Animals range from simple sponges to insects, fishes, birds and mammals. To classify them, biologists compare their body plans and important structural features. This chapter introduces the major non-chordate phyla and the main chordate classes.
Class 11 Biology Chapter 4 Animal Kingdom Notes
A good way to study Animal Kingdom is to connect each group with one identifying feature and two examples.
Table of Contents
Basis of Animal Classification
Animals can be compared using these features:
- Level of organisation: whether cells form tissues, organs and organ systems.
- Symmetry: how the body can be divided into similar halves.
- Germ layers: the layers formed during early development.
- Body cavity: whether a coelom is present.
- Segmentation: whether the body is divided into repeated units.
- Notochord: a supporting structure present at some stage in chordates.
Symmetry
An asymmetrical animal cannot be divided into matching halves in a regular way. Many sponges are examples.
A radially symmetrical animal can be divided into similar parts through several planes passing through its central axis. A sea anemone illustrates this arrangement.
A bilaterally symmetrical animal has left and right halves that can be matched along one main plane. An earthworm is an example.
Body Cavity
An acoelomate has no body cavity between the digestive tract and body wall. A pseudocoelomate has a cavity that is not completely lined by mesoderm. A coelomate has a true coelom lined by mesoderm.
These terms help distinguish several worm-like groups.
Major Non-Chordate Phyla
Non-chordates do not possess the defining chordate notochord. The following phyla are covered in the Class 11 textbook classification.
1. Porifera
Poriferans are sponges. Their bodies contain many pores through which water passes. They have a simple cellular level of organisation and are mostly marine.
Identifying feature: a canal system that moves water through the body.
Examples: Sycon and Spongilla.
2. Cnidaria
Cnidarians include forms such as hydra, jellyfish and sea anemones. They are aquatic and mostly radially symmetrical. Their specialised stinging cells, called cnidocytes, help in defence and capturing prey.
Identifying feature: stinging cells.
Examples: Hydra and Aurelia.
3. Ctenophora
Ctenophores are marine animals often called comb jellies. Rows of ciliated comb plates help them move through water. Many can produce visible light.
Identifying feature: eight rows of comb plates.
Examples: Pleurobrachia and Ctenoplana.
4. Platyhelminthes
Platyhelminths are flatworms. Their bodies are flattened from top to bottom. They are bilaterally symmetrical and acoelomate. Some live as parasites.
Identifying feature: a flattened body without a coelom.
Examples: Taenia and Fasciola.
5. Aschelminthes
Aschelminths are roundworms. They have a cylindrical, unsegmented body and a pseudocoelom. Some are free-living, while others are parasites.
Identifying feature: round body and pseudocoelom.
Examples: Ascaris and Wuchereria.
6. Annelida
Annelids have bodies divided into repeated segments. They possess a true coelom. Many annelids have a closed circulatory system.
Identifying feature: repeated body segments.
Examples: earthworm (Pheretima) and Nereis.
7. Arthropoda
Arthropods form a very large and diverse animal phylum. Their bodies have jointed appendages and an outer skeleton containing chitin. This group includes insects, spiders and crustaceans.
Identifying feature: jointed appendages and a chitinous exoskeleton.
Examples: honey bee (Apis) and cockroach (Periplaneta).
8. Mollusca
Molluscs generally have a soft body, a muscular foot and a covering called the mantle. Many have a shell, although a prominent outer shell is not present in every member.
Identifying feature: soft body with a mantle and usually a muscular foot.
Examples: Pila and Octopus.
9. Echinodermata
Echinoderms are marine animals. Adults commonly show radial symmetry, while their larvae show bilateral symmetry. Their water vascular system is a distinctive feature involved in functions including movement.
Identifying feature: water vascular system.
Examples: starfish (Asterias) and sea urchin (Echinus).
10. Hemichordata
Hemichordates are marine, worm-like animals. Their bodies have three regions: a proboscis, collar and trunk. They are treated as a separate non-chordate phylum in the textbook classification.
Identifying feature: body divided into proboscis, collar and trunk.
Examples: Balanoglossus and Saccoglossus.
Phylum Chordata
A chordate has a notochord at least during some stage of its life. Other characteristic features include a dorsal hollow nerve cord, pharyngeal slits and a post-anal tail at some stage of development.
Many chordates are vertebrates, but not every chordate is a vertebrate. In vertebrates, a vertebral column develops. Chordates without a vertebral column include tunicates and lancelets.
