Structural Organisation in Animals

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 7.

A frog that never drinks, breathes through its skin and lays 3,000 eggs

A frog sitting at the edge of a pond looks simple. Yet it takes in water without drinking, breathes in three different ways, has a heart with only three chambers, and, in the male, passes sperm, urine and faeces out through the same opening. Every one of those oddities is an exam question. This chapter dissects one animal, the frog, to show how an animal body is organised, and each strange fact has a reason once you see the frog's lifestyle: half in water, half on land.

The ladder of organisation

In a multicellular animal, similar cells together with the material between them form a tissue, and a tissue does one specific job. Tissues combine in a fixed pattern to make an organ. Two or more organs working together on a common job, by physical and chemical interaction, form an organ system. The ladder is cell, tissue, organ, organ system, organism.

Every complex animal is built from only four basic tissues. Here is what each one does.

Why the heart is the standard example. The heart is an organ that contains all four: epithelial lining, connective tissue holding it together, cardiac muscle that pumps, and neural tissue that regulates the beat. One organ, four tissues.

Two words need to be kept apart. Morphology is the external appearance of organs or body parts. Anatomy is the study of the morphology of internal organs.

Meet the frog

Frogs belong to class Amphibia of phylum Chordata. The common Indian frog in the textbook is Rana tigrina. A frog lives both in water and on land, and it is poikilothermic, meaning its body temperature changes with the surroundings (cold blooded). It can change its skin colour to match the background and hide from enemies, which is called mimicry (protective camouflage). To survive extremes, a frog retreats into deep burrows: in summer this is aestivation (summer sleep) and in winter it is hibernation (winter sleep).

Beyond the textbook. For the exam: write Rana tigrina and call the colour-matching mimicry. In reality: the Indian bullfrog is now classed as Hoplobatrachus tigerinus, and blending into the background is camouflage in the strict sense, because mimicry usually means copying another species. Why it matters: a newer paper or news report may use the new name, and the exam wording and the strict wording are different things.

What a frog looks like

The skin is smooth, moist and slippery because of mucus. The back (dorsal side) is olive green with dark irregular spots, and the belly (ventral side) is pale yellow. A frog never drinks water. It absorbs it through the skin, which is why the skin must stay moist.

The body has only a head and a trunk. There is no neck and no tail. The eyes bulge out and are covered by a nictitating membrane that protects them in water. On each side of the eyes is a tympanum (ear drum) that receives sound. The limbs are for swimming, walking, leaping and burrowing. The forelimbs have four digits and the larger, more muscular hind limbs have five webbed digits, and the webbing is what makes swimming efficient.

Pause and predict: a frog sits in a pond all day and never drinks. How does it keep its body supplied with water?

Check your answer: it absorbs water through its skin. That is also why frog skin is thin and kept moist by mucus.

Male and female frogs differ in two ways, a pattern called sexual dimorphism.

Digestion: a carnivore's short gut

The frog is a carnivore, and meat is easy to digest, so its alimentary canal is short. Food follows this route.

The route in one line: mouth → buccal cavity → pharynx → oesophagus → stomach → intestine (first part = duodenum) → rectum → cloaca → outside

A bilobed tongue, attached at the front and free at the back, flicks out to catch insects. In the stomach, the walls secrete HCl and gastric juice, and the partly digested food (chyme) moves into the duodenum. Here two secretions arrive through a common bile duct: bile from the gall bladder and pancreatic juice from the pancreas. Bile does not break fat down chemically, it emulsifies it into tiny droplets, and pancreatic juice digests carbohydrates and proteins. Final digestion happens in the intestine, whose inner wall is folded into finger-like villi and microvilli that absorb the digested food. Undigested waste goes to the rectum and out through the cloaca.

Be exact about the bile route: the liver secretes bile, the gall bladder stores it, and the common bile duct carries it to the duodenum.

Respiration: three routes, chosen by where the frog is

A frog breathes differently depending on its situation.

The lungs are a pair of elongated, pink, sac-like organs in the upper part of the trunk (thorax). Air enters through the nostrils into the buccal cavity and then to the lungs. Remember that on land the frog uses skin, buccal cavity and lungs together. It is not "lungs only".

