NCERT - STRUCTURAL ORGANISATION IN ANIMALS - 24-25

Structural Organisation in Animals

• Unicellular organisms carry out all functions like digestion, respiration, and reproduction within a single cell.
• Multicellular animals exhibit specialization, with different groups of cells performing specific functions.
• Tissues are groups of similar cells working together to perform a specific function.
• Four basic tissue types compose complex animals: epithelial, connective, muscular, and neural.
• Organs are formed by the organization of different tissues.
• Organ systems are groups of two or more organs working together to fulfill a common function.

Morphology and Anatomy of Frogs

• Frogs belong to the class Amphibia within the phylum Chordata.
• The most common frog species in India is Rana tigrina.
• Frogs are poikilothermic (cold-blooded), meaning their body temperature fluctuates with the environment.
• Frogs exhibit camouflage, changing their colour to blend in with their surroundings.
• They undergo aestivation (summer sleep) and hibernation (winter sleep) to survive extreme temperatures.

Frog Morphology

• Frog skin is smooth and slippery due to mucus production.
• The dorsal side of the frog is generally olive green with dark irregular spots.
• The ventral side of the frog is uniformly pale yellow.
• Frogs lack a neck and tail, and their bodies are divided into a head and trunk.
• They have bulging eyes protected by a nictitating membrane.
• Membranous tympanums (ears) are located on either side of the eyes, receiving sound signals.
• Forelimbs and hind limbs are used for swimming, walking, leaping, and burrowing.
• The hind limbs are larger and more muscular than the forelimbs.
• Digits are webbed to aid in swimming.
• Male frogs have vocal sacs for sound production and a copulatory pad on their first forelimb digit, absent in females.

Frog Anatomy

• The frog's body cavity houses various organ systems including digestive, circulatory, respiratory, nervous, excretory, and reproductive.
• The digestive system consists of the alimentary canal and digestive glands.
• The alimentary canal is short due to the frog's carnivorous diet, resulting in a reduced intestinal length.
• The mouth leads to the buccal cavity, then the pharynx, oesophagus, stomach, intestine, rectum, and finally the cloaca.
• The liver secretes bile, which is stored in the gallbladder.

Frog Circulatory System

• The frog's heart has three chambers: two atria and one ventricle.
• The heart is covered by a pericardium membrane.
• A sinus venosus joins the right atrium, receiving blood from the vena cava.
• The ventricle opens into a conus arteriosus.
• Arteries carry blood from the heart to the body.
• Veins collect blood from the body and return it to the heart.
• The hepatic portal system connects the liver and intestine.
• The renal portal system connects the kidneys and lower body parts.
• Frog blood consists of plasma and cells: red blood cells (RBCs or erythrocytes), white blood cells (WBCs or leucocytes), and platelets.
• Frog RBCs are nucleated and contain haemoglobin.
• Lymph is a fluid similar to blood but lacking some proteins and RBCs.

Frog Excretory System

• The excretory system includes kidneys, ureters, cloaca, and urinary bladder.
• Kidneys are bean-shaped and situated posteriorly on both sides of the vertebral column.
• Each kidney contains numerous uriniferous tubules or nephrons.
• In males, ureters act as urinogenital ducts, opening into the cloaca.
• In females, ureters and oviducts open separately into the cloaca.
• The urinary bladder is located ventral to the rectum and also opens into the cloaca.
• Frogs are ureotelic, meaning they excrete urea as a waste product.

Frog Nervous System

• The frog's nervous system includes a central nervous system (CNS), peripheral nervous system (PNS), and autonomic nervous system.
• The CNS consists of the brain and spinal cord.
• The PNS includes cranial and spinal nerves.
• The autonomic nervous system has sympathetic and parasympathetic branches.
• Ten pairs of cranial nerves arise from the brain.
• The brain is enclosed in a bony structure called the cranium.
• The brain is divided into the forebrain, midbrain, and hindbrain.
• The forebrain includes olfactory lobes, cerebral hemispheres, and the diencephalon.
• The midbrain is characterized by optic lobes.
• The hindbrain consists of the cerebellum and medulla oblongata.
• The medulla oblongata extends into the spinal cord, which is encased in the vertebral column.

Frog Sensory Organs

• Frogs possess various sensory organs, including touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), and hearing (tympanum with internal ears).
• Eyes are spherical structures situated in the orbits of the skull.
• Frogs lack external ears; only the tympanum is visible externally.
• The ear functions in hearing and balance.

