Structural Organisation in Animals NEET Notes 2027: Key Points and PDF

Structural Organisation in Animals is Chapter 7 of NCERT Class 11 Biology and part of Unit 2, Structural Organisation in Animals and Plants, of the NEET UG syllabus. The rationalised chapter covers tissues, organs and organ systems, and the morphology and anatomy of the frog: digestive, respiratory, circulatory, excretory, nervous and reproductive systems, with a free notes PDF.

Structural Organisation in Animals NEET Notes

Section-wise key points from NCERT Chapter 7, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.

Structural Organisation in Animals at a Glance

The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.

The rationalised NCERT chapter has two sections: 7.1 Organ and Organ System and 7.2 Frogs. Older editions also described animal tissues, the earthworm and the cockroach in detail; those sections are no longer in the chapter.

How are tissues, organs and organ systems organised?

A tissue is a group of similar cells along with intercellular substances that performs a specific function.

• All complex animals have only four basic types of tissue: epithelial, connective, muscular and neural.
• Tissues are organised in a specific proportion and pattern to form organs such as the stomach, lung, heart and kidney. The heart contains all four tissue types.
• Two or more organs that perform a common function through physical and/or chemical interaction form an organ system, such as the digestive or respiratory system.
• Cells, tissues, organs and organ systems split up the work, which shows division of labour.
• The increasing complexity of organs and organ systems shows a discernible trend called the evolutionary trend.
• The body of Hydra has thousands of cells of each type; the human body has billions of cells.
• Morphology is the study of form or external features; in animals, anatomy is the study of the morphology of internal organs.

Animal Tissues: What the Chapter Says

• Epithelia are sheet-like tissues lining the body’s surface and its cavities, ducts and tubes.
• Epithelia have one free surface facing a body fluid or the outside environment, and their cells are structurally and functionally connected at junctions.

The NEET syllabus still lists animal tissues, but the rationalised chapter treats them only briefly. Revise the four tissue types in more detail from the notes PDF linked below.

Frog: Classification, Habitat and Adaptations

Frogs live both on land and in freshwater and belong to class Amphibia of phylum Chordata. The most common species in India is Rana tigrina, the Indian bullfrog.

• Frogs are cold-blooded (poikilotherms): their body temperature changes with the environment.
• They change colour to hide from enemies (camouflage); NCERT calls this protective coloration mimicry.
• They shelter in deep burrows during peak summer (aestivation, summer sleep) and winter (hibernation, winter sleep).

Morphology of the Frog

The frog’s body is divisible into a head and a trunk; a neck and a tail are absent.

• Skin is smooth and slippery because of mucus and is always kept moist.
• Dorsal side is olive green with dark irregular spots; ventral side is uniformly pale yellow.
• The frog never drinks water; it absorbs water through its skin.
• A pair of nostrils lies above the mouth. Bulging eyes are covered by a nictitating membrane that protects them in water.
• A membranous tympanum (ear) on either side of the eyes receives sound signals.
• Fore limbs end in four digits; hind limbs are larger and more muscular and end in five digits. Webbed digits help in swimming.
• Limbs help in swimming, walking, leaping and burrowing.

Digestive System of the Frog

The digestive system has an alimentary canal and digestive glands; the canal is short because frogs are carnivores.

• Path of food: mouth, buccal cavity, pharynx, oesophagus (short tube), stomach, intestine, rectum and cloaca.
• Food is captured by the bilobed tongue.
• HCl and gastric juices from the stomach wall digest food; the partly digested chyme passes to the duodenum.
• The duodenum receives bile from the gall bladder and pancreatic juice from the pancreas through a common bile duct.
• Liver secretes bile, which is stored in the gall bladder; bile emulsifies fat. Pancreatic juice digests carbohydrates and proteins.
• Final digestion happens in the intestine; villi and microvilli absorb digested food.
• Undigested solid waste moves into the rectum and passes out through the cloaca.

Respiration and Circulation in the Frog

Frogs respire on land and in water by different methods, and have a well-developed closed circulatory system.

• The lungs are a pair of elongated, pink, sac-like structures in the upper trunk (thorax). Air enters through the nostrils into the buccal cavity and then the lungs.

Circulatory System

• The blood vascular system is a well-developed closed type; frogs also have a lymphatic system of lymph, lymph channels and lymph nodes.
• The heart has three chambers, two atria and one ventricle, and is covered by the pericardium.
• A triangular sinus venosus joins the right atrium and receives blood through the vena cava. The ventricle opens into a sac-like conus arteriosus on the ventral side.
• Hepatic portal system: venous link between the liver and intestine. Renal portal system: venous link between the kidneys and the lower parts of the body.
• Blood has plasma and cells: RBCs, WBCs and platelets. RBCs are nucleated and contain haemoglobin.
• Lymph differs from blood: it lacks a few proteins and RBCs.
• The circulation is a single circulation.

Excretion, Coordination and Sense Organs of the Frog

The frog has a well-developed excretory system and highly evolved neural and endocrine control.

• Excretory system: a pair of kidneys, ureters, cloaca and urinary bladder.
• Kidneys are compact, dark red and bean-like, a little posterior in the body cavity on both sides of the vertebral column. Each is made of uriniferous tubules or nephrons.
• The thin-walled urinary bladder lies ventral to the rectum and opens into the cloaca.
• The frog excretes urea, so it is ureotelic.

