Basis of Classification
The basis of classification of the Animal Kingdom is a set of fundamental features shared by groups of animals: level of organisation, symmetry, germ layers, coelom, segmentation and notochord. Over a million animal species have been described, so these features are needed to place each species in its correct group. This page explains every basis of classification in the order of the NCERT Class 11 chapter, with figures, tables and practice. NEET often links these features to phyla in match-the-list and statement questions.
- ★ Must learn Levels of organisation: cellular (sponges), tissue (coelenterates), organ (Platyhelminthes), organ system (annelids to chordates).
- ★ Must learn Incomplete digestive system: one opening that acts as both mouth and anus (Platyhelminthes). A complete one has a mouth and an anus.
- Open circulation: cells are bathed directly in blood. Closed circulation: blood flows in arteries, veins and capillaries.
- ★ Must learn Radial symmetry: coelenterates, ctenophores, echinoderms. Bilateral: annelids, arthropods and others. Sponges are mostly asymmetrical.
- Diploblastic: ectoderm and endoderm, with mesoglea between them (coelenterates). Triploblastic: mesoderm added (platyhelminthes to chordates).
- ★ Must learn Coelom: body cavity lined by mesoderm. Pseudocoelom: not lined by mesoderm (aschelminthes). Acoelomate: no body cavity (platyhelminthes).
- Metameric segmentation: external and internal segments with serial repetition of organs, as in the earthworm. The phenomenon is metamerism.
- ★ Must learn Notochord: mesodermal, rod-like, dorsal, formed in the embryo. Present in chordates; absent in non-chordates (porifera to echinoderms).
- Adult echinoderms are radially symmetrical, but their larvae are bilaterally symmetrical.
- The broad classification chart places a phylum by level of organisation, then symmetry, then body cavity.
1. Why Animals Need to be Classified
- ★ Exam imp Over a million species of animals have been described so far, so classification is essential.
- Classification also helps in assigning a systematic position to newly described species.
- Animals differ in structure and form, yet groups of them share fundamental features.
- These features are the basis of animal classification:
- arrangement of cells (level of organisation)
- body symmetry
- nature of the coelom
- patterns of the digestive, circulatory and reproductive systems
2. Levels of Organisation
- All members of Animalia are multicellular, but they do not all show the same pattern of cell organisation.
- Level of organisation: the way cells are arranged and grouped in an animal's body.
| Level | How cells are arranged | Examples |
|---|---|---|
| Cellular level | Cells form loose cell aggregates; some division of labour occurs among the cells | Sponges |
| Tissue level | Cells performing the same function are arranged into tissues | Coelenterates |
| Organ level | Tissues are grouped to form organs, each specialised for a particular function | Platyhelminthes and other higher phyla |
| Organ system level | Organs associate to form functional systems, each for a specific physiological function | Annelids, arthropods, molluscs, echinoderms, chordates |
- Aschelminthes and hemichordates also show the organ-system level of organisation.
- Organ systems in different groups show various patterns of complexity, as the digestive and circulatory systems show.
2.1 Patterns of the digestive system
- ★ Exam imp Incomplete digestive system: only one opening to the outside, which serves as both mouth and anus. Example: Platyhelminthes.
- Complete digestive system: two openings, a mouth and an anus.
2.2 Patterns of the circulatory system
Blood is pumped out of the heart.
The cells and tissues are directly bathed in it.
Blood circulates through a series of vessels of varying diameters.
The vessels are arteries, veins and capillaries.
Read the levels as a ladder in which each step groups the one before it: Cells, Tissues, Organs, Systems. Sponges stop at cells, coelenterates at tissues, flatworms at organs, and the higher phyla reach organ systems.
3. Symmetry
- Animals can be grouped on the basis of their symmetry.
- Asymmetrical: any plane that passes through the centre does not divide the body into equal halves. Sponges are mostly asymmetrical.
- ★ Exam imp Radial symmetry: any plane passing through the central axis divides the body into two identical halves. Coelenterates, ctenophores and echinoderms have this body plan.