Frog Reproductive System

• Frog reproductive systems are well organized, with distinct male and female structures.
• The male reproductive system includes a pair of testes, vasa efferentia, kidney, adrenal gland, urinogenital duct, rectum, cloaca, and cloacal aperture.
• The female reproductive system includes a pair of ovaries, oviducts, uterus, ureter, and cloaca.

Frog Anatomy

• Frogs belong to class Amphibia of phylum Chordata.
• The most common species of frog found in India is Rana tigrina.
• Frogs are cold-blooded or poikilotherms, their body temperature fluctuates with the environment.
• Frogs are capable of camouflage, changing their color to blend with their surroundings.
• Frogs undergo aestivation (summer sleep) and hibernation (winter sleep) to survive extreme temperatures.
• Frog skin is smooth and slippery due to the presence of mucus.
• Frogs have a well-developed closed circulatory system including a heart, blood vessels, and blood.
• Frogs have a lymphatic system consisting of lymph, lymph channels, and lymph nodes.
• Frog heart has three chambers: two atria and one ventricle.
• The heart is covered by a membrane called pericardium.
• A triangular structure called sinus venosus joins the right atrium.
• The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart.
• The blood from the heart is carried to all parts of the body by the arteries (arterial system).
• Veins collect blood from different parts of the body to the heart and form the venous system.
• Frogs have a hepatic portal system (liver and intestine) and a renal portal system (kidney and lower body).
• Frog blood is composed of plasma and cells (red blood cells, white blood cells, platelets).
• Frog red blood cells are nucleated and contain haemoglobin.
• Lymph is different from blood as it lacks proteins and red blood cells.
• Blood carries nutrients, gases, and water to various parts of the body during circulation.
• Frog’s excretory system consists of kidneys, ureters, cloaca, and urinary bladder.
• Kidneys are located in the body cavity, on both sides of the vertebral column.
• Kidneys are composed of nephrons, responsible for filtering waste.
• Ureters connect the kidneys to the cloaca.
• The urinary bladder is located ventral to the rectum and opens into the cloaca.
• Frogs are ureotelic, excreting urea as a nitrogenous waste product.

Frog Digestive System

• Frogs have a complete digestive system consisting of a mouth, esophagus, stomach, small intestine, large intestine, and cloaca.
• Pancreas is a digestive gland that produces pancreatic juice containing digestive enzymes.
• Food is captured by the bilobed tongue.
• HCl and gastric juices secreted from the stomach walls digest food.
• Partially digested food, called chyme, is transferred to the duodenum, the first section of the small intestine.
• The duodenum receives bile from the gall bladder and pancreatic juices through a common bile duct.
• Bile emulsifies fat, and pancreatic juices digest carbohydrates and proteins.
• Final digestion takes place in the intestine.
• Digested food is absorbed by villi and microvilli, finger-like folds found in the intestine walls.
• Undigested waste moves into the rectum and exits through the cloaca.

Frog Respiratory System

• Frogs use both cutaneous respiration (skin) and pulmonary respiration (lungs) for breathing.
• In water, the skin acts as the respiratory organ, exchanging dissolved oxygen through diffusion.
• On land, the buccal cavity, skin, and lungs act as respiratory organs.
• Lungs are elongated, pink, sac-like structures located in the thorax.
• Air enters through the nostrils, into the buccal cavity, and then to the lungs.
• During aestivation and hibernation, gas exchange occurs through the skin.

Frog Nervous System

• Frog’s nervous system includes a central nervous system (brain and spinal cord), peripheral nervous system (cranial and spinal nerves), and autonomic nervous system (sympathetic and parasympathetic).
• The brain is divided into forebrain, midbrain, and hindbrain.
• Forebrain includes olfactory lobes, cerebral hemispheres, and diencephalon.
• Midbrain is characterized by optic lobes.
• Hindbrain consists of cerebellum and medulla oblongata.
• Frog has various sense organs: touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), and hearing (tympanum and internal ears).
• Eyes are spherical, simple eyes located in the skull’s orbit.
• Frogs lack external ears; only the tympanum is visible externally.
• The ear serves for hearing and balance.

