Structural Organisation in Animals – Class 11 NEET Notes

1.🧫Introduction to Animal Tissues

In the earlier chapters, we studied many kinds of animals, including those made of a single cell and those made of many cells. In single-celled organisms, one cell does everything, like digestion, breathing, and reproduction. But in multicellular animals, different groups of cells handle these jobs in an organized way. Even simple animals like Hydra have different types of cells, and each type can have thousands of cells. In humans, there are billions of cells, each performing specific functions. So, how do all these cells work together? In multicellular animals, similar cells group together with some substances between them to do a particular job, and this group is called a tissue. Surprisingly, all complex animals have only four basic types of tissues. These tissues are arranged in a particular way to form organs like the stomach, lungs, heart, and kidneys. When two or more organs work together to perform a common task, they form an organ system, like the digestive system or respiratory system. This way, cells, tissues, organs, and organ systems share the work, showing a division of labor, which helps the whole body survive.

2. Body Organization – Organs and Organ Systems

In multicellular organisms, basic tissues come together to form organs, and these organs work together to form organ systems. This kind of organization is very important because it allows millions of cells in the body to work more efficiently and in a well-coordinated way. Each organ in our body is made up of one or more types of tissues. For example, the human heart has all four main types of tissues: epithelial, connective, muscular, and nervous tissue. When we study organs and organ systems carefully, we notice that as we move from simpler to more complex animals, there is a clear pattern or trend in their complexity. Scientists call this an evolutionary trend, which you will learn more about in higher classes. In this chapter, the focus is on the frog, which is a vertebrate. Here, we study two things: morphology and anatomy. Morphology is the study of the form or outward appearance of an organism or its body parts. In plants or microbes, morphology usually just means external features, but in animals, it refers to the external appearance of organs or body parts. Anatomy, on the other hand, usually means the study of internal organs and their structure in animals. In this chapter, you will learn about both the external features (morphology) and internal structure (anatomy) of the frog.

3. Frogs – Structure and Characteristics

Frogs can live both on land and in freshwater and belong to class Amphibia
of phylum Chordata. The most common species of frog found in India is
Rana tigrina.
They do not have constant body temperature i.e., their body
temperature varies with the temperature of the environment. Such animals
are called cold blooded or poikilotherms. You might have also noticed
changes in the colour of the frogs while they are in grasses and on dry
land. They have the ability to change the colour to hide them from their
enemies (camouflage). This protective coloration is called mimicry. You
may also know that frogs are not seen during peak summer and winter.
During this period they take shelter in deep burrows to protect them
from extreme heat and cold. This is known as summer sleep (aestivation)
and winter sleep (hibernation) respectively.

Morphology Of Frog 🐸

If you have ever touched a frog, you must have noticed that its skin is smooth and slippery because it is covered with mucus. The skin always stays moist, which is very important for the frog. The back (dorsal) side of a frog is usually olive green with dark, irregular spots, while the belly (ventral) side is pale yellow. Interestingly, frogs do not drink water with their mouths; instead, they absorb water directly through their skin. The body of a frog can be divided into two main parts: the head and the trunk. Frogs do not have a neck or a tail. Above the mouth, there is a pair of nostrils, and the eyes are bulging and protected by a special membrane called the nictitating membrane, which covers the eyes while they are in water. On each side of the eyes, there is a membranous structure called the tympanum, which functions as the ear and helps them hear sounds. Frogs have forelimbs and hind limbs that help them swim, walk, jump, and dig. The hind limbs are larger and more muscular than the forelimbs and end in five toes, while the forelimbs have four fingers. The toes on the feet are webbed, which makes swimming easier. Frogs also show sexual dimorphism, meaning males and females look different. Male frogs have vocal sacs that produce sound and a special pad called the copulatory pad on the first finger of the forelimb, which female frogs do not have.

Anatomy Of Frog 🥰

The body of a frog has a body cavity that holds all the major organ systems, including the digestive, circulatory, respiratory, nervous, excretory, and reproductive systems, each with well-developed structures and functions. The digestive system includes the alimentary canal and digestive glands. Since frogs are carnivores, their alimentary canal is short because they eat easily digestible food. The mouth opens into the buccal cavity, which leads to the pharynx and then to the short oesophagus. The oesophagus opens into the stomach, which continues as the intestine and rectum, finally opening to the outside through the cloaca. The liver produces bile, which is stored in the gall bladder, while the pancreas produces pancreatic juice with digestive enzymes. Frogs catch their food using a bilobed tongue. Digestion begins in the stomach with hydrochloric acid and gastric juices, turning food into a semi-digested substance called chyme. Chyme then moves into the first part of the small intestine, the duodenum, which receives bile from the gall bladder and pancreatic juices from the pancreas. Bile helps break down fats, and pancreatic juices digest carbohydrates and proteins. Final digestion occurs in the intestine, where nutrients are absorbed by tiny finger-like structures in the inner wall called villi and microvilli. Undigested food moves into the rectum and is expelled through the cloaca.

