Chapter 7: Structural Organization In Animals

Division of Labour

• Unicellular animals: A single cell performs functions such as digestion, respiration and reproduction.
• Multicellular animals: Different groups of cells carry out functions in an organised way.
• Tissues: Complex animals have four basic tissue types, combined in different proportions and patterns.
• Example: The heart contains epithelial, connective, muscular and neural tissues.
• Importance: Organisation divides work and allows the body's cells to function more efficiently and in coordination.

• Organs: Each organ consists of one or more tissue types and performs particular functions.
• Organ systems: Organs associate and coordinate to carry out common functions; examples include the digestive and respiratory systems.
• Evolutionary trend: Complexity in organs and organ systems shows a discernible trend called the evolutionary trend.
• Morphology: The study of form and externally visible features; in animals, it includes the external appearance of body parts and organs.
• Anatomy: The study of the internal organs of animals.

• Habitat: Frogs live both on land and in freshwater.
• Classification: Frogs belong to class Amphibia of phylum Chordata; a common Indian species is *Rana tigrina*.
• Poikilotherms: Frogs do not maintain a constant body temperature; it varies with the environmental temperature.
• Camouflage: Frogs can change colour to blend with their surroundings and hide from enemies; this protective coloration is called mimicry in the text.
• Seasonal dormancy: They shelter in deep burrows during extreme weather—summer sleep is aestivation, and winter sleep is hibernation.

Skin and Body Plan

• Skin: Smooth, slippery because of mucus, and kept moist.
• Colour: The dorsal side is generally olive green with dark irregular spots; the ventral side is uniformly pale yellow.
• Water uptake: Frogs do not drink water; they absorb it through their skin.
• Body divisions: The body has a head and trunk; a neck and tail are absent.

Sense Organs and Limbs

• Nostrils: A pair is present above the mouth.
• Eyes: Bulging eyes are protected in water by a nictitating membrane.
• Tympanum: A membranous structure on each side of the eyes receives sound signals.
• Forelimbs: End in four digits.
• Hind limbs: Larger and more muscular than the forelimbs; they end in five digits.
• Movement: Limbs help in swimming, walking, leaping and burrowing; webbed feet aid swimming.

Sexual Dimorphism

• Male frogs: Have sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs.
• Female frogs: Lack vocal sacs and the copulatory pad.

Body and limbs

• Body division: The body has a head and trunk; a neck and tail are absent.
• Skin: Mucous glands are present, and the highly vascularised skin also helps in respiration.
• Head: A pair of nostrils lies above the mouth. Bulging eyes are protected in water by a nictitating membrane.
• Hearing: A membranous tympanum on each side of the eyes receives sound signals.
• Limbs: Forelimbs have four digits; larger, more muscular hind limbs have five digits. Webbed feet aid swimming.
• Movement: Limbs help the frog swim, walk, leap and burrow.

Food digestion and absorption

• Food capture: The bilobed tongue captures prey.
• Canal sequence: Mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca.
• Digestive glands: The liver secretes bile, which is stored in the gall bladder; the pancreas produces enzyme-containing pancreatic juice.
• Stomach: Hydrochloric acid and gastric juices digest food; partially digested food, called chyme, passes to the duodenum.
• Duodenum: Receives bile and pancreatic juice through a common bile duct. Bile emulsifies fats; pancreatic juice digests carbohydrates and proteins.
• Intestine: Final digestion occurs here. Villi and microvilli absorb digested food through the intestinal wall.
• Egestion: Undigested solid waste enters the rectum and passes out through the cloaca.

Respiratory surfaces

• In water: Dissolved oxygen diffuses through the skin.
• On land: The buccal cavity, skin and lungs act as respiratory organs; lung-based respiration is pulmonary respiration.
• Lungs: A pair of elongated, pink, sac-like structures in the upper trunk (thorax).
• Air pathway: Nostrils → buccal cavity → lungs.
• During dormancy: Gaseous exchange during aestivation and hibernation takes place through the skin.

Heart, blood vessels and blood

• Heart: Located in the upper body cavity; it has two atria and one ventricle and is enclosed by the pericardium.
• Heart connections: The triangular sinus venosus joins the right atrium and receives blood through the venae cavae. The ventricle opens into the conus arteriosus.
• Vessels: Arteries carry blood away from the heart; veins return blood to it.
• Portal systems: The hepatic portal system connects the intestine and liver; the renal portal system connects the kidneys and lower parts of the body.
• Blood: Contains plasma, red blood cells (RBCs/erythrocytes), white blood cells (WBCs/leucocytes) and platelets.
• RBCs: Nucleated and contain the red pigment haemoglobin.
• Transport: Blood carries nutrients, gases and water to their respective sites.

Lymphatic system

• Lymph: Unlike blood, it lacks RBCs and contains fewer proteins.

Excretory organs and pathways

• Organs: A pair of kidneys, ureters, cloaca and urinary bladder form the excretory system.
• Kidneys: Compact, dark-red, bean-shaped organs located behind the main body region, on either side of the vertebral column.
• Nephrons: Each kidney contains many uriniferous tubules, also called nephrons; these are its structural and functional units.
• Waste removal: Blood carries excretory wastes to the kidneys, where they are separated and excreted.
• Urinary bladder: Thin-walled and situated ventral to the rectum; it opens into the cloaca.

Nervous system

• Central nervous system: Brain and spinal cord.
• Peripheral nervous system: Cranial and spinal nerves; the brain gives rise to ten pairs of cranial nerves.
• Autonomic nervous system: Sympathetic and parasympathetic divisions.
• Brain covering: The brain is enclosed in the cranium (brain box).
• Brain regions: Forebrain, midbrain and hindbrain.
• Forebrain: Olfactory lobes, paired cerebral hemispheres and unpaired diencephalon.
• Midbrain: Characterised by a pair of optic lobes.
• Hindbrain: Contains the cerebellum and medulla oblongata. The medulla passes through the foramen magnum and continues as the spinal cord within the vertebral column.

Endocrine glands

• Main glands: Pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.

Senses and their structures

• Touch: Sensory papillae.
• Taste: Taste buds.
• Smell: Nasal epithelium.
• Vision: A pair of simple, spherical eyes, each with one unit, lies in the skull's orbits.
• Hearing and balance: The tympanum is visible externally; internal ears are involved in hearing and equilibrium.
• External ear: Absent; only the tympanum can be seen externally.
• Organisation: Eyes and internal ears are well-organised structures; the other sense organs are cellular aggregations around nerve endings.

Male reproductive system

• Testes: A pair of yellowish, ovoid testes is attached to the upper kidneys by a double fold of peritoneum called the mesorchium.
• Sperm pathway: Testes → 10–12 vasa efferentia → kidneys → Bidder's canal → urinogenital duct → cloaca.
• Cloaca: A small median chamber through which faeces, urine and sperms pass to the exterior.

Female reproductive system

• Ovaries: A pair lies near the kidneys but has no functional connection with them.
• Oviducts: A pair of oviducts arising from the ovaries opens separately into the cloaca.
• Egg production: A mature female can lay 2,500–3,000 ova at a time.

• Crop protection: Frogs eat insects that can damage crops.
• Ecological role: Frogs are important links in food chains and food webs, helping maintain ecological balance.
• Human use: In some countries, people eat the muscular legs of frogs.