Beyond the textbook. For the exam: adult frogs have skin, buccal cavity and lungs, and skin works alone in water and during aestivation and hibernation. In reality: frogs do not have gills as adults, but the tadpole does, and it breathes through them until metamorphosis. Why it matters: it explains why the same species can live fully underwater early in life and half on land later.

Circulation: a closed system with three chambers

The vascular system is closed, so blood stays in vessels. The heart is muscular, has three chambers (two atria and one ventricle) and is enclosed in a membrane, the pericardium. A triangular sinus venosus joins the right atrium and receives blood from the vena cava. The ventricle opens into a sac-like conus arteriosus on the ventral side, from which arteries carry blood away, while veins bring it back.

The frog has two special venous links, called portal systems. The hepatic portal system connects the intestine and the liver, and the renal portal system connects the lower parts of the body with the kidneys. Blood from the gut thus goes through the liver first, and blood from the hind limbs goes through the kidneys first, before it returns to the heart.

Blood is plasma plus cells: RBCs, WBCs and platelets. Unlike human RBCs, frog RBCs are nucleated and contain haemoglobin. The frog also has a lymphatic system of lymph, lymph channels and lymph nodes. Lymph is similar to blood but lacks a few proteins and the RBCs.

Beyond the textbook. For the exam: the frog heart has two atria and one ventricle. In reality: oxygen-rich blood from the lungs and skin and oxygen-poor blood from the body both reach the single ventricle, so mixing is possible, and it is kept low by the timing of contraction and a spiral valve in the conus arteriosus. Why it matters: this is why textbooks call the frog heart "partly separated" rather than "completely mixed", and it explains why a four-chambered heart is more efficient for animals with high energy needs.

Excretion: kidneys, urea and a shared exit

The excretory system has a pair of kidneys, ureters, a urinary bladder and the cloaca. The kidneys are compact, dark red and bean shaped, lying a little behind the middle of the body cavity on both sides of the backbone. Each kidney is made of many structural and functional units called uriniferous tubules (nephrons). The thin-walled bladder lies on the ventral side of the rectum and opens into the cloaca.

Here the sexes differ. In males, the ureters also carry sperm, so they act as urinogenital ducts opening into the cloaca. In females, the ureters and the oviducts open separately into the cloaca. Blood carries nitrogenous waste to the kidneys, where it is separated and excreted as urea, which makes the frog ureotelic.

Beyond the textbook. For the exam: the frog is ureotelic. In reality: this is true of the adult. The aquatic tadpole mainly excretes ammonia, and the switch to urea comes with metamorphosis, as the animal spends more time away from water. Why it matters: it shows that excretory mode follows habitat, which is a pattern you will meet again in the excretion chapter.

Control and coordination

The frog's nervous system has three parts: central (brain and spinal cord), peripheral (cranial and spinal nerves) and autonomic (sympathetic and parasympathetic). The brain sits inside the cranium and has ten pairs of cranial nerves arising from it.

The medulla oblongata leaves the skull through the foramen magnum and continues as the spinal cord. The endocrine glands are the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.

The sense organs are tied to the frog's two worlds.

Eyes and internal ears are well-organised organs. The rest are cellular aggregations around nerve endings. There is no external ear, only the tympanum is visible, and the ear works for balance as well as hearing.

Reproduction: from tadpole to frog

The two sexes compare as follows.

The cloaca is a small median chamber that passes faeces, urine and sperm to the exterior. A mature female lays 2,500 to 3,000 ova at a time, and fertilisation is external, in water. The embryo develops into a larva, the tadpole, which undergoes metamorphosis to become the adult.

Pause and predict: a frog lays thousands of eggs, but a bird lays only a few. What does fertilisation outside the body tell you about the odds for each egg?

Check your answer: with external fertilisation and no parental care, many eggs are lost to predators and water, so the female produces thousands to ensure that some survive.

Why frogs matter. Frogs eat insects and so protect crops. They are also a link in food chains and food webs, which makes them important for ecological balance.

Exam traps

60-second revision

Where this chapter connects

• Previous: Anatomy of Flowering Plants did the same job for plants: how tissues build organs.
• Next: Cell: The Unit of Life drops one level down to the cell, the unit that every tissue here is made from.
• Related: Animal Kingdom places amphibians among chordates. Body Fluids and Circulation, Excretory Products and their Elimination and Neural Control and Coordination give the human versions of the systems you just met in the frog.