What is the basis of Animal Kingdom Classification?
Animals are cosmopolitan organisms that exhibit different structures and forms. Since the discovery of animals species is increasing at an exponential rate, the need for animal classification becomes the need of the hour. While talking of animal kingdom classification, a lot of common fundamental features including the arrangement of cells, body symmetry, patterns of the digestive system, nature of coelom, and patterns of circulatory or reproductive systems can be considered.
Levels of Organization
All the members of Kingdom Animalia are multicellular. The pattern of organization of cells is variable in different animals. These include the following –
1) Cellular level of organization – This is exhibited by Sponges (Phylum Porifera). There are loosely arranged cell aggregates and some division of labour (activities) occurs among the cells.
2) Tissue level of organization – This is exhibited by Coelenterates. In these organisms, a complex arrangement of cells is seen as tissues where cells perform the same function.
3) Organ level of organization – This is exhibited by members of Platyhelminthes and other higher phyla. In these, tissues are grouped particularly together to form organs. These organs perform specialised functions.
4) Organ system level of organization – This is exhibited by Annelids, Arthropods, Molluscs, Echinoderms, and Chordates. In these, organs have combined to form functional organ systems, and it is seen that each organ system performs a specific physiological function.
Patterns of the digestive system
Incomplete digestive system
Firstly, in this, there is only a single opening to the outside of the body that serves as both mouth and anus. For instance – Platyhelminthes.
Complete digestive system
Secondly, in this, the organism has two openings, mouth, and anus. For instance – Aschelminthes, Annelids, Arthropods, Molluscs, Echinoderms, Hemichordates, and Chordates.
Patterns of the circulatory system
1) Open type– In this, blood is pumped out of the heart where cells and tissues are directly bathed in it.
2) Closed type– In this, blood is flowing through a series of blood vessels of varying diameters (arteries, veins, and capillaries).
Animal kingdom classification can also be done on the basis of symmetry. These include-
Any plane that passes through the centre does not divide the body into equal halves. For instance- Sponges.
In this symmetry, any plane that passes through the central axis of the body divides the organism into two halves that are identical. For instance- Coelenterates, Ctenophores, and Echinoderms.
In this symmetry, the body can be divided into left and right halves that are identical in only one plane. For instance- Annelids, Arthropods, etc.
Organization of embryonic layers
Different animals also differ in the organization of embryonic layers. The two patterns exhibited are as following-
If the cells in the embryo are arranged in two embryonic layers, namely the external ectoderm, and internal endoderm, the animals are called diploblastic animals. In these animals, an undifferentiated specialised layer, mesoglea, is seen in between the ectoderm and the endoderm. For instance- Coelenterates.
Animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm layer and endoderm layer, are called triploblastic animals. For instance- Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordates.
Nature of coelom
The body cavity in animals, which is lined by mesoderm is called coelom. There can be a presence or absence of a coelom cavity between the body wall and the gut wall. Following are the three different categories of animals based on this-
Animals that possess coelom are called coelomates. For instance- Annelids, Molluscs, Arthropods, Echinoderms, Hemichordates, and Chordates.
The animals in which the body cavity is not lined by mesoderm rather the mesoderm is seen as scattered pouches in between the ectoderm and endoderm are called pseudocoelomate animals and such a body cavity is called pseudocoelom. For instance- Aschelminthes.
The animals which lack the body cavity are called acoelomates. For instance- Platyhelminthes.
While talking about animal kingdom classification, we must not forget about body segmentation. It is equally important as other features. The body of animals is externally and internally divided into segments. There is a serial repetition of at least some organs. For instance, in Earthworm, there is a metameric segmentation pattern (also termed as metamerism).
Presence of notochord
A rod-like structure that is derived from mesoderm is called a notochord. It is formed on the dorsal side of the animal during embryonic development. Animals with notochord are categorized as chordates and those animals which lack this structure are categorized as non-chordates. For instance- Porifera to Echinoderms.
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