Frog: Morphology and Anatomy

NCERT grounding

NCERT Class XI Biology, Chapter 7, treats the frog as the vertebrate counterpart of the earthworm and cockroach — the case study used to introduce the morphology (external form) and anatomy (internal organs) of a chordate. Section 7.2 specifies the species as Rana tigrina, the common Indian frog, and places it in class Amphibia of phylum Chordata. Sub-sections 7.2.1 Morphology and 7.2.2 Anatomy then move methodically from skin and external features to the digestive, respiratory, vascular, excretory, nervous and reproductive systems.

The frog is a cold-blooded (poikilotherm) animal whose body temperature varies with the temperature of the environment. It changes colour to merge with its surroundings — a protective coloration called mimicry — and during peak summer and winter it takes shelter in deep burrows, behaviours called aestivation (summer sleep) and hibernation (winter sleep) respectively. This page treats each anatomical system in the order NCERT presents it, going deeper than the chapter article on the structure-level details NEET targets.

External morphology of Rana tigrina

The skin of a frog is smooth and slippery because of mucus secreted by cutaneous glands; it is always maintained in a moist condition. The dorsal surface is generally olive green with dark irregular spots and the ventral surface is uniformly pale yellow. A frog never drinks water; it absorbs water through the skin. The body is divisible into two regions — a head and a trunk. There is no neck and no tail, the second feature distinguishing the adult from its tadpole.

Head: nostrils, eyes, tympanum

Above the mouth lies a pair of nostrils. The eyes are bulged and are covered by a transparent nictitating membrane that draws across the eye while the frog is in water and protects it. On either side of the eyes a circular, drum-like tympanum (the membranous ear) lies flush with the skin and receives sound signals — the external ear is absent. Inside the mouth a bilobed tongue is attached at the front and is flicked out to capture prey; this single feature is responsible for the frog's role as an insect-eating predator.

Limbs and sexual dimorphism

The forelimbs and hind limbs together support swimming, walking, leaping and burrowing. The hind limbs are larger and muscular and end in five digits; the digits are joined by skin webs that act as paddles in water. The forelimbs are smaller and end in four digits. Frogs exhibit clear sexual dimorphism: male frogs carry two diagnostic structures absent in the female.

Digestive system

The digestive system of the frog consists of an alimentary canal and a set of digestive glands. The alimentary canal is short because the frog is a carnivore — a short gut suffices for a protein-rich diet, so the length of the intestine is reduced compared with that of herbivores. Food is captured by the bilobed tongue, which is flipped out and pulled back with the prey adhering to it.

Digestive glands — liver and pancreas

Two major digestive glands serve the gut. The liver is large and bilobed; it secretes bile, which is stored in the gall bladder and released into the duodenum to emulsify fat. The pancreas is a digestive gland that produces pancreatic juice; the juice contains enzymes that digest carbohydrates and proteins. Bile and pancreatic juice reach the duodenum through a common bile duct. Partially digested food (chyme) passes from the stomach into the first part of the small intestine, the duodenum; final digestion takes place in the intestine, and the digested products are absorbed through the numerous finger-like folds called villi and microvilli on the inner intestinal wall. Undigested solid waste moves into the rectum and passes out through the cloaca.

Respiratory system

The frog has the rare distinction of respiring by two different methods at two different stations of its life. In water, the moist, vascular skin acts as the aquatic respiratory organ — dissolved oxygen in the water is exchanged through the skin by simple diffusion in a process called cutaneous respiration. On land, three structures act as respiratory organs in concert: the buccal cavity lining (buccopharyngeal respiration), the skin, and a pair of lungs. Respiration by the lungs is called pulmonary respiration.

The lungs are a pair of elongated, pink-coloured, sac-like structures lying in the upper part of the trunk (the thorax). Air enters through the nostrils into the buccal cavity and is forced down to the lungs by floor-of-mouth contractions — the frog has no rib cage and no diaphragm, so true negative-pressure breathing is absent. During aestivation and hibernation, when the frog is buried in burrows and the lungs are inactive, gaseous exchange takes place exclusively through the skin. This is why the skin must be kept continuously moist by mucus and the cutaneous capillary bed must remain richly vascularised.

Circulatory system

The vascular system of the frog is a well-developed closed type. The frog also has a lymphatic system. The blood vascular system involves the heart, blood vessels and blood; the lymphatic system consists of lymph, lymph channels and lymph nodes. The heart is a muscular structure lying in the upper part of the body cavity and is enclosed in a membrane called the pericardium. It has three chambers — two atria and one ventricle.