Frog Reproductive System

• Frogs have distinct male and female reproductive systems.
• Male: A pair of yellowish ovoid testes are attached to the upper part of the kidneys.
• Male: Vasa efferentia connect the testes to the kidneys and open into the Bidder’s canal.
• Male: The urinogenital duct connects to the cloaca.
• Female: A pair of ovaries are situated near the kidneys.
• Female: A pair of oviducts arise from the ovaries and open separately into the cloaca.
• Female: A mature female can lay 2500-3000 eggs at a time.
• Fertilization is external and takes place in water.
• Development involves a larval stage called tadpole.
• Tadpoles undergo metamorphosis to become adult frogs.

### Frog Importance

• Frogs play beneficial roles in maintaining ecological balance and are important for the food chain and food web.
• Frogs control insect populations, protecting crops.
• Some cultures consider frog legs a food source.

Frog Anatomy and Morphology

• Frogs belong to the class Amphibia of phylum Chordata.
• The most common frog species in India is Rana tigrina.
• Frogs are poikilotherms, meaning their body temperature varies with the environment.
• Frogs exhibit a process called mimicry, changing their colour to camouflage themselves from predators.
• Frogs undergo aestivation during summer (summer sleep) and hibernation during winter (winter sleep), seeking shelter to avoid extreme temperatures.

Frog Morphology

• Frogs have smooth and slippery skin due to the presence of mucus.
• The dorsal side of the frog is typically olive green with dark spots.
• The ventral side of the frog is uniformly pale yellow.
• Frogs absorb water through their skin.
• The frog's body is divided into a head and trunk, with no neck or tail.
• Frogs have a pair of nostrils located above the mouth, bulged eyes with a nictitating membrane for protection in water, and a membranous tympanum (ear) on either side of the eyes.
• Forelimbs and hind limbs are used for swimming, walking, leaping, and burrowing.
• Hind limbs are larger and more muscular than forelimbs, ending in five digits.
• Forelimbs end in four digits.
• Feet have webbed digits that aid in swimming.
• Male frogs can be distinguished by the presence of vocal sacs for sound production and a copulatory pad on the first digit of their forelimbs. These features are absent in female frogs.

Frog Anatomy

• The body cavity of frogs houses various organ systems including digestive, circulatory, respiratory, nervous, excretory, and reproductive systems.

Digestive System

• The digestive system consists of the alimentary canal and digestive glands.
• The alimentary canal is short due to the frog's carnivorous diet, resulting in a reduced length of the intestine.
• The mouth leads to the buccal cavity, which connects to the esophagus via the pharynx.
• The esophagus is a short tube that opens into the stomach.
• The stomach continues as the intestine, rectum, and finally opens outside through the cloaca.
• The liver secretes bile, which is stored in the gallbladder.

Circulatory System

• The frog's heart has three chambers: two atria and one ventricle.
• The heart is enclosed by a membrane called the pericardium.
• A triangular structure called the sinus venosus joins the right atrium.
• The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart.
• Arteries carry blood from the heart to the body (arterial system).
• Veins collect blood from the body and transport it back to the heart (venous system).
• Frogs have special venous connections between the liver and intestine (hepatic portal system) and between the kidneys and the lower body (renal portal system).
• Frog blood consists of plasma and cells: red blood cells (RBCs or erythrocytes), white blood cells (WBCs or leukocytes), and platelets.
• RBCs are nucleated and contain the red pigment hemoglobin.
• Lymph is similar to blood but lacks some proteins and RBCs.
• Blood transports nutrients, gases, and water throughout the body during circulation.
• The heart's muscular pumping action drives blood circulation.

Excretory System

• The excretory system eliminates nitrogenous wastes and consists of a pair of kidneys, ureters, cloaca, and urinary bladder.
• Kidneys are compact, dark red, bean-shaped structures located posteriorly in the body cavity on either side of the vertebral column.
• Each kidney is composed of uriniferous tubules or nephrons, the structural and functional units of the kidney.
• In male frogs, two ureters emerge from the kidneys and act as urinogenital ducts, opening into the cloaca.
• In female frogs, ureters and oviducts open separately into the cloaca.
• The thin-walled urinary bladder is located ventral to the rectum and also opens into the cloaca.
• Frogs excrete urea, making them ureotelic animals.
• Blood carries excretory wastes to the kidneys for filtration and excretion.

