There are almost 8 million different species on the planet today. However, as new species are found at an incredible rate, this number is continually changing. There are traits that make it easier to understand the evolutionary history of species and provide information about their origins and genetic relationships. Based on these characteristics, all organisms are classified into various groups. All living species are classified into several groups based on extremely basic features that they all share. The species in each category are then subdivided into smaller groups based on closer similarities between organisms in the same group. Classification is a sophisticated system for categorising living organisms into different groups.

Classification of Animal Kingdom

Based on various fundamental features like –

1. Levels of Organisation
2. Symmetry
3. Diploblastic and Triploblastic Organisation
4. Coelom development
5. Segmentation of the body and
6. Presence or absence of Notochord.

Basis of Classification

1. The animal kingdom is the largest kingdom amongst the five kingdoms consisting of all animals.
2. Animals are multicellular eukaryotes that lack a cell wall and chlorophyll, but share the same mode of feeding as plants, namely heterotrophic nutrition.
3. They are related to their cell arrangement, body symmetry, level of organisation, coelom, presence/absence of notochord, and other factors.Based on these features, the animal kingdom has been classified into 11 different phyla.
4. The different criteria for animal classification are explained below.

Levels of Organisation

1. Though animals are multicellular, the level of organization of cells varies from one animal to another.
2. Some animals have a loose mass of cells that display cellular order.Porifera is the phylum that includes such creatures. 
   As time goes on, the intricacy of bodily architecture increases, and the tissues begin to divide labour.
3. These animals are categorised as coelenterate because they have tissue levels of structure.
   Organ level organisation exists in the phylum Platyhelminthes and Aschelminthes.
4. Nor-chordates such as Annelids, Arthropods, Molluscs, Echinoderms, and Chordates have a specialised organ system for their physiological activities.
5. They are organised at the organ-system level. Despite the fact that these creatures have organ-system levels of organisation, the complexity of organ systems varies and is classified into distinct phyla.

Blind sac body plan

The blind sac body plan is the body plan where there is a single opening of the digestive system. The tube-like digestive system begins with a single aperture through which food is ingested and digested material is ejected. The gut is lined with a single layer of endodermal cells that absorb and digest food. Cnidaria and Platyhelminthes exhibit this type of body plan.

Symmetry

Fig: Radial Symmetry and Bilateral symmetry

Another reason for animal classification is symmetry. Animals, in general, exhibit two types of symmetry: –

1. Radial symmetry: A symmetry where any plane passing through the central axis divides the body into two equal halves, is called radial symmetry. Examples are Coelenterates, ctenophores and echinoderms.
2. Bilateral symmetry: In bilateral symmetry, only one plane splits the body into two equal halves, as seen in Annelids, Arthropods, Molluscs, and other organisms.
   Some species, on the other hand, lack symmetry, meaning that their bodies cannot be divided into two halves in any plane passing through the centre. Asymmetrical animals, such as Poriferans, are described as such.

Diploblastic and Triploblastic Organisation

Tissues, organs, and organ systems develop in all animals from the cells that make up the embryonic layers. Animals are divided into two groups based on the number of embryonic layers: diploblastic and triploblastic.

Fig: Diploblastic and triploblastic animals

1. Diploblastic animals: Diploblastic animals have two embryonic layers, one on the outside (ectoderm) and one on the inside (endoderm), as shown in the Phylum Coelenterata.
2. Triploblastic animals: Triploblastic animals have three embryonic layers: ectoderm, endoderm, and an intermediate layer called mesoderm. Phyla Platyhelminthes to Chordata animals have a triploblastic organisation.

Body Cavities and Tissues

1. Another important feature that differentiates animals is whether or not they have a true body cavity, called a coelom.
2. This is a space between the body tissues and internal organs, and it allows for independent movement and growth of those organs.
3. In animals that don’t have a coelom, the internal organs are attached to the tissues of the body wall, so they are not independent.
4. Of course, some animals also fall somewhere in between. These animals have a pseudocoelom, which is a body cavity not completely lined by tissue.
5. Pseudocoeloms work just like true coeloms, even though this name meaning ‘false cavity’ may suggest otherwise. Most bilaterally symmetrical animals (including us) are coelomates.
6. Animals, such as roundworms, fall into the pseudocoelomate group, while other worms, such as flatworms, do not have a coelom at all.
7. In addition to body cavities, we can also differentiate animals by whether or not they have true tissues. Animals that have true tissues are known as eumetazoans.

Coelom

The coelom is a key feature for classification. The coelom is a hollow bordered by mesoderm that lies between the body wall and the gut wall. Animals have been divided into three groups based on the presence or lack of coelo

1. Coelomate: A coelomate contains coelom. Coelomates include animals from the phylum Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata, and Chordata..
2. Pseudocoelomate: Mesoderm appears as pouches between the endoderm and the ectoderm in pseudocoelomates. However, unlike Aschelminthes, the hollow produced is not bordered with mesoderm.
3. Acoelomate: Coelom is absent in an acoelomate, e.g. Platyhelminthes.

Segmentation

1. In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs.
2. For example, in an earthworm, the body shows this pattern called metameric segmentation and the phenomenon is known as metamerism.

Notochord

The notochord is a cartilaginous, longitudinal support rod that runs beneath the nerve cord. Based on the existence or lack of a notochord, the animal kingdom has been divided into two groups: Non-Chordata and Chordata. Chordates have a notochord, but non-chordates don’t. Non-chordates are animals from the Phylum Porifera through the Phylum Echinoderms.

Summary

The animal kingdom, which includes all animals, is the largest of the five kingdoms. A body plan is a set of structural and developmental characteristics that can be used to distinguish a phylum of animals. At some time throughout their development, whether in the embryonic, larval, or adult stage, all members of a group have the same body layout. Tissues, organs, and organ systems develop in all animals from the cells that make up the embryonic layers. Animals are divided into two groups based on the number of embryonic layers: diploblastic and triploblastic. The coelom is a key feature for classification. The coelom is a cavity between the body wall and gut wall, lined by mesoderm. The notochord is a cartilaginous, longitudinal support rod that runs beneath the nerve cord. Based on the existence or lack of a notochord, the animal kingdom has been divided into two groups: Non-Chordata and Chordata.

Frequently Asked Questions (FAQs) on Animal Construction

Q.1. What is the basis of classification?
Ans: The living things are classified into seven different levels, these are kingdom, phylum, classes, order, families, genus, and species. Kingdoms: These are the most basic classification of living things.

Q.2. What are the 5 levels of the organization?
Ans: These parts are divided into levels of the organization. Cells, tissue, organs, organ systems, and organisms are the five tiers.

Q.3. Why is body symmetry important?
Ans: They are better able to react to environmental stimuli coming from all around their bodies because of the equal distribution of bodily parts and sense organs. Because the forms of organs and cells are inextricably linked to their activities and functions, symmetry is vital at those scales as well.

Q.4. What is notochord and what is its function?
Ans: The notochord is the chordates’ defining structure, and it plays an important role in vertebrate development. It is a major skeletal element of the developing embryo and a source of midline signals that design surrounding tissues.

Q.5. What is a coelom in animals?
Ans: In most animals, the coelom (or celom) is the major bodily cavity that surrounds and contains the digestive tract and other organs inside the body.

Learn the difference between a Plant Cell and an Animal Cell here

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