebrata (notochord replaced by a vertebral column). Vertebrates are further classified into Cyclostomata (jawless fish like Petromyzon), Pisces — divided into Chondrichthyes (cartilaginous fish like shark) and Osteichthyes (bony fish like Rohu) — Amphibia (frog, salamander), Reptilia (lizards, snakes, turtles), Aves (birds with feathers and forelimbs modified into wings), and Mammalia (animals with hair, mammary glands, and viviparity).
Understanding animal classification is essential because it helps us study the relationships between animals, their evolution, and their roles in nature. It also lays the foundation for advanced studies in zoology, ecology, and applied biological sciences. By examining basic features such as body symmetry, coelom, and notochord, students gain insight into how animals are organised and how they have adapted to varied habitats — from deep oceans to dense forests and even to the air above us.
Question and Answers
1-Mark Questions
Q1. What is the basis of classification of animals?
Answer: Animals are classified on the basis of fundamental features such as levels of organisation, symmetry, germ layers, coelom, segmentation, and presence of notochord.
Q2. Define symmetry in animals.
Answer: Symmetry refers to the arrangement of body parts around an axis or plane that divides the body into similar halves.
Q3. Name a diploblastic animal.
Answer: Hydra is a diploblastic animal having only two germ layers — ectoderm and endoderm.
Q4. What is a coelom?
Answer: A coelom is a fluid-filled body cavity lined by mesoderm, situated between the body wall and the gut.
Q5. Give one example of a pseudocoelomate animal.
Answer: Ascaris (roundworm) belonging to phylum Aschelminthes is a pseudocoelomate.
Q6. Which is the largest phylum in the animal kingdom?
Answer: Phylum Arthropoda is the largest phylum, containing more than two-thirds of all animal species.
Q7. What is a notochord?
Answer: A notochord is a long, rod-like, flexible structure found on the dorsal side of the body of chordates during some stage of their life.
Q8. Name the special cells found in members of phylum Coelenterata.
Answer: Cnidoblasts or nematocysts are the special stinging cells found in coelenterates.
Q9. Give one example of an oviparous mammal.
Answer: Echidna (spiny anteater) and Ornithorhynchus (Platypus) are egg-laying mammals.
Q10. What is meant by metameric segmentation?
Answer: Metameric segmentation is the division of the body into a series of repetitive segments called metameres, as seen in earthworm.
2 to 3-Mark Questions
Q1. Differentiate between diploblastic and triploblastic animals with examples.
Answer: Diploblastic animals have only two embryonic germ layers — outer ectoderm and inner endoderm — with a non-cellular jelly-like mesoglea in between, e.g., Hydra and jellyfish. Triploblastic animals have three germ layers — ectoderm, mesoderm and endoderm — from which all tissues and organs develop, e.g., Planaria, earthworm, frog, and humans.
Q2. Explain the three types of coelomic conditions found in animals.
Answer: (i) Acoelomate — animals lacking a body cavity between the body wall and gut; the space is filled with parenchyma cells, e.g., flatworms. (ii) Pseudocoelomate — animals with a body cavity that is not lined by mesoderm on both sides, e.g., Ascaris. (iii) Coelomate — animals with a true coelom completely lined by mesoderm, e.g., earthworm, frog and humans.
Q3. Write the distinguishing features of phylum Porifera.
Answer: Members of phylum Porifera are commonly known as sponges. They are mostly marine, asymmetrical or radially symmetrical, multicellular animals with a cellular level of organisation. Their body has numerous pores called ostia and a large opening called osculum. Water enters through ostia, passes through canals, and goes out through the osculum. They have a skeleton of spicules or spongin fibres. Examples include Sycon, Spongilla, and Euspongia.
Q4. List the chief characteristics of class Aves.
Answer: Class Aves are warm-blooded vertebrates whose body is covered with feathers. Their forelimbs are modified into wings and the hind limbs are adapted for walking, swimming, or perching. They have a streamlined body, a beak without teeth, hollow pneumatic bones, and a four-chambered heart. They are oviparous and reproduce by laying calcareous-shelled eggs. Examples include Pavo (peacock), Columba (pigeon), and Corvus (crow).
Q5. Differentiate between Chondrichthyes and Osteichthyes.
Answer: Chondrichthyes are cartilaginous fishes with an endoskeleton made of cartilage; they have placoid scales, a ventral mouth, no operculum, and no air bladder, e.g., shark and ray. Osteichthyes are bony fishes with an endoskeleton of bone; they possess cycloid or ctenoid scales, an operculum covering the gills, and an air bladder for buoyancy, e.g., Rohu and Catla.