- Bilateral symmetry: the body can be divided into identical left and right halves in only one plane. Examples: annelids, arthropods.
| Symmetry | Planes giving identical halves | Examples |
|---|---|---|
| Asymmetrical | None | Most sponges |
| Radial | Any plane through the central axis | Coelenterates, ctenophores, echinoderms (adults) |
| Bilateral | Only one plane (left and right halves) | Annelids, arthropods and other bilateral phyla |
Radial animals spell "CCE": Coelenterates, Ctenophores, Echinoderms. Sponges are mostly asymmetrical, and the remaining phyla, Platyhelminthes to Chordata, are bilateral.
4. Diploblastic and Triploblastic Organisation
- Germ layers (germinal layers): the embryonic cell layers from which the body develops.
- ★ Exam imp Diploblastic animals: cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm. Example: coelenterates.
- Mesoglea: an undifferentiated layer present between the ectoderm and the endoderm of diploblastic animals.
- Triploblastic animals: the developing embryo has a third germinal layer, the mesoderm, between ectoderm and endoderm.
- Triploblastic animals range from platyhelminthes to chordates.
Two germ layers: ectoderm and endoderm.
Mesoglea (undifferentiated) lies between them.
Example: coelenterates.
Three germ layers: ectoderm, mesoderm and endoderm.
Mesoderm is a true germ layer.
Platyhelminthes to chordates.
Mesoglea and mesoderm both sit in the middle, but only one is a germ layer. Mesoglea is undifferentiated and is not a germ layer, so an animal with mesoglea is still diploblastic.
5. Coelom
- The presence or absence of a cavity between the body wall and the gut wall is very important in classification.
★ Very important Coelom: the body cavity that is lined by mesoderm. Animals possessing a coelom are coelomates.
- ★ Exam imp Coelomates: annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
- Pseudocoelom: a body cavity that is not lined by mesoderm. Instead, mesoderm is present as scattered pouches between ectoderm and endoderm.
- Pseudocoelomates: animals with a pseudocoelom. Example: aschelminthes.
- Acoelomates: animals in which the body cavity is absent. Example: platyhelminthes.
| Type | Body cavity | Mesoderm | Example |
|---|---|---|---|
| Coelomate | Present (true coelom) | Lines the cavity | Annelids to chordates |
| Pseudocoelomate | Present (false coelom) | Scattered pouches; does not line the cavity | Aschelminthes |
| Acoelomate | Absent | Fills the space between ectoderm and endoderm | Platyhelminthes |
Coelomates in order: annelids, molluscs, arthropods, echinoderms, hemichordates, chordates. Remember "All Monkeys Are Excellent Hill Climbers".
In Figure 3, name the cavity in panel (a).
In Figure 3, which panel has mesoderm only as scattered pouches?
Which panel in Figure 3 shows a platyhelminth body plan?
6. Segmentation
- In some animals, the body is divided externally and internally into segments.
- The segments show a serial repetition of at least some organs.
★ Very important In the earthworm, the body shows metameric segmentation, and the phenomenon is known as metamerism.
7. Notochord
★ Very important Notochord: a mesodermally derived, rod-like structure formed on the dorsal side during embryonic development in some animals.
- ★ Exam imp Chordates: animals with a notochord.
- Non-chordates: animals that do not form a notochord, from porifera to echinoderms.
8. Broad Classification of Kingdom Animalia
- The fundamental features above are used together to give the broad classification of Animalia.
- Reading the chart in Figure 4 is a fixed sequence of questions:
- Level of organisation: the cellular level leads to Porifera, which are mostly asymmetrical and acoelomate.
- Symmetry (tissue, organ or organ-system level): radial animals are acoelomate and belong to Coelenterata (Cnidaria) or Ctenophora.
- Body cavity (bilateral animals): without a body cavity, the animal is an acoelomate (Platyhelminthes).
- With a false coelom, it is a pseudocoelomate (Aschelminthes).
- With a true coelom, it is a coelomate: Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata or Chordata.
