Structural Organization In Animals
In India, the most common species of frog is Rana tigrina. The frogs are cold-blooded or poikilotherms. They have the ability to camouflage. The frogs also show mimicry as a tool for protection. During summers, the frogs live in summer sleep (aestivation) and during winters, they live in winter sleep (hibernation).
The skin of frog is moist and slippery due to the presence of mucus. The dorsal side of body is usually olive green with dark irregular spots. The skin on the ventral side is uniformly pale yellow.
The body of a frog is divisible into head and trunk. A pair of nostrils is present above the mouth. Eyes are bulged and covered by a nictitating membrane. These membranes protect the eyes while the frog is under water. Ears are represented by membranous tympanum on either side of the eyes.
The forelimbs and hind limbs help in swimming, walking, leaping and burrowing. The hind limbs have five digits. The hind limbs are large and more muscular than forelimbs. The forelimbs have four digits. Webbed digits help in swimming. Sexual dimorphism is present in frogs. Sound producing vocal sacs and a copulatory pad (on the first digit of the fore limb) are present in male frogs.
Digestive System: The alimentary canal is short because frogs are carnivorous. The alimentary canal is composed of buccal cavity, pharynx, oesophagus, stomach, intestine and rectum. The rectum opens out by cloaca. Liver produces bile and the pancreas produces pancreatic juice. Digestive enzymes are present in the pancreatic juice. The bilobed tongue helps in capturing prey.
Digestion: Gastric juices and HCl are secreted in the stomach; where partial digestion of food takes place. Bile and pancreatic juice are received in the duodenum. Bile emulsifies fat. Pancreatic juices digest carbohydrates and protein. Final digestion takes place in the intestine.
Absorption: Numerous finger-like folds are present in the inner wall of intestine. These are called villi and microvilli and facilitate absorption of food. The undigested food goes to the rectum from where it is expelled out through cloaca.
Respiration: Frogs respire through lungs when they are on land. The exchange of gases takes place through skin when the frog is in water. The lungs are a pair of elongated, pink-coloured, sac-like structures. Lungs are present in the upper part of the thorax. During aestivation and hibernation, exchange of gases takes place through skin.
Blood Vascular System: The vascular system is closed type and is well developed. Lymphatic system is also present in frogs. The blood vascular system of frog is composed of a heart, blood vessels and blood. The lymphatic system consists of lymph, lymph channels and lymph nodes.
Heart: The heart is situated in the upper part of the body cavity. There are three chambers in the heart of frog. There are two atria and one ventricle. The heart is covered by a membrane; called pericardium. A triangular structure; called sinus venosus; joins the right atrium. Blood from the vena cava reaches the sinus venosus. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart.
Arteries and Veins: Arteries carry blood from the heart to all parts of the body. The veins carry blood from all parts of the body to the heart. Hepatic portal system and renal portal system are present in frogs. The hepatic portal system is a system of special venous connection between liver and intestine. The renal portal system is a system of special venous connection between the kidneys and the lower parts of the body.
Blood: The blood is composed of plasma and cells. RBCs and WBCs and platelets are present in the blood of frogs. RBCs are nucleated and contain haemoglobin. Lymph lacks few proteins and RBCs and hence is different from blood.
Excretory System: The excretory system is composed of a pair of kidneys, ureters, cloaca and urinary bladder. The kidneys are compact, dark red and bean-like structures. The kidneys are situated a little posteriorly in the body cavity; on both sides of the vertebral column.
Each kidney is composed of several nephrons. Two ureters emerge from the kidneys in the male frogs. In males, the ureters act as urogenital duct and opens into the cloaca. In females, the ureters and oviduct open separately in the cloaca. The frog is a ureotelic animal.
Control & Coordination: Frog has a highly evolved neural system and endocrine glands.
Coordination By Hormones: Hormones are secreted by various endocrine glands and facilitate chemical coordination. The main endocrine glands in frog are; pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenal and gonads.
Nervous System: The nervous system is organized into central nervous system, peripheral nervous system and autonomic nervous system.
Central Nervous System: The central nervous system is composed of the brain and the nerve cord. The brain is enclosed in a bony structure; called brain box or cranium. The brain is divided into forebrain, midbrain and hindbrain.
Forebrain includes olfactory lobes, paired cerebral hemispheres and unpaired diencephalon.
A pair of optic lobes is present in the midbrain.
The hindbrain consists of cerebellum and medulla oblongata. The medulla passes out through the foramen magnum and continues into the spinal cord. The spinal cord is enclosed in the vertebral column.
Ten pairs of cranial nerves arise from the brain.
Sense Organs: Organs of touch (sensory papillae), taste buds, olfactory receptors (in nasal epithelium), eyes and internal ears are the sense organs of frog. The eyes and internal ears are well developed, but the rest of the sense organs are cellular aggregations around nerve endings.
Frogs have simple eyes. The ear also serves as the organ of balancing (equilibrium).
Male Reproductive Organs: The male reproductive system of frog is composed of a pair of yellowish ovoid testes. The testes are adhered to the upper part of kidneys by a double fold of peritoneum called merorchium. There 10 – 12 vasa efferentia arising from the testes. They enter the kidneys on their side and open into Bidder’s canal. Finally, it communicates with the urinogenital duct which comes out of the kidneys. The urogenital ducts open into the cloaca. The cloaca is used to pass faecal matter, urine and sperms to the exterior.
Female Reproductive Organs: There is a pair of ovaries which are situated near kidneys. There is no functional connection between the ovaries and the kidneys. The oviducts open separately into the cloaca. A female frog can lay 2500 to 3000 ova at a time.
Fertilization: Fertilisation is external and takes place in water. Development is indirect and the larva is called tadpole.
Significance for Humans: Frogs eat insects and thus protect the crops. Frogs serve as an important link in the food chain and hence maintain the ecosystem. Frog meat is used as delicacy in some countries.
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