Q6. Write the general characters of phylum Echinodermata.
Answer: Echinodermata are exclusively marine animals having a spiny skin (echinos = spiny, derma = skin). They show radial symmetry as adults but bilateral symmetry as larvae. They are triploblastic and coelomate, with a unique water vascular system used for locomotion, capture of food, and respiration. Their endoskeleton is made of calcareous plates. Examples include Asterias (starfish), Echinus (sea urchin), and Cucumaria (sea cucumber).
5 to 7-Mark Questions
Q1. Describe the basis of classification of the animal kingdom in detail.
Answer: The animal kingdom shows enormous diversity, and biologists have established several criteria for its classification:
(i) Levels of organisation: Animals are organised at the cellular level (Porifera), tissue level (Coelenterata), organ level (Platyhelminthes), or organ-system level (Annelida onwards).
(ii) Symmetry: Animals may be asymmetrical (sponges), radially symmetrical (Hydra, starfish), or bilaterally symmetrical (annelids, vertebrates).
(iii) Diploblastic and triploblastic organisation: Diploblastic animals have two germ layers (Coelenterata) while triploblastic animals have three layers (Platyhelminthes onwards).
(iv) Coelom: Acoelomates lack a body cavity (flatworms), pseudocoelomates have a false coelom (Aschelminthes), and coelomates have a true coelom (Annelida onwards).
(v) Segmentation: Some animals show metameric segmentation where the body is divided into similar segments, as in earthworms.
(vi) Notochord: Animals having a notochord are placed in phylum Chordata, while those lacking it are non-chordates.
Q2. Discuss the salient features of phylum Arthropoda with examples.
Answer: Phylum Arthropoda is the largest phylum in the animal kingdom, containing nearly 80% of all known animals. The salient features are:
1. They have a bilaterally symmetrical, segmented, and triploblastic body.
2. They are coelomate animals with organ-system level of organisation.
3. The body is covered by a chitinous exoskeleton that is shed periodically (moulting).
4. The body is divided into head, thorax, and abdomen, and they bear jointed appendages, after which the phylum is named (arthros = jointed, podos = foot).
5. Respiration occurs through gills, book gills, book lungs, or tracheal system.
6. The circulatory system is open and the excretory organs are Malpighian tubules or green glands.
7. Sensory organs include compound eyes, antennae, and statocysts. Sexes are usually separate, and fertilisation is internal.
Examples: Apis (honey bee), Periplaneta (cockroach), Locusta (locust), Limulus (king crab), and Scorpion.
Q3. Describe the three subphyla of phylum Chordata with examples.
Answer: Phylum Chordata is divided into three subphyla on the basis of the position and persistence of the notochord:
(i) Urochordata (Tunicata): The notochord is present only in the tail of the larval form and disappears in the adult. Their body is enclosed in a tunic. They are exclusively marine, e.g., Herdmania, Salpa, Doliolum.
(ii) Cephalochordata: The notochord extends from the head to the tail throughout life. They are also marine, with a fish-like body, e.g., Branchiostoma (Amphioxus).
(iii) Vertebrata: The notochord is replaced by a bony or cartilaginous vertebral column in the adult. They have a well-developed brain enclosed in a cranium. Vertebrata is divided into classes Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia. Examples are Petromyzon, shark, Rohu, frog, lizard, pigeon, and human.
Q4. Write the distinguishing features of class Mammalia.
Answer: Class Mammalia is the most advanced class of vertebrates and shows the following features:
1. They are warm-blooded animals found in a variety of habitats — terrestrial, aquatic, aerial, and underground.
2. The body is covered with hair which provides insulation.
3. Mammary glands are present in females for the production of milk to nourish the young.
4. They have external ears (pinnae) and a muscular diaphragm separating the thoracic and abdominal cavities.
5. The heart is four-chambered, and they show double circulation.
6. Teeth are heterodont (different types) and thecodont (set in sockets).
7. Most mammals are viviparous (give birth to young ones), although a few like Echidna and Platypus are oviparous.
Examples: Macropus (kangaroo), Pteropus (flying fox), Camelus (camel), Macaca (monkey), Panthera tigris (tiger), and Homo sapiens (human).
Q5. Give a comparative account of phyla Platyhelminthes, Aschelminthes, and Annelida.