Echinodermata are placed with the bilateral coelomates, although they show radial or bilateral symmetry depending on the stage (adults radial, larvae bilateral).
Phyla in order, Porifera to Chordata: "Please Come Carefully, Pack All Animal Anatomy Models; Every Hall Counts" for Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, Chordata.
9. Exam Essentials: Pairs, Exceptions and Numbers
- Statement traps often swap coelom and pseudocoelom: only the coelom is lined by mesoderm.
- Platyhelminthes are triploblastic yet acoelomate, and their gut is incomplete.
- Coelenterates are both diploblastic and at the tissue level; ctenophores are also diploblastic.
- "Notochord present" defines chordates; porifera to echinoderms never form one.
Exceptions
- Sponges are mostly, not always, asymmetrical.
- Echinoderms are triploblastic coelomates, yet the adults are radially symmetrical; only the larvae are bilateral.
- Platyhelminthes reach the organ level but have only one opening to the gut.
- Aschelminthes have a body cavity, but it is not lined by mesoderm.
- Mesoglea lies between two germ layers but is not itself a germ layer.
Numbers to Remember
- Over a million: animal species described so far.
- 1 opening: incomplete digestive system (mouth and anus are the same opening).
- 2 openings: complete digestive system (mouth and anus).
- 2 germ layers: diploblastic; 3 germ layers: triploblastic.
- Only 1 plane gives identical halves in bilateral symmetry.
- 3 kinds of vessels in closed circulation: arteries, veins and capillaries.
Pairs to Match
| Feature | Animal group |
|---|---|
| Cellular level of organisation | Sponges |
| Tissue level of organisation | Coelenterates |
| Organ level of organisation | Platyhelminthes |
| Incomplete digestive system | Platyhelminthes |
| Mostly asymmetrical | Sponges |
| Radial symmetry | Coelenterates, ctenophores, echinoderms |
| Bilateral symmetry | Annelids, arthropods |
| Diploblastic, with mesoglea | Coelenterates |
| Pseudocoelom | Aschelminthes |
| Acoelomate | Platyhelminthes |
| Metameric segmentation | Earthworm |
| Notochord absent | Porifera to echinoderms |
| Open circulation | Cells and tissues bathed directly in blood |
| Closed circulation | Arteries, veins and capillaries |
Which layer lies between ectoderm and endoderm in coelenterates?
Name the phenomenon shown by the segmented body of an earthworm.
Is the notochord ectodermal or mesodermal?
10. Quick Revision
- Over a million animal species are described; classification also places new species.
- Basis: cell arrangement, symmetry, coelom, and digestive, circulatory and reproductive patterns.
- Cellular level: sponges; tissue: coelenterates; organ: Platyhelminthes; organ system: annelids to chordates.
- Incomplete gut: one opening (Platyhelminthes); complete gut: mouth and anus.
- Open circulation bathes tissues in blood; closed circulation uses arteries, veins, capillaries.
- Asymmetry: most sponges; radial: coelenterates, ctenophores, echinoderms; bilateral: annelids, arthropods.
- Diploblastic: ectoderm, endoderm, mesoglea between; triploblastic: mesoderm added.
- Coelom is lined by mesoderm; pseudocoelom is not; acoelomates have no cavity.
- Coelomates: annelids, molluscs, arthropods, echinoderms, hemichordates, chordates.
- Pseudocoelomates: aschelminthes; acoelomates: platyhelminthes.
- Metamerism: serial external and internal segments, as in the earthworm.
- Notochord: mesodermal dorsal rod of the embryo; chordates have it, non-chordates do not.
- Chart order: level of organisation, symmetry, body cavity, phylum.
11. Solved Examples
List I: A. Cellular level; B. Tissue level; C. Organ level; D. Organ-system level
List II: I. Platyhelminthes; II. Coelenterates; III. Annelids; IV. Sponges
Choose the correct answer.