Frogs breathe both in water and on land using different methods. In water, their skin acts as the main respiratory organ, and oxygen from water diffuses directly through the skin, a process called cutaneous respiration. On land, they use their skin, buccal cavity, and lungs for breathing. Lung-based breathing is called pulmonary respiration. The lungs are a pair of elongated, pink, sac-like organs located in the upper trunk (thorax). Air enters through the nostrils, passes into the buccal cavity, and then reaches the lungs. Even during periods of aestivation (summer sleep) and hibernation (winter sleep), gaseous exchange continues through the skin, allowing them to survive when inactive.

The frog has a well-developed closed circulatory system and also a lymphatic system. The blood circulatory system includes the heart, blood vessels, and blood, while the lymphatic system has lymph, lymph vessels, and lymph nodes. The heart is muscular, located in the upper part of the body cavity, and has three chambers: two atria and one ventricle. It is covered by a membrane called the pericardium. A triangular structure called the sinus venosus joins the right atrium, receiving blood through major veins called vena cava. The ventricle connects to a sac-like conus arteriosus on the ventral side of the heart. Arteries carry blood from the heart to all body parts, and veins collect blood from the body back to the heart. Frogs also have special venous connections: the hepatic portal system links the liver and intestine, and the renal portal system connects the kidneys and lower body. Blood consists of plasma and cells, including red blood cells (RBCs) with hemoglobin, white blood cells (WBCs), and platelets. Lymph is different from blood as it lacks RBCs and some proteins. Blood transports nutrients, gases, and water throughout the body, pumped by the muscular heart.

The excretory system of frogs is well-developed and removes nitrogenous wastes. It includes a pair of kidneys, ureters, urinary bladder, and cloaca. Kidneys are bean-shaped, dark red, and located along the vertebral column. Each kidney has functional units called nephrons. In males, ureters act as urinogenital ducts and open into the cloaca, while in females, ureters and oviducts open separately into the cloaca. The urinary bladder is thin-walled and ventral to the rectum. Frogs excrete urea, making them ureotelic animals. Blood carries wastes to the kidneys, where they are filtered and excreted.

Frogs have a highly developed system for control and coordination, including both the nervous system and endocrine glands. Hormones from endocrine glands control chemical coordination in the body. Major endocrine glands in frogs include the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenal glands, and gonads. The nervous system includes 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, and the brain is divided into the forebrain, midbrain, and hindbrain. The forebrain has olfactory lobes, paired cerebral hemispheres, and unpaired diencephalon. The midbrain has optic lobes, and the hindbrain has the cerebellum and medulla oblongata, which continues into the spinal cord protected by the vertebral column.

Frogs have various sense organs for touch, taste, smell, vision, and hearing. Eyes are spherical, located in the skull’s orbit, and simple in structure. The external ear is absent, but the tympanum functions for hearing and balance. Male and female frogs have well-organized reproductive systems. Males have a pair of yellowish, oval testes attached to the upper kidneys. Sperm passes through vasa efferentia into Bidder’s canal and then into the urinogenital duct, which opens into the cloaca, a chamber for faeces, urine, and sperm. Females have a pair of ovaries near the kidneys with no connection to them. Oviducts from the ovaries open separately into the cloaca. A mature female can lay 2,500–3,000 eggs at a time. Fertilization is external in water, and development includes a larval stage called the tadpole, which later undergoes metamorphosis to become an adult frog.

Frogs are very useful to humans and the ecosystem. They eat insects, protecting crops, and maintain ecological balance as an important link in the food chain and food web. In some countries, people even eat frog legs as food.

4. Overview Of This Chapter 😍

In multicellular animals, cells, tissues, organs, and organ systems share work in a coordinated way, ensuring the survival of the whole body. This division of labor allows the body to function efficiently. A tissue is a group of similar cells along with intercellular substances that perform one or more specific functions. Epithelial tissues are sheet-like and line the surfaces of the body, as well as cavities, ducts, and tubes. These tissues have a free surface facing body fluids or the external environment, and their cells are connected through junctions for structural and functional support. The common frog in India is the Indian bullfrog, Rana tigrina. Its body is covered with skin containing mucous glands, which are rich in blood vessels and help in respiration both in water and on land. The frog’s body is divided into the head and trunk. It has a muscular tongue that is bilobed at the tip, used to catch prey. The digestive system consists of the oesophagus, stomach, intestine, and rectum, all opening into the cloaca, and includes digestive glands like the liver and pancreas. Frogs breathe through their skin in water and through lungs on land. Their circulatory system is closed with single circulation, and their red blood cells are nucleated. The nervous system is divided into central, peripheral, and autonomic parts. The excretory and reproductive systems share the urinogenital ducts, which open into the cloaca. Male frogs have a pair of testes, and females have a pair of ovaries, with each female capable of laying 2,500–3,000 eggs at a time. Fertilization and early development occur externally in water, where eggs hatch into tadpoles, which later undergo metamorphosis to become adult frogs.