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Basis of Animal Classification

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Basis of Animal Classification - Lesson Summary

With over a million animal species on our earth, it would be difficult to categorise them into different groups. However, with the help of basic fundamental features such as level of organisation, symmetry, cell organisation, nature of coelom, segmentation and notochord, scientists have classified animals into various groups.

Level of organisation is the first basic fundamental feature that deals with the pattern of cell organisation in an animal. There are four patterns of cell organisation namely, cellular, tissue, organ and organ system. 

a) Cellular level of organisation is exhibited by Sponges where cells are loosely arranged. 

b) Tissue level of organisation is more complex and is displayed in coelenterates. Here, the cells that perform the same function are arranged into tissues.

c) Organ level of organisation is exhibited by members of Platyhelminthes and other higher phyla. Here, the tissues are grouped to form organs and each organ performs a particular function.

d) Organ system level of organisation is the last level of organisation.
  • Here, the organs are associated with one another to form a functional system where each system performs a specific physiological function.
  • Animals like annelids, arthropods, molluscs, echinoderms and chordates exhibit this level of organisation.
  • Organ systems can also differ in complexity in various animal groups.For example, Platyhelminthes have an incomplete digestive system, where a single opening outside the body serves as both the mouth and the anus. Human beings have a complete digestive system that has two openings – mouth and anus.
  • Two types of circulatory systems in animals are also seen as open and closed.

Symmetry is another feature by which animals are classified. 
They can be asymmetrical, like sponges, or have radial symmetry, like coelenterates, ctenophores and echinoderms.  However, most animals like annelids and arthropods have bilateral symmetry.

Cell organisation is the next feature used for classification.There are two types of cell organisation–diploblastic and triploblastic. 
  • Coelenterates have diploblastic organisation, where the germinal layers of the cell consist of an external ectoderm and an internal endoderm.
  • Members from phylum platyhelminthes to chordates have triploblastic organisation, where a third germinal layer, the mesoderm, is found between the ectoderm and the endoderm.

Nature of coelom is another feature based on which organisms are classified. The coelom is the body cavity lined by mesoderm. The presence or absence of this cavity helps in classifying the animals. There are three categories, coelomates, acoelomates and pseudocoelomates. 
  • Coelomates are the animals that possess a body cavity. They include annelids, molluscs, arthropods, echinoderms, hemichordates and chordates. 
  • Acoelomates are the animals in which the body cavity is absent. They include organisms belonging to platyhelminthes. 
  • Pseudocoelomates are the animals that possess a pseudocoelom, such as aschelminthes. A pseudocoelom is a body cavity in which the mesoderm is not lined, but rather scattered between the ectoderm and the endoderm.

Segmentation is another feature used for classification of organisms.
  • The body of some animals can be divided externally and internally into segments. 
  • A serial repetition of the segments in animals along with some organs is called as metamerism. This is observed in earthworms. 

Notochord is another feature used  for classification of animals. Based on the presence or absence of a notochord, animals can be classified into non-chordates and chordates.

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