(A) A-IV, B-II, C-I, D-III
(B) A-II, B-IV, C-I, D-III
(C) A-IV, B-II, C-III, D-I
(D) A-I, B-II, C-IV, D-III
Answer: (A). Sponges are cellular, coelenterates are at the tissue level, Platyhelminthes are at the organ level, and annelids reach the organ-system level.
A. Mesoglea is the third germ layer of coelenterates.
B. Platyhelminthes are triploblastic acoelomates.
C. A pseudocoelom is lined by mesoderm.
D. Echinoderms are coelomates.
E. The segmentation of the earthworm is called metamerism.
Choose the correct answer.
(A) B, D and E only
(B) A, B and C only
(C) B and E only
(D) A, D and E only
Answer: (A). A is false: mesoglea is undifferentiated, not a germ layer. B is true. C is false: a pseudocoelom is not lined by mesoderm. D and E are true.
A. Organ-system level; B. Cellular level; C. Organ level; D. Tissue level
Choose the correct order.
(A) B, D, C, A
(B) D, B, C, A
(C) B, C, D, A
(D) A, C, D, B
Answer: (A). Cells group into tissues, tissues into organs, and organs into organ systems.
(A) It is mesodermally derived.
(B) It is a rod-like structure on the dorsal side.
(C) It is formed during embryonic development.
(D) It is present in all animals from porifera to echinoderms.
Answer: (D). Porifera to echinoderms are non-chordates; they never form a notochord.
Statement II: An incomplete digestive system has a single opening that serves as both mouth and anus.
Choose the correct answer.
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct
Answer: (A). Both statements are correct; Statement II states what makes the flatworm gut incomplete.
(A) Ascaris
(B) Fasciola
(C) Pheretima
(D) Hydra
Answer: (C). A mesoderm-lined cavity is a true coelom, found in annelids such as Pheretima. Ascaris is a pseudocoelomate, Fasciola an acoelomate and Hydra a diploblastic coelenterate.
12. Practice Questions
- What difficulties would you face in classifying animals if common fundamental features were not taken into account?Answer: Animals that differ in form could not be grouped, every species would need a separate description, new species could not be given a systematic position, and relationships between groups would be missed.
- If you are given a specimen, what steps would you follow to classify it?Answer: Check its level of organisation, then symmetry, germ layers and body cavity (acoelomate, pseudocoelomate or coelomate), then segmentation and notochord, and finally the distinctive features of the phylum.
- How useful is the study of the nature of the body cavity and coelom in classifying animals?Answer: It divides bilateral triploblastic animals into acoelomates (Platyhelminthes), pseudocoelomates (Aschelminthes) and coelomates (Annelida to Chordata).
- Segmentation in the body is first observed in which of the following?
(a) Platyhelminthes
(b) Aschelminthes
(c) Annelida
(d) ArthropodaAnswer: (c) Annelida, which are metamerically segmented. - Match List I with List II. List I: A. Mostly asymmetrical; B. Radial; C. Bilateral; D. Radial adult, bilateral larva. List II: I. Arthropods; II. Echinoderms; III. Sponges; IV. Ctenophores.
(1) A-III, B-IV, C-I, D-II
(2) A-IV, B-III, C-I, D-II
(3) A-III, B-I, C-IV, D-II
(4) A-III, B-IV, C-II, D-IAnswer: (1). Sponges are mostly asymmetrical, ctenophores radial, arthropods bilateral, and echinoderms radial as adults. - Read the statements. A. In open circulation, blood is pumped out of the heart and bathes the cells. B. Closed circulation uses arteries, veins and capillaries. C. A complete digestive system has a single opening. Choose:
(1) A and B only
(2) B and C only
(3) A and C only
(4) A, B and CAnswer: (1). C is wrong: a complete digestive system has two openings, the mouth and the anus. - Arrange the questions used in the broad classification chart in order. A. Body cavity; B. Level of organisation; C. Symmetry; D. Phylum.
(1) B, C, A, D
(2) C, B, A, D
(3) B, A, C, D
(4) A, C, B, DAnswer: (1). Level of organisation comes first, then symmetry, then body cavity, which gives the phylum. - Which pair is NOT correctly matched?
