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Structural Organization in Animals

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Every living organism is made up of cells. The basic structural organization of any multicellular living organism is as follows:

Cells-Tissue-Organ-Organ System

Structural Organization in Animals

For a living thing to be called living, it must have a cell. Every organism starts with the fundamental unit of life i.e., the cell. A bunch of cells that perform the same function accumulate and form the tissue. Different tissue forms different organs. And organs coordinate with each other and perform various functions, this is known as the organ system.


The cell is the basic unit of life. It is the functional and structural unit of all living organisms. Where organisms are of two types unicellular or multicellular organisms. Unicellular means the organisms are made up of single cells like bacteria, protists, and unicellular fungi yeasts while multicellular can be made up of two cells (diplococci), two thousand cells, or a number of cells can be in billions like in a human being. Cells can work as independent entities or come together to perform a specific function as in tissue.




A group of similar cells performing similar functions consists of intercellular substances. The organization of tissue is different to form different organs like lungs, stomach, and heart. It is important to note that different organs and organ systems are basically made up of only four types of tissue.

Types of Tissue


Animal Tissue

The four types of animal tissue are namely epithelial tissue, connective tissue, muscular tissue, and neural tissue.

Epithelial Tissue 

This type of tissue form covers the internal and external surfaces of the body. The cells are compactly packed with a little intercellular matrix.

Epithelial tissue is of two types simple and compound epithelial tissue. Simple epithelium comprises a single layer of cells and functions as a protective lining for body cavities. The compound epithelium comprises more than one layer of cells and functions are limited to secretion and absorption.

Connective Tissue

This tissue is named connective tissue because of its specific function of supporting and linking with other tissues/organs of the body. It is the most abundant and widely distributed type of tissue in the body. It can be soft tissue or hard types like cartilage and bone. Connective tissue is of three types loose connective, dense connective, and specialized connective tissues. 

  • Loose connective tissue has loosely arranged cells and fibers in a semi-fluid ground substance.
  • Dense connective tissue is compact fibers and fibroblasts in which the orientation of fibers can be regular like in tendons and ligaments or irregular like in skin.
  • Specialized connective tissue is cartilage, bones, and blood. Blood is the fluid connective tissue that functions as the main circulating fluid. Bones are hard and non-pliable rich in calcium salts that function as a structural framework. Cartilage is solid, pliable, and resists compression.

Muscle Tissue

Muscles are made of long, cylindrical parallel arranged numerous fine fibers called myofibrils. Muscles are of three types skeletal, smooth and cardiac. 

  • Skeletal muscles: they are attached to skeletal bones.
  • Smooth muscles: these muscles are concerned with involuntary functioning organs.
  • Cardiac muscles are related to the heart muscle.

Neural Tissue

Neural tissue controls the body’s response system to changing external environments. Neurons are the basic unit of the nervous system which are excitable in nature and send information from one place to other. When stimuli are exerted an electrical disturbance travels from neuron to neuron.

Organ and Organ System

The organ system is known to be a biological system consisting of a group of organs working together to perform one or more functions. The basic tissue organizes to form organs which in turn organize to form the organ system. Each organ is made up of one or more than one basic tissue. The human organism includes eleven organ systems. In multicellular organisms, a combination of such organs will then interrelate with one another to form organ systems and perform various functions.

The organization of the body into tissues, organs, and organ systems is essential for the body to function in an effective manner. Each organ has a specific role in a plant or animal body. Some organs perform activities that directly participate in the functioning of more than one system. The organ system includes the Integumentary system, Skeletal system, Muscular system, Nervous system, Digestive system, and so on. The simple formation of the body of organisms includes the following step: Cell – tissue – organ – organ system – organisms.

Also Read: Level of organization


Common Indian earthworms are Pheretima and Lumbricus. The earthworm is a reddish-brown terrestrial invertebrate that inhabits the uppermost moist soil layer. 