Nervous System

• The nervous system controls and coordinates body functions.
• It includes the central nervous system (brain and spinal cord), peripheral nervous system (cranial and spinal nerves), and autonomic nervous system (sympathetic and parasympathetic).
• Ten pairs of cranial nerves arise from the brain.
• The brain is enclosed in a bony structure called the brain box (cranium).
• The brain is divided into the forebrain, midbrain, and hindbrain.
• The forebrain includes olfactory lobes, paired cerebral hemispheres, and an unpaired diencephalon.
• The midbrain is characterized by a pair of optic lobes.
• The hindbrain consists of the cerebellum and medulla oblongata.
• The medulla oblongata extends through the foramen magnum and continues into the spinal cord.
• The spinal cord is enclosed within the vertebral column.

Sensory Organs

• Frogs have various sensory organs: organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), and hearing (tympanum with internal ears).
• Eyes are spherical structures located in the orbits of the skull, functioning as simple eyes (with a single unit).
• External ears are absent in frogs, only the tympanum is visible externally.
• Ears function in both hearing and balance (equilibrium).

Reproductive Systems

• Male Reproductive System:
  • Consists of a pair of yellowish ovoid testes attached to the upper part of the kidneys by a peritoneal fold called the mesorchium.
  • 10-12 vasa efferentia arise from the testes, enter the kidneys, and open into Bidder's canal.
  • The urinogenital duct connects to the cloaca, which serves as a chamber for the passage of faecal matter, urine, and sperm.
• Female Reproductive System:
  • Includes a pair of ovaries situated near the kidneys.
  • A pair of oviducts arise from the ovaries and open into the cloaca separately.
  • A mature female can lay 2500 to 3000 ova at a time.
  • Fertilization is external and occurs in water.
  • Development involves a larval stage called a tadpole.
  • Tadpoles undergo metamorphosis to transform into adults.

Ecological Importance of Frogs

• Frogs are beneficial to humans as they consume insects and protect crops.
• They maintain ecological balance by serving as an important link in food chains and webs within ecosystems.
• In some countries, the muscular legs of frogs are consumed as food.

Summary

• Cells, tissues, organs, and organ systems demonstrate division of labor to ensure the survival of the organism as a whole.
• Tissue is defined as a group of cells with intercellular substances, performing one or more functions.
• Epithelia are sheet-like tissues that line body surfaces, cavities, ducts, and tubes.
• Epithelia possess a free surface facing body fluid or the external environment.
• Their cells are interconnected structurally and functionally at junctions.

Organ and Organ Systems

• The text highlights that complex animals, like frogs, have different cell types organized to form tissues. These tissues then form organs and multiple organs working together become organ systems.
• The text says that the different organ systems in a frog's body are: digestive, circulatory, respiratory, nervous, excretory, and reproductive.
• The text emphasizes that the arrangement of different organs within an organism is essential for the efficient and coordinated function of the millions of cells that make up the body.
• The text briefly mentions how the complexity of organ systems within different animals demonstrates an evolutionary pattern that will be studied in greater detail in later classes.
• The text makes a distinction between the terms "morphology" and "anatomy". Morphology refers to the study of the external appearance of an organism. Anatomy, on the other hand, studies the internal organs of the organism.

Frogs

• Frogs belong to the class Amphibia of the phylum Chordata.
• They are poikilothermic, meaning they have a body temperature that fluctuates with the surrounding environment.
• Frogs exhibit protective coloration called mimicry, which allows them to blend with their surroundings.
• Frogs can survive extreme weather conditions by entering states of hibernation (winter sleep) and aestivation (summer sleep).

Morphology

• Frog skin is smooth and slippery, and always maintained in a moist condition.
• The frog's dorsal (back) side is olive green with dark spots, while the ventral (bottom) side is pale yellow.
• Frogs don't drink water; they absorb it through their skin.
• The frog's body can be divided into a head and a trunk, but it lacks a neck and a tail.
• The frog has a pair of nostrils above the mouth, bulged eyes covered by a nictitating membrane, external tympanum (ear), and webbed feet.
• The hind limbs are larger and more muscular than the forelimbs.
• Sexual dimorphism exists in frogs, with male frogs having vocal sacs and a copulatory pad on the first digit of the forelimbs, which are absent in females.