Answer: The three phyla represent successive stages of evolutionary advancement among invertebrates:
Platyhelminthes: Flatworms with dorsoventrally flattened bodies. They are triploblastic, acoelomate, and bilaterally symmetrical. Most are parasitic with hooks and suckers for attachment. Excretion occurs through flame cells. Examples are Planaria, Liver fluke, and Tapeworm.
Aschelminthes: Roundworms with cylindrical bodies. They are triploblastic, pseudocoelomate, and bilaterally symmetrical. Many are parasitic in plants and animals. The alimentary canal is complete. Examples are Ascaris, Wuchereria (filaria worm), and Ancylostoma (hookworm).
Annelida: Segmented worms with metameric segmentation. They are triploblastic, true coelomate, and bilaterally symmetrical. They show organ-system level of organisation. Locomotion occurs through setae or parapodia. Excretion is by nephridia. Examples are Nereis, Pheretima (earthworm), and Hirudinaria (leech).
Multiple Choice Questions (MCQs)
Q1. Which one of the following is a diploblastic animal?
(a) Earthworm (b) Hydra (c) Frog (d) Ascaris
Answer: (b) Hydra
Q2. The largest phylum of the animal kingdom is —
(a) Mollusca (b) Annelida (c) Arthropoda (d) Chordata
Answer: (c) Arthropoda
Q3. Cnidoblasts are characteristic of —
(a) Porifera (b) Coelenterata (c) Mollusca (d) Echinodermata
Answer: (b) Coelenterata
Q4. Water vascular system is found in —
(a) Annelida (b) Mollusca (c) Arthropoda (d) Echinodermata
Answer: (d) Echinodermata
Q5. Notochord is replaced by vertebral column in —
(a) Urochordata (b) Cephalochordata (c) Vertebrata (d) Hemichordata
Answer: (c) Vertebrata
Q6. Which of the following is a cartilaginous fish?
(a) Rohu (b) Catla (c) Shark (d) Hilsa
Answer: (c) Shark
Q7. Egg-laying mammal is —
(a) Kangaroo (b) Bat (c) Platypus (d) Whale
Answer: (c) Platypus
Q8. Pseudocoelom is found in —
(a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Chordata
Answer: (b) Aschelminthes
Q9. Comb plates are found in —
(a) Porifera (b) Coelenterata (c) Ctenophora (d) Mollusca
Answer: (c) Ctenophora
Q10. Flame cells are excretory organs of —
(a) Earthworm (b) Planaria (c) Cockroach (d) Frog
Answer: (b) Planaria
Fill in the Blanks
Q1. Sponges belong to the phylum __________.
Answer: Porifera
Q2. The body of arthropods is covered by a __________ exoskeleton.
Answer: chitinous
Q3. Balanoglossus belongs to the phylum __________.
Answer: Hemichordata
Q4. __________ is the only cyclostome example commonly studied.
Answer: Petromyzon
Q5. Mammary glands are characteristic of class __________.
Answer: Mammalia
True or False
Q1. Hydra is a triploblastic animal.
Answer: False — Hydra is a diploblastic animal.
Q2. Earthworm shows metameric segmentation.
Answer: True
Q3. Birds are cold-blooded animals.
Answer: False — Birds are warm-blooded animals.
Q4. Octopus is an example of phylum Mollusca.
Answer: True
Q5. All mammals are viviparous.
Answer: False — A few mammals like Platypus and Echidna are oviparous.
Glossary
| Term | Meaning |
|---|---|
| Symmetry | Arrangement of body parts around an axis or plane |
| Diploblastic | Animals with two embryonic germ layers (ectoderm and endoderm) |
| Triploblastic | Animals with three germ layers (ectoderm, mesoderm, endoderm) |
| Coelom | True body cavity lined by mesoderm |
| Acoelomate | Animal without a body cavity, e.g., flatworm |
| Pseudocoelomate | Animal with a false body cavity, e.g., Ascaris |
| Notochord | Rod-like flexible structure on the dorsal side of chordates |
| Cnidoblast | Stinging cell found in coelenterates |
| Metameric segmentation | Body divided into similar repeated segments |
| Open circulatory system | Blood flows freely in body cavities, found in arthropods and most molluscs |
| Water vascular system | Unique system of canals in echinoderms used for locomotion and feeding |
| Operculum | Bony plate that covers the gills in bony fishes |
| Oviparous | Animals that lay eggs, e.g., birds, most reptiles |
| Viviparous | Animals that give birth to young ones, e.g., most mammals |
| Mammary glands | Milk-producing glands found in female mammals |