Morphology of Earthworm

The body is divided into repeated segments called metameres about 100-120 in number. The body can be differentiated into three regions pre-clitellar (1st to 13th segment), clitellar (14th 15th 16th segment), and post-clitellar region (17th to the last segment). A single female genital pore is present at the mid-ventral line of the 14th segment and a pair of male genital pores are on the ventrolateral sides of the 18th segment.

Also, Read: Morphology of Earthworm

Anatomy of Earthworm

The epidermis is made up of single-layer columnar epithelial cells which contain secretory glands. A mouth open into the buccal cavity leads to a muscular pharynx. The small narrow tube esophagus continues to the muscular gizzard which helps in grinding then continues to the stomach. Then the alimentary canal leads its way to the intestine and at last opens to the exterior by a small aperture called the anus.


Brown or black-bodied animals that are included in class Insecta of phylum Arthropoda seen in the tropical regions have a body range from 1/4 inches to 3 inches.

Morphology of Cockroach

The body of the cockroach is divided into three regions head, thorax, and abdomen. A hard chitinous exoskeleton covers the entire body. The Head is triangular in shape and lies anteriorly at right angles to the longitudinal body axis. Thorax is divided into prothorax, mesothorax (first pair of wings), and metathorax (second pair of wings).

Anatomy of Cockroach

The alimentary canal is divided into three regions foregut, midgut, and hindgut. The mouth opens into a short tubular pharynx leading to a passage called the esophagus which opens into a sac-like structure called a crop used for storing food. The crop is followed by gizzards which have six chitinous plates called teeth. A ring of 6-8 blind tubules called hepatic or gastric caecae at the junction of the foregut and midgut. At the junction of the midgut and hindgut, Malpighian tubules are present. The hindgut is differentiated into the ileum, colon, and rectum. The blood vascular system of the cockroach is open type and open space where visceral organs are bathed in the blood are called hemocoel and the colorless plasma and hemocytes are called hemolymph.

Anatomy of Cockroach

Anatomy of Cockroach


Frogs can live on both lands and in freshwater and belong to the class Amphibia of phylum Chordata. The common species of frog found in India is Rana tigrina. Frogs are cold-blooded animals.



Morphology of Frog

The skin is smooth and slippery due to the presence of mucus. The body of the frog is divided into the head and trunk. A neck and tail are absent. A membranous tympanum receives sound signals. The forelimbs and hindlimbs help in swimming, walking, leaping, and burrowing. The hind limbs end in 5 digits and they are larger and more muscular than the forelimbs which end in 4 digits. Feet have webbed digits and frogs exhibit sexual dimorphisms. For more details about the Morphology of Frogs.

Anatomy of Frog

The alimentary canal is short as frogs are carnivores and hence the length of the intestine is small. The vascular system of frogs is a well-developed closed type involving the heart, blood vessels, and blood. Respiratory systems are cutaneous type in water and pulmonary type on land. The excretory system involves the elimination of nitrogenous waste by a pair of kidneys.  For more details about the Anatomy of Frogs.

FAQs on Structural Organisation in Animals

Question 1: What is the structural organization of an animal? 


Cells-Tissue-Organ-Organ System

Question 2: Define tissue.


Tissue is the segregation of similar cells that perform the same functions and follow the same cellular pattern.

Question 3:Name the five kingdoms classified by the Whittaker.


Monera, Protista, Fungi, Plantae,  Animalia 

Question 4: What is a tissue? And explain the tissue level of organization.


Tissue is a group of cells together is called tissue. The tissue level of organization is the level in between the levels for the formation of an organism and in that level, the cellular level turns into the tissue level and all the cells together and forming the tissues is the tissue level of organization.

Question 5: What is the organ level of the organization?


The organ level of organization is the level at which all the tissues are completely formed together to form an organ and at this level, some organs do the same thing to maintain the balance in the body(eg. metabolism, excretion, elimination, etc.) is organ level of organization.

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Last Updated : 03 Feb, 2023
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