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1800-102-2727Have 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.
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.
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 |
|
The organisation of cells into different layers during embryonic development is called 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 |
|
The body cavity present between the body wall and gut wall which has mesodermal linings is termed as a 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 |
|
Based on the origin of coelom, the organisms having true coelom (eucoelomates) can be categorised as follows:
Examples: Members of phylum Annelida, phylum Arthopoda and phylum Mollusca.
Examples: Members of phylum Echinodermata, phylum Hemichordata and phylum Chordata.
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?
A. Radially symmetrical
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
B. Presence of radial symmetry
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.
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:
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 |