Structural Organisation in Animals - Notes | Class 11

Rana tigrina is the most common species in India.
They are poikilotherms (cold blooded).
They can change colour to hide from their enemies (camouflage). This protective coloration is called mimicry.
During summer and winter, they undergo aestivation (summer sleep) and hibernation (winter sleep) respectively to protect them from extreme heat and cold.

Morphology of Frog

Body is divisible into head and trunk. Neck and tail absent.
Skin is moist, smooth, and slippery due to the mucus.
Colour of dorsal side is olive green with dark irregular spots, and ventral side is pale yellow.
The frog never drinks water but absorbs it through the skin.
A mouth, paired nostrils, and bulged eyes (covered by nictitating membrane) are present.
On either side of eyes have a membranous tympanum (ear).
The forelimbs (4 digits) and hind limbs (5 digits) help in swimming, walking, leaping, and burrowing. The hind limbs are larger and muscular than forelimbs.
Feet have webbed digits that help in swimming.
Frogs exhibit sexual dimorphism. Male frogs have sound-producing vocal sac and also a copulatory (nuptial) pad on the first digit of forelimbs, which are absent in female frogs.

Consists of alimentary canal and digestive glands.
The alimentary canal is short because frogs are carnivores, hence the length of intestine is reduced.
Mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca.
Liver secretes bile that is stored in gall bladder. Pancreas produces pancreatic juice containing digestive enzymes.
Food is captured by the bilobed tongue.
Digestion: Gastric juice and HCl secreted from gastric wall digest the food. Partially digested food (chyme) is passed from stomach to the duodenum.
Duodenum receives bile and pancreatic juices through a common bile duct.
Bile emulsifies fat. Pancreatic juice digests carbohydrates and proteins. Digestion completes in the intestine.
Finger-like villi and microvilli in intestine absorb digested food. The undigested solid waste moves into the rectum and passes out through cloaca.

Skin acts as aquatic respiratory organ (cutaneous respiration). Dissolved oxygen in the water is exchanged through the skin by diffusion. During aestivation and hibernation, respiration takes place through skin.
On land, the buccal cavity, skin, and lungs (pulmonary respiration) act as the respiratory organs.
The lungs are a pair of elongated, pink coloured sac-like structures present in the thorax. Air enters through the nostrils into the buccal cavity and then to lungs.

Closed type. Includes Blood vascular system (heart, blood vessels & blood) and lymphatic system (lymph, lymph channels & lymph nodes).
Heart is 3-chambered, (two atria and one ventricle) and is covered by a membrane called pericardium.
A triangular structure called sinus venosus joins the right atrium. It receives blood through major veins (vena cava).
The ventricle opens into a saclike conus arteriosus on the ventral side of the heart.
The blood pumped from the muscular heart is carried to all parts of the body by the arteries (arterial system).
The veins collect blood from different parts of body to the heart and form the venous system.
Hepatic portal system (venous connection between liver and intestine) and renal portal system (between kidney and lower parts of the body) are present in frogs.
Blood contains plasma and cells (RBC, WBC & platelets). RBCs are nucleated and contain haemoglobin.
Blood transports nutrients, gases, and water to tissues.

Includes kidneys (2), ureters (2), cloaca & urinary bladder.
Kidneys are dark red and bean-shaped. Found posteriorly in the body cavity on both sides of vertebral column. Each kidney is formed of uriniferous tubules (nephrons).
2 ureters emerge from the kidneys. In male frogs, the ureters act as urinogenital duct which opens into cloaca.
In females, ureters & oviduct open separately in cloaca.
The thin-walled urinary bladder is present ventral to the rectum which also opens in the cloaca.
The frog is a ureotelic animal (excretes urea). Nitrogenous wastes are carried by blood into the kidney where it is separated and excreted.

The endocrine glands secrete hormones.
Endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals & gonads.

Fore-brain: Includes olfactory lobes, paired cerebral hemispheres, and unpaired diencephalon.
Mid-brain: Includes a pair of optic lobes.
Hind-brain: Includes cerebellum & medulla oblongata.

Medulla oblongata passes out through the foramen magnum and continues into spinal cord, which is enclosed in the vertebral column.
Sense organs include organs of:

Male reproductive organs consist of a pair of yellowish ovoid testes, which are found adhered to the upper part of kidneys by a double fold of peritoneum (mesorchium).
Vasa efferentia (10-12 in number) arise from testes. They enter the kidneys on their side and open into Bidder’s canal. It communicates with urinogenital duct that comes out of the kidneys and opens into cloaca.
The cloaca is a small, median chamber that is used to pass faecal matter, urine, and sperms to the exterior.

The female reproductive organs include a pair of ovaries. The ovaries are situated near kidneys, and there is no functional connection with kidneys.
A pair of oviduct arising from the ovaries opens into the cloaca separately.
A mature female can lay 2500 to 3000 ova at a time.
Fertilisation is external and takes place in water.
Development involves a larval stage called tadpole.
Tadpole undergoes metamorphosis to form the adult.

Frogs are beneficial for mankind because they eat insects and protect the crop.
Maintain ecological balance by serving as an important link of food chain and food web in the ecosystem.
In some countries, the muscular legs of frog are used as food by man.

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