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Basis of Classification: Body Plan, Segmentation, Notochord and Broad Classification of Kingdom Animalia

Basis of Classification: Body Plan, Segmentation, Notochord and Broad Classification of Kingdom Animalia

Have you visited any construction site in your neighbourhood? Can you guess what’s the first thing done before assembly of bricks and other construction work? It’s planning. Like the way every building has a plan, our body also has a plan. Amazingly, this plan is actually one of the bases of classification.

Like a train with repetitive compartments, certain creatures have compartments in their bodies which get repeated. The nature of compartmentalisation varies in our bodies just like in trains. Some trains have differently coloured and structured compartments while some have uniform. Similarly, the type of compartmentalisation in our body can be taken as a basis of classification.

You must have felt the vertebral column at the back? Have you ever investigated if an earthworm has a spiny structure at its back or not! That’s of biologists interest. How? So, the presence or absence of notochord is a basis of classification of animals.

Let’s dig holes into body plan, segmentation and notochord and understand how these are helping in the classification of animals.

Body Plan as a Basis of Classification

The set of morphological features which are common to the organisms belonging to a particular phyla is collectively known as body plan.

Classification Based on Body Plan

Types of body plan


Possible category (members of phyla)


Cell aggregate type

Cells are arranged as loose cell aggregates.

Porifera (sponges)

Cell aggregate body plan

Blind sac type

Incomplete digestive system where only a single external opening (blastopore) serves as both mouth and anus.

Coelenterata, Ctenophora,


Blind sac body plan

Tube-within-a-tube type

Alimentary canal extends from the mouth to anus inside the body. This appears like a tube within a tube, with fluid-filled cavities in between the two.

Aschelminthes, Annelida, Arthopoda, Mollusca, Echinodermata and Chordata

Tube-within-a-tube body plan

Segmentation as a Basis of Classification

The divisibility of the body into similar constituent units is termed as segmentation.

Classification based on segmentation

Types of animals

Nature of segmentation

Possible category (members of phyla)


No segmentation

Body is not divided into segments.

Porifera, Coelenterata etc.

 Non-segmented body of sponges

Pseudo metamerism

Body is externally divided into segments but it is not coherent with internal segmentation.


Pseudometamerism in tapeworm


Body is externally and internally divided into segments with serial repetition of organs in the segments and a coordination between them to attain a specific function. The body segments are called metameres.

Annelida, Arthopoda and Chordata

Metameric segmentation in earthworm

Notochord as a Basis of Classification

Notochord is defined as a rod-like structure derived from the mesoderm during the development of the embryo. It is present on the dorsal side of the animal body.

 Notochord in embryonic stage

Classification Based on Presence of Notochord

Types of animals

Presence or absence of notochord

Possible category (members of phyla)


Animal without a notochord.

Porifera to Hemichordata


Animal with a notochord.


Broad Classification of Kingdom Animalia


Practice Problems of Basics of Classification

  1. Solve the following riddle:

Ques:- “I am ‘Spongey’. I have a cell aggregate type body plan. Can you help me find my phyla?”

A. Coelenterata
B. Ctenophora
C. Porifera
D. Echinodermata

Solution: The organisms which have cells arranged as loose cell aggregates are said to have cell aggregate type body plan. Members of phylum Porifera have a cell aggregate type of body plan.

Hence, the correct option is c.

Ques:- Bodies of tapeworms are externally divided into many segments. How are they different from annelids which also show external segmentation?

A. Tapeworms show metamerism which is absent in annelids
B. Tapeworms show external segmentation while annelids exhibit metamerism
C. Tapeworms show internal segmentation which is absent in annelids
D. Tapeworms show external segmentation while annelids show only internal segmentation

Solution: Tapeworms have their bodies externally divided into several repeating segments. This is also called pseudometamerism. Annelids exhibit true metamerism with bodies having segments with external and internal partitions. 

Hence, the correct option is b.

Ques:- An organism has an organ system level of body organisation but shows radial symmetry like ctenophores. Predict the phyla under which it falls.

A. Coelenterata
B. Platyhelminthes
C. Echinodermata
D. Chordata

Solution: Members of phylum Echinodermata have organ system level of body organisation. Adult echinoderms can be divided into two equal halves across multiple planes passing through the central body axis. 

Hence, the correct option is c.

Ques:- Which of the following body plans is found in organisms having an organ system level of body organisation?

A. Cell aggregate type
B. Tube-within-a-tube
C. Blind sac
D. Either a or c

Solution: Organisms having organ system level of body organisation can be members of phylum Aschelminthes/ Annelida/ Arthopoda/ Mollusca/ Echinodermata or Chordata. These members have alimentary canals extending from the mouth to anus inside the body giving the appearance of a tube lying within another tube.

Hence, the correct option is b.

FAQs of Basics of Classification

Ques:- Define metamerism. 

Solution: Segmentation in which there is recurrence of certain organs in each segment is known as metamerism. Besides, the external segmentation coincides with internal segmentation. It is also known as true segmentation. Example: Members of the phylum Annelida.

Ques:- Name the fundamental feature(s) used to classify the animals.

Solution: List of fundamental feature(s) used to broadly classify the animal kingdom are as follows:

  • Levels of organisation
  • Symmetry 
  • Germ layer organisation
  • Presence of coelom
  • Body plan
  • Segmentation
  • Notochord

Ques:-Name the phylum that has a cell aggregate body plan.

Solution: Cells are arranged as loose cell aggregates in a cell aggregate body plan. It is found in the members of phylum Porifera.

Ques:- Define notochord along with its functions.

Solution: Notochord is a mesodermally derived rod-like structure which is formed on the dorsal surface of the body during embryonic development in some animals. Animals having notochord are termed chordates. 

The notochord gives rise to the vertebral column in vertebrates. It provides protection to the spinal cord.

Other Related Topics

The Living World Biological Classification Plant Kingdom
Animal Kingdom Morphology of Flowering Plants Anatomy of Flowering Plants
Structural Organization in Animals Cells: The Unit of Life Biomolecules
Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

Other Related Topic Of Biology

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