Chapter 7: Structural Organisation in Animals | Summary

Frogs can live both on land and in freshwater and belong to class Amphibia of phylum Chordata. The most common species found in India is Rana tigrina. Frogs are poikilotherms (cold-blooded), meaning their body temperature varies with that of the environment.

Frogs have the ability to change their body colour to blend with their surroundings, a phenomenon called mimicry, which serves as a form of camouflage. During peak summer they undergo aestivation (summer sleep) and during winter they undergo hibernation (winter sleep), taking shelter in deep burrows to protect themselves from extreme temperatures.

The skin of a frog is smooth and slippery due to the presence of mucus and is always kept moist. The dorsal side is generally olive green with dark irregular spots, while the ventral side is uniformly pale yellow. The frog never drinks water but absorbs it through the skin. The body is divisible into head and trunk; neck and tail are absent.

A pair of nostrils is present above the mouth. The eyes are bulged and covered by a nictitating membrane for protection in water. A membranous tympanum on either side of the eyes receives sound signals. Forelimbs end in four digits and hind limbs in five digits; hind limbs are larger and more muscular and have webbed digits for swimming. Male frogs possess vocal sacs and a copulatory pad on the first digit of the forelimb, features absent in females, indicating sexual dimorphism.

The digestive system consists of the alimentary canal and digestive glands. Since frogs are carnivores, the alimentary canal is short. The pathway of food is: mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca → cloacal aperture. Food is captured by the bilobed tongue. HCl and gastric juices secreted by the stomach walls digest food, forming partially digested chyme which passes to the duodenum.

The duodenum receives bile from the gall bladder and pancreatic juice from the pancreas through a common bile duct. Bile emulsifies fats while pancreatic juice digests carbohydrates and proteins. Final digestion and absorption take place in the intestine via villi and microvilli. Undigested waste moves to the rectum and exits through the cloaca.

Frogs respire by two different methods depending on their environment. In water, the skin acts as the aquatic respiratory organ (cutaneous respiration) and dissolved oxygen is exchanged by diffusion. On land, the buccal cavity, skin and lungs all serve as respiratory organs. Respiration through the lungs is called pulmonary respiration.

The lungs are a pair of elongated, pink, sac-like structures present in the upper part of the trunk region (thorax). Air enters through the nostrils into the buccal cavity and then to the lungs. During aestivation and hibernation, gaseous exchange takes place exclusively through the skin.

The vascular system of frogs is a well-developed, closed type. The blood vascular system includes the heart, blood vessels and blood, while the lymphatic system consists of lymph, lymph channels and lymph nodes. The heart is a three-chambered muscular structure (two atria and one ventricle) covered by the pericardium. A triangular sinus venosus joins the right atrium and receives blood via the vena cava. The ventricle opens into the conus arteriosus on the ventral side.

Special venous connections exist between the liver and intestine (hepatic portal system) and between the kidney and lower body (renal portal system). Blood is composed of plasma, RBCs (nucleated erythrocytes containing haemoglobin), WBCs (leucocytes) and platelets. Lymph differs from blood in that it lacks a few proteins and RBCs. The heart pumps blood to carry nutrients, gases and water throughout the body.

The excretory system of the frog consists of a pair of kidneys, ureters, cloaca and urinary bladder. The kidneys are compact, dark red, bean-shaped structures located posteriorly in the body cavity on both sides of the vertebral column. Each kidney is composed of structural and functional units called uriniferous tubules or nephrons.

The frog excretes urea and is therefore a ureotelic animal. In male frogs, ureters act as urinogenital ducts opening into the cloaca. In females, the ureters and oviducts open separately into the cloaca. The urinary bladder is thin-walled, situated ventral to the rectum, and also opens into the cloaca. Excretory wastes are carried by blood to the kidneys where they are separated and excreted.

The control and coordination system of the frog includes both the neural system and endocrine glands. The nervous system is organised into the central nervous system (brain and spinal cord), peripheral nervous system (cranial and spinal nerves) and autonomic nervous system (sympathetic and parasympathetic). There are ten pairs of cranial nerves. The brain is enclosed in the cranium and divided into forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (optic lobes) and hind-brain (cerebellum and medulla oblongata).

The medulla oblongata continues into the spinal cord, which is enclosed in the vertebral column. Prominent endocrine glands in the frog include the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads. Chemical coordination of organ functions is achieved through hormones secreted by these glands.

Frogs possess different types of sense organs: organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with internal ears). Eyes and internal ears are well-organised structures, while the rest are cellular aggregations around nerve endings.

The eyes are a pair of spherical structures situated in orbits in the skull and are simple eyes (with only one unit). External ears are absent; only the tympanum is visible externally. The ear functions as both an organ of hearing and of balance (equilibrium).

Male reproductive organs consist of a pair of yellowish ovoid testes adhered to the upper part of the kidneys by a double fold of peritoneum called mesorchium. Vasa efferentia (10–12 in number) arise from testes, enter the kidneys, open into Bidder's canal and ultimately communicate with the urinogenital duct that opens into the cloaca.

Female reproductive organs include a pair of ovaries situated near the kidneys, with no functional connection to them. A pair of oviducts opening separately into the cloaca arise from the ovaries. A mature female can lay 2500–3000 ova at a time. Fertilisation is external, occurring in water. Development involves a larval stage called a tadpole, which undergoes metamorphosis to form the adult frog.

Frogs are beneficial to humans because they feed on insects and thus protect crops. They maintain ecological balance by serving as important links in food chains and food webs within ecosystems.

In some countries, the muscular legs of frogs are used as food by humans. The ecological role frogs play as both predators of insects and prey for larger animals makes them essential components of balanced ecosystems.

Chapter 7 establishes the principle of structural organisation in animals through the study of the frog, Rana tigrina. The hierarchical arrangement of cells into tissues, tissues into organs, and organs into organ systems enables division of labour, ensuring efficient functioning and survival of the whole organism. The frog exemplifies this perfectly, possessing well-developed digestive, circulatory, respiratory, excretory, nervous, endocrine, sensory and reproductive systems, each with distinct structures and coordinated functions.

From the CBSE examination perspective, this chapter is highly significant as it tests both factual recall and conceptual understanding of frog anatomy. Students must be thorough with the pathway of the alimentary canal, the three-chambered heart, portal systems, modes of respiration, excretory ureotelic nature, brain divisions, and reproductive differences between male and female frogs. Diagrams of the digestive system and reproductive systems are frequently asked in board exams.