(1) Pseudocoelomate: Aschelminthes
(2) Acoelomate: Platyhelminthes
(3) Diploblastic: Coelenterates
(4) Coelomate: CtenophoraAnswer: (4). Ctenophores are radial acoelomates on the chart; coelomates are annelids to chordates. - Assertion: Sponges show the cellular level of organisation. Reason: Their cells form loose aggregates with some division of labour.
(1) Both are true and the Reason explains the Assertion
(2) Both are true but the Reason does not explain it
(3) The Assertion is true, the Reason is false
(4) Both are falseAnswer: (1). Loose cell aggregates, without true tissues, define the cellular level. - Name the undifferentiated layer present between the ectoderm and the endoderm of diploblastic animals.Answer: Mesoglea.
Common Mistakes to Avoid
- Calling mesoglea a germ layer. It is undifferentiated, so coelenterates remain diploblastic.
- Writing that a pseudocoelom is lined by mesoderm. Only a true coelom is lined by mesoderm.
- Calling Platyhelminthes diploblastic because they lack a body cavity. They are triploblastic acoelomates.
- Saying all sponges are asymmetrical. Sponges are mostly asymmetrical.
- Calling echinoderms bilateral. Adults are radial; only their larvae are bilateral.
- Thinking an incomplete digestive system has no mouth. Its single opening works as both mouth and anus.
- Treating metamerism as outer rings only. The segments are external and internal, with organs repeated in series.
- Describing the notochord as ectodermal or ventral. It is mesodermal and dorsal, formed in the embryo.
Frequently Asked Questions
What is the basis of classification of animals?
Animals are classified on fundamental features shared by groups: the arrangement of cells or level of organisation, body symmetry, germ layers, the nature of the coelom, segmentation, the notochord, and the patterns of the digestive, circulatory and reproductive systems. These features also help place newly described species in the correct group.
What are the levels of organisation in animals?
There are four. Sponges show the cellular level, with loose cell aggregates. Coelenterates show the tissue level. Platyhelminthes show the organ level, with tissues grouped into organs. Annelids, arthropods, molluscs, echinoderms and chordates show the organ-system level, where organs form systems for specific physiological functions.
What is the difference between radial and bilateral symmetry?
In radial symmetry, any plane passing through the central axis divides the body into two identical halves, as in coelenterates, ctenophores and adult echinoderms. In bilateral symmetry, only one plane divides the body into identical left and right halves, as in annelids and arthropods. Most sponges are asymmetrical.
What is the difference between diploblastic and triploblastic animals?
Diploblastic animals have two embryonic layers, an outer ectoderm and an inner endoderm, with an undifferentiated mesoglea between them; coelenterates are an example. Triploblastic animals have a third germ layer, the mesoderm, between the ectoderm and the endoderm. Platyhelminthes to chordates are triploblastic.
What is the difference between a coelom and a pseudocoelom?
A coelom is a body cavity lined by mesoderm, found in annelids, molluscs, arthropods, echinoderms, hemichordates and chordates. A pseudocoelom is a body cavity not lined by mesoderm; the mesoderm lies as scattered pouches between ectoderm and endoderm, as in aschelminthes. Platyhelminthes have no body cavity.
What is metamerism?
Metamerism is the phenomenon in which the body is divided externally and internally into segments, with a serial repetition of at least some organs. This pattern is called metameric segmentation. The earthworm is the standard example, and segmentation is first seen in the phylum Annelida.
What is a 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 chordates. Animals that never form it, from porifera to echinoderms, are non-chordates. The notochord is one of the key features used to classify animals.
Which basis of classification points are most asked in NEET?
NEET most often asks the coelom types with examples, diploblastic versus triploblastic animals, the levels of organisation, radial versus bilateral symmetry, and the incomplete gut of Platyhelminthes. These points usually appear in match-the-list questions and in statement sets that mix true and false lines about each feature.
Previous year questions on Basis of Classification
8 questions from past papers, each with a step-by-step solution.
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