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Major Groups of Animals

Animals are broadly grouped based on whether they are,

(i) Single-celled (Unicellular) or many celled (Multicellular), and

(ii) Vertebrates or invertebrates


The phyla or groups are as follows:

(i) Single cell – Protozoa e.g. Amoeba


(ii) Many celled – Invertebrate (animals without backbones)
(a) Porifera – e.g. Sponges
(b) Coelenterata – e.g. Jellyfish
(c) Platyhelminthes – e.g. Tapeworm
(d) Nemathelminthes – e.g. Ascaris
(e) Annelida – e.g. Earthworm
(f) Arthropoda – e.g. Crab
(g) Mollusca – e.g. Snail
(h) Echinodermata – e.g. Starfish




Jelly fish

Tape worm


Earth worm




Vertebrata (Animals with backbones)

(a) Class: Pisces (e.g., fish)
(b) Class: Amphibia (e.g., frog)
(c) Class: Reptilia (e.g., lizard)
(d) Class: Aves (e.g., pigeon)
(e) Class: Mammalia (e.g., rat)










Protozoa :-

This group consists of unicellular organisms where a single cell carries out all the functions. They are usually motile with special locomotory structures like pseudopodia or cilia. Food vacuoles and contractile vacuoles are present . They reproduce by binary fission and conjugation.

Example : Amoeba, Paramecium, Plasmodium.



The word means organisms with holes. These are non-motile animals attached to some solid support. There are holes or ‘pores’ all over the body. These lead to a canal system that helps in circulating water throughout the body to bring in food and oxygen. These animals are covered with a hard outside layer or skeleton. The body design involves very minimal differentiation and division into tissues. They are commonly called sponges and are mainly found in marine habitats.


These are animals living in water. They show more body design differentiation. There is a cavity in the body called the gastrovascular cavity . The body is made of two layers of cells, one makes up cells on the outside of the body(ectoderm) and the other makes the inner lining of the body(endoderm) separated by a jelly like mesoglea. Some of these species live in colonies, while others have a solitary life-span. Jellyfish and sea anemones are common examples.


The body of animals in this group is far more complexly designed than in the two other groups we have considered so far. The body is bilaterally symmetrical, meaning that the left and the right halves of the body have the same design. There are three layers of cells – the ectoderm, endoderm and mesoderm from which differentiated tissues can be made, which is why such animals are called triploblastic. This allows outside and inside body linings as well as some organs to be made. There is thus some degree of tissue formation. However, there is no true internal body cavity or coelom, in which well developed organs can be accommodated. The body is flattened dorso -ventrally -meaning from top to bottom, which is why these animals are called flatworms. They are either free living or parasitic.


Example : Free living animals are planarians and parasitic animals like tapeworms and liverflukes.



Body is also bilaterally symmetrical and triploblastic. However, the body is cylindrical rather than flattened. There are tissues, but no real organs, although a sort of body cavity or a pseudo coelom is present. These are very common as parasitic worms causing diseases, like elephantiasis.


Example : include Ascaris ,Wuchereria.




Annelids are also bilaterally symmetrical and triploblastic, but in addition they have a true body cavity. This allows true organs to be packaged in the body structure. There is, thus, extensive organ differentiation. Thus, differentiation occurs in a segmental fashion, with the segments lined up one after the other from head to tail. These animals are found in a variety of habitats – fresh water, marine water as well as land. Earthworms and leeches are common examples.


This probably is the largest group of animals. These animals are bilaterally symmetrical, segmented and characterised by the presence of jointed legs. There is an open circulatory system and so the blood does not flow in well defined blood vessels. The coelomic cavity is filled with blood. Some familiar examples are prawns, butterflies, houseflies, spiders, scorpions and crabs.


In the animals of this group, there is bilateral symmetry. The coelomic cavity is reduced. There is little segmentation. They have an open circulatory system and kidney like organs for excretion. There is a foot that is used for moving around. Examples are snails and mussels.


In Greek, echinos means ‘hedgehog’ and derma means ‘skin.’ Thus, these are spiny skinned organisms. These are exclusively marine animals. They are triploblastic and have a coelomic cavity. They also have a peculiar water driven tube system that they use for moving around. They have hard calcium carbonate structures that they use as a exoskeleton.


Example : Are starfish and sea urchins.




These animals are bilaterally symmetrical, triploblastic and have a coelom. In addition, they show a new feature of body design, namely a notochord, at least some stages of their lives. The notochord is a long rod-like support structure that runs along the back of the animal separating the nervous tissue from the gut. It provides a place for muscles to attach for ease of movement. Protochordates may not have a proper notochord present at all stage in their lives or for the entire length of the animal. Protochordates are marine animals.


Example : Are Balanoglossus, Herdemania and Amphioxus.




These animals have a true vertebral column and internal skeleton, allowing a completely different distribution of muscle attachment points to be used for movement.
Vertebrates are bilaterally symmetrical, tirploblastic, coelomic and segmented, with complex differentiation of body tissues and organs. 

All chordates possess the following features:

(a) have a notochord
(b) have a dorsal nerve cord
(c) are triploblastic
(d) have paired gill pouches
(e) are coelomate

Vertebrates are grouped into five classes.



Consists of fishes and are are exclusively aquatic . Their skin is covered with scales/plates. They obtain oxygen dissolved in water by using gills. The body is streamlined and provided with a tail for movement. They are cold- blooded and their hearts have only two chambers, unlike the four that humans have. They lay eggs. We can think of many kinds of fish, some with skeletons made entirely of cartilage, such as sharks and some with a skeleton made of both bone and cartilage, such as tuna or rohu.


These animals differ from the fish in the lacking of scales, in having mucous glands in the skin, and a three chambered heart. Respiration is through either gills or lungs. They lay eggs. These animals are found both in water and on land.


Example : Frogs, toads and salamanders.



These animals are cold blooded, have scales and breathe through lungs. While most of them have a three-chambered heart, crocodiles have four heart chambers. They lay eggs with tough coverings and do not need to lay their eggs in water, unlike amphibians Snakes, turtles, lizards and crocodiles fall in this category.

These are warm blooded animals and have a four chambered heart. They lay eggs. There is an outside covering of feathers and the two forelimbs are modified for flight. They breathe through lungs. All birds fall in this category.

Mammals are warm blooded animals with four chambered hearts. They have mammary glands for the production of milk to nourish their young. Their skin has hair as well as sweat glands. Most mammals familiar to us produce live young ones. However, a few of them, like the Platypus and Echidna lay eggs and some like kangaroos give birth to small sized young ones which are kept in a marsupium or pouch till they grow .



The system of scientific naming or nomenclature we use today was introduced by Carolus Linnaeus in the eighteenth century. The scientific name of an organism is the result of the process of classification, which puts it along with the organisms it is most related to. But when we actually name the species, we do not list out the whole hierarchy of groups it belongs to. Instead, we limit ourselves to writing the name of the genus and species of that particular organism. The world over, it has been agreed that both these names will be used in Latin forms.


Carolus Linnaeus 

Certain conventions are followed while writing the scientific names:

(i) The name of the genus begins with a capital letter.
(ii) The name of the species begins with a small letter.
(iii) When printed, the scientific name is given in italics.
(iv) When written by hand, the genus name and the species name have to be underlined separately.


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