Introduction
The human body, like that of other multicellular organisms, is composed of various cells that form tissues, organs, and organ systems. This structural organisation allows for efficient division of labour, ensuring the survival and functionality of an organism. In this chapter, we explore the organ and organ system, focusing on the morphology and anatomy of frogs as a representative of vertebrates.
Organ and Organ System in Animals
Multicellular animals exhibit a hierarchical structural organisation, where cells group together to form tissues, which further combine to create organs. These organs function together as organ systems to perform specific physiological roles.
For example, the human heart consists of epithelial, connective, muscular, and neural tissues, working together to pump blood. Organ systems such as the digestive system, respiratory system, circulatory system, and nervous system coordinate to maintain body functions.
Frogs: Morphology and Anatomy
Frogs belong to the Class Amphibia under Phylum Chordata. The most commonly found species in India is Rana tigrina. Frogs are cold-blooded (poikilothermic), meaning their body temperature fluctuates with environmental conditions. They undergo aestivation (summer sleep) and hibernation (winter sleep) to survive extreme temperatures.
Morphology of Frogs
Skin: Smooth, slippery, and moist due to the presence of mucus.
Coloration: The dorsal side is olive green with irregular dark spots, while the ventral side is pale yellow.
Water Absorption: Frogs do not drink water but absorb it through their skin.
Body Structure: The body is divided into head and trunk (no neck or tail).
Nostrils: Positioned above the mouth for respiration.
Eyes: Bulged and protected by a nictitating membrane.
Ears: Tympanum (membrane covering external ears) receives sound signals.
Limbs: Forelimbs and hindlimbs are used for swimming, walking, leaping, and burrowing.
Sexual Dimorphism: Male frogs have vocal sacs and copulatory pads on their forelimbs, absent in females.
Anatomy of Frogs
The internal structure of frogs includes multiple organ systems, each performing distinct functions:
1. Digestive System
The alimentary canal is short since frogs are carnivorous, requiring less digestion time.
The mouth leads to the buccal cavity, followed by the pharynx, oesophagus, stomach, small intestine, and cloaca.
Liver and pancreas aid digestion by secreting bile and pancreatic juices.
Digestion occurs via HCl and gastric juices in the stomach.
Nutrients are absorbed through villi and microvilli in the intestine.
Undigested waste exits through the cloaca.
2. Respiratory System
Frogs exhibit dual-mode respiration:
Cutaneous respiration: Through the skin in water.
Pulmonary respiration: Through lungs on land.
Buccal respiration: Exchange of gases through the buccal cavity.
During aestivation and hibernation, respiration occurs entirely through the skin.
3. Circulatory System
Closed circulatory system with a three-chambered heart (two atria, one ventricle).
Blood circulation is achieved through arteries and veins.
Frogs possess a hepatic portal system and renal portal system.
Blood components: Red Blood Cells (RBCs), White Blood Cells (WBCs), and platelets.
RBCs are nucleated and contain hemoglobin.
Lymphatic system includes lymph, lymph nodes, and lymphatic vessels.
4. Excretory System
Ureotelic excretion: Frogs excrete urea as their primary nitrogenous waste.
The excretory system includes kidneys, ureters, cloaca, and urinary bladder.
In males, the ureters function as urinogenital ducts, while in females, the oviducts and ureters are separate.
5. Nervous System
Divided into Central Nervous System (CNS), Peripheral Nervous System (PNS), and Autonomic Nervous System (ANS).
Brain regions: Forebrain (cerebrum, diencephalon, olfactory lobes), Midbrain (optic lobes), Hindbrain (cerebellum, medulla oblongata).
Ten pairs of cranial nerves emerge from the brain.
6. Sense Organs
Frogs have well-developed sensory structures for detecting environmental stimuli:
Touch (Sensory papillae)
Taste (Taste buds)
Smell (Nasal epithelium)
Vision (Eyes with nictitating membrane)
Hearing (Tympanum and internal ears)
7. Reproductive System
Frogs have distinct male and female reproductive systems.
Males: A pair of testes produces sperm, which passes through vasa efferentia, kidneys, and urinogenital ducts before being released via the cloaca.
Females: A pair of ovaries produces ova, which travel through the oviducts into the cloaca.
Fertilization is external, occurring in water.
A female frog lays 2500–3000 eggs at a time.
The tadpole larva undergoes metamorphosis to develop into an adult frog.
Importance of Frogs in Ecosystem
Frogs act as natural pest controllers by consuming insects, thus protecting crops.
They serve as a crucial link in the food chain, maintaining ecological balance.
In some regions, frog legs are consumed as a delicacy.
Frequently Asked Questions (FAQs)
Q1: What is the structural organisation in animals?
A: The structural organisation in animals refers to the hierarchical arrangement of cells, tissues, organs, and organ systems to perform various functions efficiently.
Q2: What is the function of the cloaca in frogs?
A: The cloaca in frogs is a common chamber that receives digestive waste, excretory waste, and reproductive fluids before they exit the body.
Q3: What type of circulatory system do frogs have?
A: Frogs have a closed circulatory system with a three-chambered heart (two atria and one ventricle), ensuring oxygenated and deoxygenated blood are circulated efficiently.
Q4: How do frogs respire?
A: Frogs exhibit cutaneous respiration (through the skin in water), pulmonary respiration (through lungs on land), and buccal respiration (via the buccal cavity).
Q5: What is the role of the nictitating membrane in frogs?
A: The nictitating membrane is a transparent protective layer over the frog’s eyes, helping them see underwater while preventing damage.
Conclusion
The structural organisation of animals ensures efficient body functioning through the division of labour among cells, tissues, organs, and organ systems. Studying frogs as a representative organism provides deep insights into vertebrate anatomy and physiology. Understanding these concepts is crucial for competitive exams like NEET, UPSC, and Class 11 Biology NCERT syllabus.
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