Symmetry
Symmetry is another basis of classification of animals. Generally, animals show two types of symmetry-
Radial symmetry: A symmetry where any plane passing through the central axis divides the body into two equal halves is called the radial symmetry. Examples are Coelenterates, ctenophores and echinoderms.
Bilateral symmetry: In bilateral symmetry, only a single plane divides the body into two equal halves, e.g. annelids, arthropods, molluscs, etc.
However, some animals do not show any symmetry i.e. their body can’t be divided into two halves in any plane passing through the centre. Such animals are said to be asymmetrical, e.g. Poriferan.
Diploblastic and Triploblastic Organisation
All animals develop their tissues, organs and organ system from the cells that form the embryonic layers. Based on the number of embryonic layers, animals are classified into two- diploblastic and triploblastic animals.
Diploblastic animals: In diploblastic animals, the <a href="https://byjus.com/biology/cells/">cells</a> form two embryonic layers, i.e., an outer layer- ectoderm and an inner layer – endoderm, e.g. Phylum Coelenterata.
Triploblastic animals: In triploblastic animals, cells are arranged in three embryonic layers- ectoderm, endoderm and a middle layer – mesoderm. Animals belong to Phyla Platyhelminthes to Chordata have a triploblastic organisation.
Coelom
Coelom is a key feature for classification. Coelom is a cavity between the body wall and gut wall, lined by mesoderm. Depending on presence/absence of coelom, animals have been classified into three:
- Coelomate: Coelom is present in a coelomate. Animals belonging to phyla Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata and Chordata are examples of coelomates.
- Pseudocoelomate: In pseudocoelomates, mesoderm appears as pouches between endoderm and ectoderm. But the cavity formed is not lined by mesoderm, e.g. Aschelminthes.
- Acoelomate: Coelom is absent in an acoelomate,e.g. Platyhelminthes.
Notochord
The notochord is a longitudinal, cartilaginous supporting rod running beneath the nerve cord. The animal kingdom has been broadly classified into two, based on the presence and absence of notochord- Non-Chordata and Chordata. Chordates possess notochord while non-chordates lack it. Animals belonging to the Phylum Porifera to Phylum Echinoderms are non-chordates.
Kritika Trehan 7 years, 4 months ago
Symmetry
Symmetry is another basis of classification of animals. Generally, animals show two types of symmetry-
Radial symmetry: A symmetry where any plane passing through the central axis divides the body into two equal halves is called the radial symmetry. Examples are Coelenterates, ctenophores and echinoderms.
Bilateral symmetry: In bilateral symmetry, only a single plane divides the body into two equal halves, e.g. annelids, arthropods, molluscs, etc.
However, some animals do not show any symmetry i.e. their body can’t be divided into two halves in any plane passing through the centre. Such animals are said to be asymmetrical, e.g. Poriferan.
Diploblastic and Triploblastic Organisation
All animals develop their tissues, organs and organ system from the cells that form the embryonic layers. Based on the number of embryonic layers, animals are classified into two- diploblastic and triploblastic animals.
Diploblastic animals: In diploblastic animals, the <a href="https://byjus.com/biology/cells/">cells</a> form two embryonic layers, i.e., an outer layer- ectoderm and an inner layer – endoderm, e.g. Phylum Coelenterata.
Triploblastic animals: In triploblastic animals, cells are arranged in three embryonic layers- ectoderm, endoderm and a middle layer – mesoderm. Animals belong to Phyla Platyhelminthes to Chordata have a triploblastic organisation.
Coelom
Coelom is a key feature for classification. Coelom is a cavity between the body wall and gut wall, lined by mesoderm. Depending on presence/absence of coelom, animals have been classified into three:
Notochord
The notochord is a longitudinal, cartilaginous supporting rod running beneath the nerve cord. The animal kingdom has been broadly classified into two, based on the presence and absence of notochord- Non-Chordata and Chordata. Chordates possess notochord while non-chordates lack it. Animals belonging to the Phylum Porifera to Phylum Echinoderms are non-chordates.
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