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# Basis of Classification: Symmetry, Germ Layer Organisation and Coelom

Have you ever wondered if mathematics can be applied in the field of biology? Yes! That’s something possible. You must be having ideas about symmetry! Lines of symmetry, planes of symmetry and many more.

Astonishingly, symmetry can be used as a basis of classification of living things. Why? Don’t worry, we have an answer to it. The living entities are three-dimensional solids. So, these can be divided across planes into halves. Now the question is how many planes? So, this is something variable and the number of planes across which an organism can be divided into identical halves serves as the basis of classification.

Have you ever noticed slices of bread in a sandwich? You will be surprised to know that the bodies of living organisms can be compared to sandwiches in their embryonic stages. Yes! We all are composed of layers of cells. The number of layers in the embryonic stages is another basis of classification. It is also called germ layer organisation.

Let’s again revise geometrical shapes. Cylinders are of different types, right? Solid and hollow. The difference lies in the nature of cavities. Living bodies can also be compared to cylinders. The nature of cavities inside the living bodies is another basis of classification.

Now, we will be peeping into a few more bases of classification like symmetry, germ layer organisation and coelom.

## Symmetry as a Basis of Classification

The quality of being composed of multiple similar parts which can be broken down into the constituent parts along certain planes is termed as symmetry.

### Classification Based on Symmetry

 Types of symmetry Description Possible category (members of phyla) Figures Asymmetrical - Body cannot be divided into two equal halves through any plane passing through the centre of the body. Porifera Symmetrical Radial symmetry Body can be divided into two equal halves through any plane passing through the centre of the body. Coelenterata, Ctenophora, Echinodermata (adult) Bilateral symmetry Body can be divided into two equal halves, i.e., right half and left half, through only one plane. Platyhelminthes, Aschelminthes, Annelida, Arthopoda, Mollusca, Echinodermata (larva) and Chordata

## Germ layer organisation as a basis of classification

The organisation of cells into different layers during embryonic development is called germ layer organisation.

### Classification based on germ layer organisation

Types of germ layer organisation

Description

Possible category (members of phyla)

Figures

#### Diploblastic

Cells are arranged in two embryonic layers, outer ectoderm and inner endoderm. An undifferentiated layer is sandwiched between them called mesoglea.

Coelenterata, Ctenophora

#### Triploblastic

Cells are arranged in three embryonic layers, outer ectoderm, inner endoderm and middle mesoderm.

Platyhelminthes, Aschelminthes, Annelida, Arthopoda, Mollusca, Echinodermata and Chordata

## Presence of Coelom as a Basis of Classification

The body cavity present between the body wall and gut wall which has mesodermal linings is termed as a coelom.

### Classification Based on Presence of Coelom

Types of animals

Presence or absence of coelom

Possible category (members of phyla)

Figures

#### Acoelomate

Body cavity is absent.

Platyhelminthes

#### Pseudocoelomate

Body cavity is not lined by mesoderm, but mesoderm is present as scattered pouches between ectoderm and endoderm.

Aschelminthes

#### Eucoelomate

True body cavity is present lined by mesoderm.

Annelida, Arthopoda, Mollusca, Echinodermata and Chordata

### Bifurcation of eucolomates

Based on the origin of coelom, the organisms having true coelom (eucoelomates) can be categorised as follows:

• Schizocoelomate: Body cavity which is formed by splitting of mesodermal mass is termed as schizocoelom. Organisms having schizocoelom are called schizocoelomates.

Examples: Members of phylum Annelida, phylum Arthopoda and phylum Mollusca.

• Enterocoelomate: Body cavity which is formed by gut pouches is termed as enterocoelom. Organisms having enterocoelom are called enterocoelomate.

Examples: Members of phylum Echinodermata, phylum Hemichordata and phylum Chordata.

## Practice Problems of Classification

Ques:- Suppose you find a new creature while walking in the morning. You observed that the body of the creature can be divided into two equal parts along the vertical plane. Which of the following terms best describes the organism?

B. Bilaterally symmetrical
C. Asymmetrical
D. Both b and c

Solution: An organism that can be divided into two equal halves, i.e., right half and left half, through only one plane is said to be bilaterally symmetrical. Hence, the new creature is bilaterally symmetrical.

Hence, the correct option is b.

Ques:- Which of the following features is common in earthworms, cockroaches and starfishes?

A. Presence of true coelom
C. Presence of open circulatory system
D. Diploblastic nature

Solution: Organisms having a true body cavity lined by mesoderm are called eucoelomates. Earthworms, cockroaches and starfishes have true coelom.

Hence, the correct option is a.

Ques:- Which of the following phyla has members with both bilateral and radial symmetry?

A. Coelenterata
B. Ctenophora
D. Platyhelminthes
D. Echinodermata

Solution: Echinoderm adults show radial symmetry while the larval echinoderms have bilateral symmetry. Hence, phylum Echinodermata shows both radial and bilateral symmetry.

Hence, the correct option is d.

Ques:- Triploblastic organisation and bilateral symmetry starts from which phylum during evolution

A. Porifera
B. Ctenophora
C. Annelida
D. Platyhelminthes

Solution: Platyhelminthes is the first phylum to show bilateral symmetry (one plane dividing body into identical halves) with triploblastic organisation (presence of three germ layers in the body).

Hence, the correct option is d.

## FAQs of Classification

Que:- Define radial symmetry with examples.

Solution: Radial symmetry refers to the type of symmetry in which an individual can be divided into two equal halves through any plane passing through its centre. Examples include coelenterates, ctenophores and echinoderm adults.

Que:- Describe coelom along with its function.

Solution: The body cavity present between the body wall and gut wall which has mesodermal linings is termed as a coelom. It is usually fluid-filled and functions like a protective cushion around the internal organs.

Que:- Mention the types of coelom based on origin.

Solution: Based on origin, coelom can be of two types based on origin:

• Schizocoelom: Body cavity which is formed by splitting of mesodermal mass is termed as schizocoelom. Example of schizocoelomate: Annelids.
• Enterocoelom: Body cavity which is formed by gut pouches is termed as enterocoelom. Example of eucoelomate: Echinoderms.

Que:- Mention the three germ layers of a triploblastic animal and two body parts that are formed from each of the germ layers.

Solution

 Germ layers Body parts formed Ectoderm Nervous system, epidermal skin cells Mesoderm Muscle cells, bones Endoderm Stomach, lungs

## Other Related Topics

### Other Related Topic Of Biology

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