Sinus venosus, conus arteriosus and the portal systems

A triangular structure called the sinus venosus joins the right atrium and receives deoxygenated blood through the major veins called vena cava. The ventricle, on contracting, opens into a sac-like conus arteriosus (also called the truncus arteriosus) on the ventral side of the heart, from which the arterial system distributes blood to all parts of the body. The veins collecting blood from the body and returning it to the heart form the venous system. In addition to the standard arterio-venous loop, the frog has two special portal systems — venous circuits that pass through a second capillary bed before returning to the heart.

Blood and lymph

The blood is composed of plasma and cells. The blood cells are RBCs (erythrocytes), WBCs (leucocytes) and platelets. Unlike mammalian erythrocytes, the RBCs of the frog are nucleated and contain the red-coloured pigment haemoglobin. Lymph is different from blood — it lacks a few proteins and RBCs. The blood carries nutrients, gases and water to the respective sites during circulation; the circulation of blood is achieved by the pumping action of the muscular heart. Because the ventricle is single, oxygenated and deoxygenated streams partially mix — a defining feature of amphibian "single circulation" (the NCERT summary explicitly notes "circulatory system is closed with single circulation").

Excretory system

The elimination of nitrogenous wastes is carried out by a well-developed excretory system that consists of a pair of kidneys, two ureters, the cloaca and a urinary bladder. The kidneys are compact, dark red and bean-like structures situated a little posteriorly in the body cavity on either side of the vertebral column; they belong to the mesonephric type. Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons.

Two ureters emerge from the kidneys. In male frogs the ureters act as urinogenital ducts — they carry both urine and sperms and open into the cloaca. In females, the ureters and the oviduct open separately into the cloaca. The thin-walled urinary bladder is present ventral to the rectum and also opens into the cloaca. The frog excretes urea and is therefore a ureotelic animal. Excretory wastes are carried by the blood into the kidney where they are separated and excreted.

Nervous system and sense organs

The system for control and coordination is highly evolved in the frog and includes both the neural system and a set of endocrine glands. The chemical coordination of various organs is achieved by hormones secreted by the endocrine glands. The prominent endocrine glands of the frog are the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads. The nervous system is organised into a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves) and an autonomic nervous system (sympathetic and parasympathetic).

There are ten pairs of cranial nerves arising from the brain. The brain is enclosed in a bony structure called the brain box (cranium) and is divided into three regions. The forebrain contains the olfactory lobes, the paired cerebral hemispheres and an unpaired diencephalon; the midbrain is characterised by a pair of optic lobes; and the hindbrain consists of the cerebellum and the medulla oblongata. The medulla oblongata passes out through the foramen magnum and continues as the spinal cord, which is enclosed in the vertebral column.

Sense organs

The frog has five types of sense organs — touch (sensory papillae on the skin), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with internal ears). Of these, only the eyes and the internal ears are well-organised structures; the rest are cellular aggregations around nerve endings. The eyes are a pair of spherical structures situated in the orbit in the skull and are simple eyes — possessing only one optical unit (in contrast to the compound eye of the cockroach). An external ear is absent; only the tympanum can be seen externally. The ear is an organ of both hearing and balancing (equilibrium).

Reproductive system

The frog has well-organised male and female reproductive systems. The male reproductive organs consist of a pair of yellowish ovoid testes, which are found adhered to the upper part of the kidneys by a double fold of peritoneum called the mesorchium. Vasa efferentia are 10–12 in number; they arise from the testes, enter the kidneys on their own side and open into Bidder's canal. Finally Bidder's canal communicates with the urinogenital duct that comes out of the kidney and opens into the cloaca. The cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms to the exterior through the cloacal aperture.

The female reproductive organs include a pair of ovaries situated near the kidneys; there is no functional connection between the ovaries and the kidneys. A pair of oviducts arising from the ovaries open into the cloaca separately (not as a single common duct). A mature female can lay 2,500 to 3,000 ova at a time. Fertilisation is external and takes place in water during amplexus, when the male grips the female with the nuptial pad on his first forelimb digit and releases sperms over the eggs as they are shed. Development involves a free-swimming, gilled larval stage — the tadpole — which subsequently undergoes metamorphosis to form the adult frog.

Worked examples

Common confusion & NEET traps