Animal kingdom – Classification

Animal kingdom – Classification

The kingdom is the second-highest taxonomic rank below the domain. Under the kingdom Animalia, the multicellular,  eukaryotic organisms with few exceptions are placed.

Basis of Classification of Animal Kingdom

Some common features are used as the basis of the classification of animals. These are common to many individuals related to the arrangement of the cell- level of organization, the symmetry of the body, the nature of coelom (body cavity), patterns of the digestive system, and the circulatory or reproductive system.

The Level of Organisation

We know that all members of Animalia are multicellular, but all of them do not possess the same pattern of body organization of cells. Some organisms have cellular levels, some tissue levels, and organ and organ system levels.

• As in Sponges, the cellular level of organization is found.
• In Coelenterates, the tissue level of organization is found.
• The organ level of organization starts from Platyhelminthes and is found in other higher phyla.
• The organ system level of Organisation is found in animals like Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.

Organ systems show variations in pattern complexities in different animals.

1. Incomplete digestive system: As we can see the digestive system in Platyhelminthes consists of a single opening outside of the body, that functions for both mouth and anus, so it is called incomplete.
2. The complete digestive system: In the complete digestive system two openings i.e. mouth and anus are found.

The circulatory system may be of two types i.e. Open type and Closed type.

1. Open type of circulatory system: The blood is pumped out of the heart and the direct bath of cells and tissue takes place.
2. Closed type of circulatory system: In this type of circulation, the blood circulates through a series of vessels like arteries, veins, and capillaries.

Symmetry

The body may or may not be divided into two equal and identical parts. On the basis of these features, the animal can be categorized as:

• Asymmetrical– Any plane that passes through the center does not divide into equal parts. Ex. Sponges are found mostly asymmetrical.
• Radial symmetry– In this type, any plane that passes through the axis of the center and the body divides into two equal halves, then it is called radial symmetry. For example- Coelenterates, Ctenophores, and Echinoderms show this type of symmetry.

Radial symmetry of an organism

• Bilateral symmetry– the body divides into identical left and right planes in one plane only then it is called bilateral symmetry. For example-Annelids, Arthropods, etc show this type of symmetry.

Bilateral Body Symmetry-

The Diploblastic and Triploblastic Organisation

1. Diploblastic animals– The cells found arranged in two embryonic layers i.e. external ectoderm and an internal endoderm. Mesoglea is an undifferentiated layer found between the ectoderm and endoderm.  Example- Coelenterates
2. Triploblastic animals– In a developing embryo, the third germinal layer called mesoderm is found in between ectoderm and endoderm. These animals are called triploblastic animals. These are Platyhelminthes to chordates.

Coelom or Body cavity

Coelomates– The coelom is the body cavity lined by a mesoderm. The animals consisting of coeloms are called coelomates. Examples- Annelids, molluscs, arthropods, and chordates.

Pseudocoelomates– The body cavity is not lined by mesoderm in some animals, but mesoderm is present as scattered pouches in between ectoderm and endoderm. This type of body cavity is called pseudo-coelom and the organism is called pseudocoelomate. As in Aschelminthes.

Acoelomate– In these organisms, the body cavity is not found. As in Platyhelminthes.

Segmentation

The body of some organisms is externally and internally divided into segments with synchronized repetition of some organs. Such as in earthworms, the body shows a pattern which is termed metameric segmentation, and the phenomenon is called metamerism.

On the basis of Notochord

It is a mesodermally derived structure found in a rod-like shape on the dorsal side during embryonic development. Animals with notochords are called Chordates and without notochords are called non-chordates. Porifera to Echinodermata are non-chordates.

Classification of Animal Kingdom

The major phyla are:

• Phylum – Porifera
• Phylum – Coelenterata (Cnidaria)
• Phylum- Ctenophora
• Phylum- Platyhelminthes
•  Aschelminthes,
• Phylum-Annelida
• Phylum-Arthropoda
• Phylum-Mollusca
• Phylum – Echinodermata
• Hemichordata
• Phylum-Chordata

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