Anatomy

• The frog's internal body contains organ systems that perform various functions.
• The digestive system consists of an alimentary canal and glands.
• The alimentary canal is short because frogs are carnivores.
• The mouth opens into the buccal cavity, which connects to the esophagus and then the stomach.
• The stomach continues into the intestine, rectum, and finally the cloaca.
• The liver secretes bile, which is stored in the gallbladder.
• The circulatory system has a three-chambered heart, with two atria and one ventricle, that is covered by a membrane called the pericardium.
• The heart receives blood from the sinus venosus, which is connected to the vena cava (major veins).
• The ventricle opens into a conus arteriosus that pumps blood through arteries (the arterial system) to different parts of the body.
• The blood is carried back to the heart through veins, forming the venous system.
• The circulatory system also includes hepatic portal and renal portal systems, which connect the liver and intestines with the heart and the kidneys with lower body parts, respectively.
• Frog blood is composed of plasma, red blood cells (RBCs), white blood cells (WBCs), and platelets. RBCs in frogs have a nucleus and contain hemoglobin.
• The lymph fluid is similar to blood but lacks a few proteins and RBCs.
• The excretory system clears nitrogenous wastes from the body.
• This system consists of paired kidneys, ureters, a cloaca, and a urinary bladder.
• The kidneys are compact, dark red, and bean-shaped, situated on both sides of the vertebral column.
• Each kidney contains uriniferous tubules or nephrons, which are the functional units responsible for filtering waste.
• The ureters, which act as urinogenital ducts in males, carry urine and sperm to the cloaca.
• In females, the ureters and oviducts open separately into the cloaca.
• The urinary bladder is situated ventral to the rectum and opens into the cloaca.
• Frogs are ureotelic, which means they excrete urea as their primary nitrogenous waste.
• The nervous system controls and coordinates body functions.
• The nervous system includes the neural system (nerves) and endocrine glands that secrete hormones.
• The male reproductive system is composed of two testes, vasa efferentia, the Bidder's canal, and a urinogenital duct.
• The testes are yellowish and ovoid, connected to the kidneys by a fold of peritoneum called mesorchium.
• Vasa efferentia, 10-12 in number, connect the testes to the kidneys. They open into the Bidder's canal, which ultimately connects to the urinogenital duct that empties into the cloaca.
• The cloaca is a common chamber for the elimination of feces, urine, and sperm.
• The female reproductive system includes two ovaries, oviducts, and a cloaca.
• The ovaries are located near the kidneys.
• The oviducts connect the ovaries to the cloaca, where eggs are released.
• A mature female frog can lay up to 2,500-3,000 eggs at once.
• Frogs have external fertilization that takes place in water, and their development involves a larval tadpole stage.
• The tadpole undergoes metamorphosis into an adult frog.
• Frogs are beneficial to humans because they eat insects and protect crops.
• They play a significant role in ecological balance, forming a crucial link in the food chain and food web.
• Frogs are considered a food source for humans in some parts of the world.

Structural Organization in Animals

• Multicellular organisms have a well-organized structure to carry out various functions.
• Four basic types of tissues make up complex animals: epithelial, connective, muscular, and neural.
• Tissues combine to form organs, and multiple organs working together form organ systems.

Morphology and Anatomy of Frog

• Frogs belong to the class Amphibia of phylum Chordata.
• Common species in India is Rana tigrina.
• They are poikilotherms, meaning their body temperature changes with the environment.
• They can change color for camouflage, called mimicry.
• They hibernate in winter (winter sleep) and aestivate in summer (summer sleep).

Frog Morphology

• Frogs have smooth, slippery skin due to mucus.
• Skin is always in a moist condition.
• Tongue is bilobed for capturing food.

Frog Digestion

• The pancreas produces pancreatic juice with digestive enzymes.
• Food is broken down by HCl and gastric juices in the stomach.
• Chyme (partially digested food) travels to the duodenum.
• Bile from the gall bladder and pancreatic juices digest fats, carbohydrates, and proteins.
• Final digestion occurs in the intestine.
• Digested nutrients are absorbed by villi and microvilli in the intestine.
• Undigested waste moves to the rectum and exits through the cloaca.

Frog Respiration

• Frogs use two methods: cutaneous respiration (through skin) and pulmonary respiration (through lungs).
• In water, skin acts as an aquatic respiratory organ.
• On land, the buccal cavity, skin, and lungs are responsible for respiration.
• During aestivation and hibernation, gas exchange occurs through the skin.

Frog Circulation

• Well-developed closed circulatory system: heart, blood vessels, blood.
• Lymphatic system: lymph, lymph channels, lymph nodes.
• Heart has three chambers: two atria and one ventricle.
• Blood is composed of plasma, red blood cells (RBCs), white blood cells (WBCs), and platelets.
• RBCs are nucleated and contain hemoglobin, a red pigment.
• Lymph lacks some proteins and RBCs.
• Blood carries nutrients, gases, and water to various parts of the body.