For school-level classification, learn the following vertebrate classes and their distinguishing characteristics.
Major Chordate Classes
1. Cyclostomata
Cyclostomes are jawless, fish-like animals. They have a circular sucking mouth and lack paired fins.
Examples: lamprey (Petromyzon) and hagfish (Myxine).
2. Chondrichthyes
These are cartilaginous fishes: their internal skeleton is mainly cartilage. They breathe through gills.
Examples: dogfish (Scoliodon) and stingray (Trygon).
3. Osteichthyes
These are bony fishes. Their internal skeleton is mainly bone, and they breathe through gills.
Examples: Catla and Labeo.
Easy comparison: Chondrichthyes have a cartilaginous skeleton; Osteichthyes have a bony skeleton.
4. Amphibia
Amphibians commonly live part of their lives in water and part on land. Their skin is generally moist and lacks scales.
Examples: frog and toad.
5. Reptilia
Reptiles generally have dry skin covered with scales or scutes. They include lizards, snakes, turtles and crocodiles.
Examples: garden lizard (Calotes) and cobra (Naja).
6. Aves
Aves includes birds. Feathers are their most recognisable feature. Their forelimbs are modified into wings, although some birds do not fly.
Examples: pigeon (Columba) and peacock (Pavo).
7. Mammalia
Mammals have mammary glands that produce milk for their young. Hair is another characteristic feature. Mammals include land animals as well as aquatic forms.
Examples: human and whale.
Fast Revision: Feature → Group
- Pores and water canal system → Porifera
- Stinging cells → Cnidaria
- Comb plates → Ctenophora
- Flattened, acoelomate body → Platyhelminthes
- Round body and pseudocoelom → Aschelminthes
- Repeated body segments → Annelida
- Jointed appendages → Arthropoda
- Soft body and mantle → Mollusca
- Water vascular system → Echinodermata
- Proboscis, collar and trunk → Hemichordata
- Notochord at some stage → Chordata
Common Mistakes to Avoid
- Do not place Hemichordata within Chordata in this textbook classification.
- Do not assume every chordate has a vertebral column.
- Do not confuse the spore-like openings of a sponge with the stinging cells of a cnidarian.
- Do not classify every soft-bodied animal as a mollusc; check its other features.
- Do not say all birds fly; ostriches and penguins are birds.
Check Your Understanding
1. Which phylum has jointed appendages?
Arthropoda.
2. Which phylum has a water vascular system?
Echinodermata.
3. What distinguishes Chondrichthyes from Osteichthyes?
Chondrichthyes have a cartilaginous skeleton; Osteichthyes have a bony skeleton.
4. Why are all vertebrates chordates?
They possess chordate features, including a notochord at a stage of development.
5. Are all chordates vertebrates?
No. Some chordates do not develop a vertebral column.
Continue Learning
Return to the CBSE Class 11 Biology – Complete Study Guide to browse the chapters. Continue with Chapter 5: Morphology of Flowering Plants when its notes are available.
Frequently Asked Questions
What criteria help classify animals?
Level of organisation, symmetry, germ layers, body cavity, segmentation and presence or absence of a notochord.
What is radial symmetry?
Body parts are arranged around a central axis, allowing several planes to divide the body into similar halves.
What is bilateral symmetry?
Only one plane divides the body into roughly equal left and right halves.
What does diploblastic mean?
The embryo develops two primary germ layers: ectoderm and endoderm.
What does triploblastic mean?
The embryo develops ectoderm, mesoderm and endoderm.
What is a coelom?
A coelom is a body cavity lined by mesoderm.
What are the identifying features of Porifera?
Sponges have a porous body with a canal system and generally show cellular-level organisation.
What are cnidocytes?
They are specialised stinging cells characteristic of cnidarians.
Which phylum includes flatworms?
Platyhelminthes includes flatworms such as planarians and tapeworms.
Which phylum includes roundworms?
Aschelminthes, also called Nematoda, includes roundworms such as Ascaris.
Which feature characterises annelids?
They have a segmented body; the earthworm is a familiar example.
What is the largest animal phylum?
Arthropoda is the largest animal phylum and includes insects, spiders and crustaceans.
What is a notochord?
It is a flexible supporting rod present at least during some stage of a chordate’s development.
How do chordates differ from non-chordates?
Chordates possess a notochord at some stage, along with a dorsal hollow nerve cord and pharyngeal slits at some stage.
Name the major vertebrate classes.
Cyclostomata, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia are the classes covered in NCERT.