Criteria For Animal Classification :
Kingdom Animalia is classified into different phyla on the basis of following criteria :
1. Number of germ layers : Every multicellular organism begins life from a single cell called zygote. The zygote divides and re-divide to form the mass of cells.. These cells get arranged into two or three layers called "Germinal layers'.
When an organism shows only two layer i.e. outer ectoderm and inner endoderm separated by non living substance called mesogloea, it is called diploblastic animal. Members of phylum Cnidaria are diploblastic.
When an organism shows three germinal layers i.e. outer ectoderm, inner endoderm and middle mesoderm, it is called triploblastic animal. Members of phylum Platyhelminthes onwards are triploblastic.
2. Animal body plan : There are three basic body plans among the animals, namely :
a) Cell aggregate plan : It includes the simplest types of animals like sponges. The animals show aggregation of cells with least division of labour. The cells do not form tissues or organs. There is no nervous coordination between them. Members of phylum Porifera belong to this category.
b) Blind sac body plan : It is shown by little complex animals. There is a digestive cavity with a single opening which acts as a mouth as well as anus (i.e. incomplete digestive system). The food is ingested and egested through the same opening (mouth). Members of phylum Cnidaria belong to this type.
c) Tube within tube body plan : This body plan is present in all advanced and highly evolved animals. digestive system is complete having two separate openings, one at each end. e.g., Animals belonging to phylum annelida onwards belong to this type.
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| Animal body plan |
3. Body symmetry : Symmetry means similarity in shape, size and number of parts on the opposite sides of a median line. It implies that the two halves obtained by drawing an imaginary line along different planes are similar and identical. Animals may be asymmetrical, radially symmetrical or bilaterally symmetrical.
a) Asymmetrical animals : The body of asymmetrical animals cannot be divided into two equal parts by any place. e.g., certain sponges, snails.
b) Radially symmetrical animals : In certain animals body can be cut or divided into two similar halves in a number of planes. All these cuts (planes) pass through the centre and appear like radii. This type of symmetry is called radial symmetry. e.g. Hydra, Starfish.
c) Bilaterally symmetrical animals : In this type the body of an animal can be bisected by a single median or vertical plane. The two halves are identical. This type of symmetry is called bilateral symmetry. e.g. Fish, Frog.
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| Body symmetry |
4. Body cavity or Coelom : It is the shape between the body wall and the alimentary canal which is produced by splitting of mesoderm, during embryonic development. Depending on the nature of coelom, there are three types of animals.
a) Acoelomates : In certain animals, there is no body cavity or coelom. Hence they are called acoelomates. The space between body wall and alimentary canal is filled with parenchymatous tissue. For e.g. members of phylum Platyhelminthes.
b) Pseudocoelomates : Members of phylum Aschelminthes show false body cavity or pseudocoel. i.e. the cavity between the body wall and alimentary canal is lined by patches of mesodermal cells.
c) Coelomates : All animals from phylum Annelida onwards, have true body cavity or corlom. In triploblastic animals, the mesoderm splits into two layers enclosing a cavity called coelom. The outer layer lines, the body wall while the inner layer covers the alimentary canal. The cavity is filled by coelomic fluid. In some animals like cockroach, body cavity is filled with blood. Hence it is called haemocoel.
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| Body cavity or coelom |
5. Body Segmentation (metamerism) : It is a series of successive units or segments arranged along the length of body of the animals. When the external segmentation of the body coincides with internal segmentation, such animals are called metamerically segmented and the phenomenon is called metamerism. e.g. Earthworm.
6. Body Support : The external or internal framework which provides support to the body is called skeleton. It is of two types, exoskeleton and endoskeleton. The lower group of animals possess only exoskeleton for protection. e.g. cockroach while higher animals possess both the exoskeleton as well as endoskeleton. e.g. fish, cobra, parrot.