Structural Organisation in Animals


Probably the best example of an amphibian that you remember right from your childhood is the frog. Did you know just like the butterfly, a frog also undergoes complete metamorphosis. From a tadpole that breathes with gills and has a tail to an amphibian with developed organ systems, it is a fascinating journey. Let us learn more about frogs.

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Frogs are amphibians, they can live both on land and in water. Their lives begin in water when they are eggs. From the eggs, tadpoles emerge, which also live in water. When these tadpoles fully develop they begin to live on land.

A frog is a poikilotherm, which means that it is a cold-blooded animal. The body temperature is not constant and varies according to their surrounding environment. So, when it is very cold, they lay in the sun to get warmer and when it is very hot, they get into the water bodies to cool themselves.

The habitat of a frog is quite varied and can be found all over the world, except maybe, of course, the Antarctic region. They are not marine animals. They can be found only in freshwater habitat. A frog goes into summer sleep called aestivation and winter sleep called hibernation, to escape the peak summer and winter conditions. The common Indian frog goes by the scientific name, Rana tigrina.

Morphology and Anatomy of Frog

Morphology of a Frog

The frog is a chordate, showing the characteristics of the Phylum Chordata. The body of the frog is divisible into head and trunk. Neck and tail are absent in a frog. The skin of the frog has mucus, which makes the skin moist, smooth and slippery. The skin of the frog has the ability to absorb water.

It is dark green in color with irregular spots on the dorsal side, while in the ventral side; it is pale yellow in color. The head of a frog is triangular in shape with a blunt snout. The eyes of a frog have a nictitating membrane that gives protection when the frog is in water. On the sides of the eyes, a membranous tympanum (ear) is present that receives the sound signals.

A frog has two forelimbs and two hind limbs. They help in walking, swimming, leaping as well as burrowing. Sexual dimorphism is seen in frogs, with the male species having vocal sacs and copulatory pad. These are absent in female frogs.

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Body Anatomy of a Frog

Inside the body cavity of a frog, there are many organ systems present such as the circulatory system, digestive system, respiratory system, nervous system, excretory system, and reproductive system. All the organs and organ systems are well developed with specific functions. Let us know more about these organ systems.

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Digestive System

It is well developed and has the alimentary canal and digestive glands. The alimentary canal of the frog is not long, but it is short.  The mouth that is present on the head opens into the buccal cavity that has the sticky tongue which is bi-lobed. The mouth opens into the pharynx and oesophagus.

This tube-like oesophagus opens into the stomach. It leads to the intestine and the rectum. The rectum opens through an opening called the cloaca. Liver and gallbladder organs are also present along with the pancreas. The liver secretes bile and the pancreas secrete the pancreatic juices.

Food is digested in the stomach by the action of HCL and other gastric juices that are secreted. The food is broken down into simpler substances and in the intestine, it is absorbed and assimilated. Any undigested food waste goes into the rectum and is passed out through the cloaca. In many ways, the functioning of the digestive system of a frog is similar to the human digestive system.

Morphology and Anatomy of Frog

Digestive System (Source – Pinterest)

Blood Vascular System

The circulatory system of a frog is very well developed. In fact, just like humans, frogs also have a lymphatic system. A closed circulatory system is seen in frogs. The different organs present in this organ system are the heart, blood vessels, and blood. Lymph, lymph nodes and channels make up the lymphatic system.

The heart has three chambers and is made up of muscle tissue. It is present in the upper part of the body. The three chambers of the heart consist of two atria and one ventricle. The pericardium covers the heart.

Blood is opaque and sticky and is red in color due to the presence of haemoglobin. It has the blood cells and plasma.  From the heart, the blood goes to all part of the body the arteries, which makes up the arterial system.

The venous system comprising of all the veins carries blood from all organs to the heart. In frogs, there is also a hepatic portal system and renal portal system, where there is a venous connection present in the organs of liver, intestine, and kidneys.

Morphology and Anatomy of Frog

Respiratory System

The skin of the frog has a role to play in the respiration process. It acts as the aquatic respiratory organ, as oxygen gets diffused through the skin when a frog is in water. Otherwise, when the frog is on land, respiration occurs through the lungs as well as the skin. The respiration through the skin is called cutaneous respiration and the respiration through the lungs is called pulmonary respiration.

There are present a pair of lungs which are elongated in shape and are sac-like structures. They are present in the trunk region of the body. Through the nostrils, air enters the buccal cavity and then reaches the lungs. When a frog goes into aestivation or hibernation, cutaneous respiration occurs.

Nervous system

The nervous system is very well organized and consists of the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord and the peripheral nervous system consists of cranial and spinal nerves. The autonomic nervous system has the sympathetic and parasympathetic nervous system.

The brain is enclosed in a cranium and there are ten pairs of cranial nerves that emerge out of the brain. The brain is divided into three parts – forebrain, midbrain, and hindbrain. The olfactory lobes, cerebral hemispheres, and unpaired diencephalon are included in the forebrain. A pair of optic lobes is present in the midbrain. Cerebellum and medulla oblongata are part of the hindbrain.

The spinal cord is enclosed in the vertebral column. There are different sense organs that present in a frog, which include sensory papillae, taste buds, nasal epithelium, eyes, and internal ears. The eyes and the ears are well developed.

Learn more about the Neural Tissue here in detail.

Endocrine System

Control and coordination are evolved to a higher extent in frogs. The nervous system along with the endocrine system achieves this. Hormones that are secreted by the endocrine glands help in the chemical coordination. Some of the endocrine glands include thymus, pituitary, thyroid, parathyroid, pineal body, pancreatic islets, gonads and adrenal glands.

Excretory System

It consists of a pair of kidneys, ureters, urinary bladder and cloaca. Kidneys are dark red in color and are present in the posterior region of the body. Several nephrons or uriniferous tubules are present in the kidney. These are the structural and functional units of the excretory system. Two ureters are present that come out of the kidney.

In male frogs, the ureters act as the urinogenital tract and open into the cloaca. But in female frogs, the ureters and cloaca open separately into the cloaca. The urinary bladder is thin-walled and opens into the cloaca. And frogs are ureotelic animals. They excrete urea.

Reproductive System

The male and the female reproductive systems are well developed. The male reproductive system includes the following organs, a pair of testes, Vasa efferentia (10-12 in number) and a urinogenital duct that opens into the cloaca. The cloaca is a chamber which is used to pass the fecal matter, urine as well as sperms to the outside.

The female reproductive system has a pair of ovaries, a pair of oviducts that open into the cloaca separately. A female frog can lay about 2500 to 3000 eggs at a time. External fertilization is seen in frogs and occurs in water.

Morphology and Anatomy of Frog

Female Reproductive System (source – Biozoomer)

Solved Questions For You

Q: How can you differentiate between a male frog and female frog?

Ans. The male frogs have a vocal sac externally. Apart from that internally also, the reproductive system is different in male and female frogs. The male frogs have a pair of testes and the female frogs have a pair of ovaries.

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