Frog Excretion

• Excretory system: kidneys, ureters, cloaca, urinary bladder.
• Kidneys are bean-shaped and composed of nephrons (functional units).
• Ureters act as urinogenital ducts in males, opening into the cloaca.
• In females, ureters and oviducts open separately into the cloaca.
• Urinary bladder is ventral to the rectum and also opens into the cloaca.
• Frogs are ureotelic animals, excreting urea as waste.

Frog Nervous System

• Central nervous system (CNS): brain and spinal cord.
• Peripheral nervous system (PNS): cranial and spinal nerves.
• Autonomic nervous system (ANS): sympathetic and parasympathetic.
• Brain is divided into forebrain, midbrain, and hindbrain.
• Sensory organs: touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), and hearing (tympanum with internal ears).

Frog Reproduction

• Well-organized male and female reproductive systems.
• Male reproductive system: testes, vasa efferentia, fat bodies, kidney, urinogenital duct, rectum, cloaca.
• Female reproductive system: ovaries, oviducts, ova, ureter, urinary bladder, cloaca.

Structural Organization in Animals - The Frog

• The body of a frog is composed of billions of cells organized into tissues, organs, and organ systems to perform various functions.
• The frog has no neck or tail, and its body is divided into a head and trunk.
• The frog's skin is highly vascularized and plays a role in respiration both in water and on land.
• The frog's skin contains mucous glands which help in keeping the skin moist.
• The frog's skin is olive green with dark irregular spots on the dorsal side, and uniformly pale yellow on the ventral side.
• The frog captures prey with its bilobed tongue, which is muscular and located at the tip of its mouth.
• The frog doesn't drink water, instead it absorbs it through its skin.
• Frogs are carnivores and have short digestive systems.
• The frog's digestive system consists of the alimentary canal and digestive glands.
• The alimentary canal includes the mouth, buccal cavity, pharynx, esophagus, stomach, intestine, rectum, and cloaca.
• The frog's digestive glands include the liver, gallbladder, and pancreas.
• The liver secretes bile, which is stored in the gallbladder.
• The pancreas produces pancreatic juice, which contains digestive enzymes.
• Frogs can breathe both on land and in water using different mechanisms.
• In water, the frog uses its skin for respiration - "cutaneous respiration".
• On land, the frog uses its buccal cavity, skin, and lungs for respiration - "pulmonary respiration".
• The frog's lungs are elongated, pink-colored sac-like structures located in the upper part of the trunk region.
• The frog's circulatory system is a closed system consisting of a heart, blood vessels, and blood.
• The frog's heart has three chambers - two atria and one ventricle.
• The frog's heart is covered by a membrane called the pericardium.
• The frog's blood is composed of plasma and cells, including red blood cells (RBCs), white blood cells (WBCs), and platelets.
• The frog's excretory system consists of a pair of kidneys, ureters, cloaca, and urinary bladder.
• The kidneys are compact, dark red, bean-like structures located on both sides of the vertebral column.
• The frog excretes urea as a nitrogenous waste product, making it a ureotelic animal.
• The frog's nervous system is responsible for control and coordination, and includes both neural and endocrine components.
• The frog's endocrine glands secrete hormones that regulate various bodily functions.
• The frog's male reproductive organs include a pair of testes, which are located near the kidneys.
• The frog's female reproductive organs include a pair of ovaries, which are also located near the kidneys.
• The frog's fertilization is external and takes place in water.
• The frog's development involves a larval stage called a tadpole.
• The tadpole undergoes metamorphosis to transform into an adult frog.
• Frogs are beneficial to humans because they eat insects and maintain ecological balance.
• Frogs are an important part of the food chain and food web in ecosystems.
• Some cultures use frog legs as food.

Important Terms

• Tissue: A group of similar cells along with intercellular substances performing one or more functions in the body.
• Organ: A structure composed of two or more tissues that work together to perform a specific function.
• Organ system: A group of organs that work together to perform a complex function.
• Cutaneous respiration: Respiration through the skin.
• Pulmonary respiration: Respiration through the lungs.
• Ureotelic: An animal that excretes urea as a nitrogenous waste product.
• Metamorphosis: A biological process in which an animal undergoes a significant physical transformation.