Structural Organisation in Animals: common doubts, answered
The questions students ask most often about Structural Organisation in Animals, each with a short answer. For the full chapter, read the Structural Organisation in Animals notes.
About the chapter
Why is frog considered a good example of the amphibian life mode?
A frog has a dual way of life, living both in water and on land, which is the meaning of amphibian. It has moist skin for gas exchange, external fertilisation in water, and a larval stage, the tadpole, that breathes in water. These features tie it to water, and the common Indian species is Rana tigrina.
Tissues, organs and organ systems
Read this section in the notes →What are the four basic types of animal tissue?
The four basic tissues are epithelial, connective, muscular and neural tissue. All complex animals are built from these. Even a single organ like the heart contains all four, with muscle for contraction, connective tissue for support, epithelium for lining, and neural tissue to control the activity. Tissues combine to form organs, and organs form organ systems.
What is an organ system and what is division of labour?
An organ system is a group of two or more organs that perform a common function through physical or chemical interaction. Division of labour is the sharing of work, so that each organ or system performs a particular task. It makes the body efficient, because no single part has to do everything on its own.
The frog and its way of life
Read this section in the notes →What is the difference between aestivation and hibernation in the frog?
Aestivation is the summer sleep and hibernation is the winter sleep. In both the frog retreats to a deep burrow to avoid extremes of temperature. During this time it takes in oxygen only through its skin, since lungs are not used. Do not confuse this with aestivation in flowers, which means arrangement of petals in a bud.
Why does a frog never drink water?
A frog absorbs water through its moist, mucus-covered skin, so it never needs to drink. The skin is thin and permeable, which also allows gas exchange. The frog is a poikilotherm, so its body temperature changes with the surroundings, and the skin changes colour to help it hide from enemies by camouflage.
External features of the frog
Read this section in the notes →How many digits are there on the fore and hind limbs of a frog?
The fore limb has four digits and the hind limb has five. The hind limb is longer, muscular and webbed, and it is used for leaping and for swimming, whereas the shorter fore limb helps in support and landing. Students often reverse the digit numbers, so remember that the hind limb is the one with five.
How do you distinguish a male frog from a female frog externally?
Male frogs have sound-producing vocal sacs and a copulatory pad on the first digit of each fore limb. Females lack both of these. The vocal sacs are used to make the calls of the breeding season. The body is divided into head and trunk, with no neck and no tail.
Digestive system
Read this section in the notes →Why is the alimentary canal of a frog short?
The frog is carnivorous, and a carnivore's food is easier to digest than plant material, so a short gut is enough, with a short intestine ending in the rectum and cloaca. The frog has a bilobed tongue that is flipped out to catch insects. Bile and pancreatic juice enter the duodenum together through a common bile duct.
Respiration
Read this section in the notes →How does a frog breathe in water and on land?
In water the frog breathes only through the skin, which is called cutaneous respiration, with dissolved oxygen diffusing across it. On land it uses the buccal cavity, the skin and the lungs. During aestivation and hibernation the lungs are not used, and the skin alone provides gas exchange.
Circulation, blood and lymph
Read this section in the notes →How many chambers does the frog heart have?
The frog heart has three chambers, two atria and one ventricle. The sinus venosus joins the right atrium and the conus arteriosus leaves the ventricle on the ventral side, but neither is counted as a chamber. A pericardium covers the heart. The circulation is of the closed type, and because there is only one ventricle, oxygenated and deoxygenated blood mix there.
What are the hepatic portal and renal portal systems in the frog?
A portal system is a vessel that begins and ends in capillaries. The hepatic portal system carries blood from the intestine to the liver, and the renal portal system carries blood from the lower body and hind limbs to the kidneys. Both are present in the frog, which is an important feature of its circulation.
Are the red blood cells of a frog nucleated?
Yes, frog red blood cells have a nucleus and contain haemoglobin, unlike mature human red cells, which have no nucleus. Frog lymph lacks some proteins and red cells found in blood. This is one more difference between amphibians and mammals, and it is a common question in the comparison of the two.
Excretory system
Read this section in the notes →What kind of nitrogenous waste does a frog excrete?
The frog excretes urea, so it is ureotelic. Its kidneys are made of many uriniferous tubules, or nephrons, which filter the waste. The ureters open into the cloaca, and a thin-walled urinary bladder lying ventral to the rectum also opens into it. Urea is far less toxic than ammonia and needs less water to remove, which suits an animal that spends much of its life on land.
What is the difference between male and female frogs in the way urine and gametes leave?
In the male, each ureter also carries sperms, so it is called the urinogenital duct and opens into the cloaca. In the female, the ureters and oviducts open separately into the cloaca. So only the male frog has a common route for excretory and reproductive products.
Nervous and endocrine control
Read this section in the notes →What are the divisions of the frog brain and where are the optic lobes?
The frog brain has a fore-brain with olfactory lobes, cerebral hemispheres and diencephalon; a mid-brain with a pair of optic lobes; and a hind-brain with the cerebellum and medulla oblongata. So the optic lobes are in the mid-brain, not the fore-brain. The medulla leaves the skull through the foramen magnum as the spinal cord.
How many pairs of cranial nerves does a frog have?
A frog has ten pairs of cranial nerves, arising from the brain. Humans, by comparison, have twelve pairs. The nervous system consists of the central, peripheral and autonomic parts. The frog also has hormone-secreting glands such as the pituitary, thyroid and adrenals.
Sense organs
Read this section in the notes →Does the frog have an external ear?
No, the frog has no external ear. Only a circular tympanum, the eardrum, is visible on the outside of the head, and it receives sound signals. The internal ear is a membranous labyrinth that is concerned with balancing as well as hearing. The eyes and internal ears are the well-organised sense organs.
Reproduction and development
Read this section in the notes →What is the path of sperms in a male frog?
Sperms made in the testes pass through about 10 to 12 fine vasa efferentia into the kidney, then into Bidder's canal, then into the urinogenital duct, and finally reach the cloaca. The testes are attached to the kidneys by a mesorchium, a fold of peritoneum. This explains why the male uses the same duct for urine and sperms.
How many eggs does a female frog lay and how does development occur?
At one time a female frog lays roughly 2500 to 3000 ova. They are fertilised outside her body, in water, and the zygote develops into a tadpole larva, which undergoes metamorphosis to become the adult frog. Frogs breed in the rainy season, and the larval stage that lives in water differs greatly from the adult.
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