How Diverse are Animals?

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How Diverse are Animals?

Hmmm....if you think they can be classified on the basis of whether they live on land, in water or in the air, Congrats! You are thinking like Aristotle! But according to the accepted system of classification, you are wrongL. To find what’s right, read on!

Robert Whittaker, in 1969 proposed ‘Five kingdom classification’ of living organisms. Till date we follow this system. Kingdoms are sub grouped into phylum for animals or division for plants, class, order, family, genus and species. Hence, the basic unit of classification is species. Species includes all the organisms that are similar to breed and produce fertile offspring. Today we will try to understand the various phyla of animal kingdom. We just classified animals on the basis of their mode of living. But in Whittaker’s Classification, animals are classified into 10 Phyla Porifera, Coelenterata, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca,  Echinodermata, Protochordata, Vertebrata on the basis of different features like:

    •  Cellular or tissue level of body organization
    •  Body symmetry
    •  Type of body cavity called as coelom
    •  Presence or absence of segmentation
    •  Presence or absence of a backbone.

It all sounds Greek to you if you don’t know it already. Don’t worry, we will consider each one separately. Let us start with Porifera. These  are multicellular organisms, now what’s multicellular? Multicellular organisms are organisms that consist of more than one cell, in contrast to unicellular organisms. Poriferans exhibit minimal level of tissue organization. They lack nervous system. Porifera get their name from two words, “pori,” meaning “holes,” and “fera,” meaning “bearing”.   Porifera includes Sycon, Spongilla and Euplectella.  e.g.  Sponges.

Coelenterates are radially symmetrical (radial symmetry is the symmetry about a central axis, as in a starfish or a tulip flower) organisms which live in marine habitat. Some are solitude and some are colonial.  Coelenterates get their name from two Greek words - “koilos,” meaning “hollow,” and “enteron,” meaning “intestine”.  e.g. Corals,  Hydra. 

Platyhelminthes are either free living or parasitic.  They are triploblastic (having a body derived from three embryonic cell layers, as in all multicellular animals) animals. Platyhelminthes get their name from two Greek words - “platy,” meaning “flat,” and “helminthes,” meaning “worms”.  E.g. Planaria. 

Nematoda are bilaterally symmetrical, triploblastic parasitic worms.  Nematoda get their name from two Greek words - “nema,” which means “thread,” and “ode,” which means “like”.  Nematodes can be free living or parasitic.  

Annelida are bilaterally symmetrical, triploblastic, schizocoelomates with segmented body.  Annelida get their name from the Latin word “anellus,” which means "little ring".  Annelids are characterised by the presence of a circulatory system.  e.g. Earthworm. 

Arthropoda are bilaterally symmetrical, triploblastic animals with true coelom. Arthropoda means “joint legs”.  This phylum gets its name from the Greek words arthron, meaning “joint”, and podos, meaning “foot”.   

Molluscs are bilaterally symmetrical, triploblastic gastropods with reduced coelom. Mollusca is derived from a Latin word, which means “thin-shelled and soft”. 

Echinodermata are triploblastic animals with true coelomic cavity.  Echinodermata are spiny skinned organisms which get their name from the Greek words “echinos,” meaning protective “spines,” and “derma,” meaning “skin”. Skeletons of echinoderms are hard calcium carbonates. They exhibit radial symmetry.

Protochordata are the organisms belonging to the phylum Chordata, and are primitive chordates.  Protochordates possess a notochord during their early stage of development.  The notochord is a long rod-like support that runs all along the back of the animal separating the nervous tissue from the gut. e.g. Balanoglossus, Herdmania.

Vertebrata are the most advanced group of animals with true vertebral column and strong endoskeleton.  Vertebrates are grouped into different classes based on bilateral symmetry, notochord, dorsal nerve cord, paired gill pouches, triploblastic, and coelomate. These classes are Pisces, Amphibia, Reptilia, Aves and Mammalia.

Now it’s your turn to find to which Phylum do the Humans belong!!


Shreenabh Agrawal

Class IX-A

The Chanda Devi Saraf School

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Posted By : ScienceIndia Administrator
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