Endocrine and Nervous System

• Endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.
• Nervous system: central (brain and spinal cord), peripheral (cranial and spinal nerves) and autonomic (sympathetic and parasympathetic).
• Ten pairs of cranial nerves arise from the brain, which is enclosed in a bony brain box (cranium).
• Fore-brain: olfactory lobes, paired cerebral hemispheres and unpaired diencephalon. Mid-brain: a pair of optic lobes. Hind-brain: cerebellum and medulla oblongata.
• The medulla oblongata passes out through the foramen magnum and continues as the spinal cord in the vertebral column.

Eyes and internal ears are well-organised structures; the other sense organs are cellular aggregations around nerve endings.

Reproductive System and Development of the Frog

Frogs have well-organised male and female reproductive systems, with external fertilisation and a tadpole larva.

• Male: 10 to 12 vasa efferentia arise from the testes, enter the kidneys and open into Bidder’s canal, which joins the urinogenital duct opening into the cloaca.
• The cloaca is a small median chamber that passes faecal matter, urine and sperms to the exterior.
• Female: a pair of oviducts arising from the ovaries opens separately into the cloaca.
• A mature female can lay 2500 to 3000 ova at a time.
• Fertilisation is external and takes place in water. Development has a larval stage, the tadpole, which undergoes metamorphosis into the adult.
• Frogs benefit humans by eating insects and protecting crops, and are an important link in food chains and food webs. In some countries their muscular legs are eaten.

Most-asked Facts for NEET: Structural Organisation in Animals

Revise these one-line facts from the NCERT chapter just before the exam.

• All complex animals have only four basic tissue types; the heart has all four.
• Frog: class Amphibia, phylum Chordata; common Indian species Rana tigrina.
• Frogs are poikilotherms; aestivation is summer sleep and hibernation is winter sleep.
• Frogs never drink water; they absorb it through the skin.
• Neck and tail are absent; eyes have a nictitating membrane.
• Fore limbs have four digits; hind limbs have five.
• Males have vocal sacs and a copulatory pad on the first digit of the fore limb.
• The tongue is bilobed and captures food.
• Cutaneous respiration in water and during aestivation and hibernation; pulmonary respiration on land.
• Heart: three chambers (two atria, one ventricle), sinus venosus and conus arteriosus.
• Frog RBCs are nucleated; circulation is closed and single.
• Hepatic portal and renal portal systems are both present.
• The frog is ureotelic.
• Ten pairs of cranial nerves; mid-brain has a pair of optic lobes.
• Testes are attached to the kidneys by the mesorchium; vasa efferentia are 10 to 12 and open into Bidder’s canal.
• A female lays 2500 to 3000 ova at a time; fertilisation is external; the larva is a tadpole.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Calling the frog warm-blooded. It is cold-blooded (poikilothermic).
• Giving the frog heart four chambers or two ventricles. It has two atria and one ventricle.
• Saying frog RBCs lack a nucleus. Frog RBCs are nucleated.
• Writing that frogs drink water. They absorb water through the skin.
• Calling the adult frog ammonotelic or uricotelic. NCERT describes it as ureotelic.
• Mixing up the ducts: in males the ureters act as urinogenital ducts; in females the ureters and oviducts open separately into the cloaca.
• Saying fertilisation is internal. It is external and takes place in water.
• Giving the frog 12 pairs of cranial nerves. It has ten pairs.
• Swapping the digits: fore limbs have four digits and hind limbs five.
• Revising earthworm and cockroach from old notes as if they were in the current chapter. The rationalised chapter and the NEET syllabus use the frog.

Structural Organisation in Animals Notes PDF and Related Pages

Download the AglaSem notes for Structural Organisation in Animals (key notes and important questions with answers) free: View PDF

This PDF was made for the older NCERT edition: it covers animal tissues and the cockroach in detail but has no frog section, so use the notes on this page for the frog.

• All chapters: NEET Biology notes
• Full topic list: NEET syllabus
• Practice: NEET question papers
• Source text: NCERT Chapter 7 (NCERT, Class 11 Biology)

NEET Biology Notes: All Chapters

Botany

• The Living World
• Biological Classification
• Plant Kingdom
• Cell: The Unit of Life
• Plant Growth and Development
• Anatomy of Flowering Plants
• Cell Cycle and Cell Division
• Respiration in Plants
• Photosynthesis in Higher Plants
• Morphology of Flowering Plants
• Principles of Inheritance and Variation
• Microbes in Human Welfare
• Sexual Reproduction in Flowering Plants
• Molecular Basis of Inheritance
• Organisms and Populations
• Ecosystem
• Biodiversity and Conservation

Zoology

• Animal Kingdom
• Biomolecules
• Breathing and Exchange of Gases
• Excretory Products and their Elimination
• Body Fluids and Circulation
• Structural Organisation in Animals
• Locomotion and Movement
• Reproductive Health
• Chemical Coordination and Integration
• Neural Control and Coordination
• Evolution
• Human Health and Disease
• Human Reproduction
• Biotechnology and its Applications
• Biotechnology: Principles and Processes

All chapters with their notes PDFs are on the NEET Biology notes page.

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