Chapter 2.3: Structural Organisation in Animals

Morphology and Anatomy of Frog (Rana tigrina)

Body: Divisible into head and trunk. Neck and tail are absent.
Skin: Moist, smooth, and slippery due to the presence of mucus. The dorsal side is olive green with dark irregular spots, and the ventral side is a uniform pale yellow.
Eyes: Bulging and are covered by a nictitating membrane that protects them in water.
Ear: A tympanum (eardrum) is present on either side of the eyes.
Limbs: Two pairs of limbs. Forelimbs are short with four digits. Hind limbs are larger and muscular with five webbed digits, adapted for swimming and leaping.
Sexual Dimorphism: Males can be distinguished by the presence of sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs.

Alimentary Canal: Short because frogs are carnivores.
- Mouth: Opens into the buccal cavity.
- Pharynx: Leads to the oesophagus.
- Oesophagus: A short tube that opens into the stomach.
- Stomach: Followed by the intestine, rectum, and finally opens outside by the cloaca.
Digestive Glands:
- Liver: Secretes bile, which is stored in the gall bladder.
- Pancreas: Produces pancreatic juice containing digestive enzymes.
Digestion: Food is captured by the bilobed tongue. Digestion occurs in the stomach and intestine. Digested food is absorbed by the villi and microvilli in the inner wall of the intestine.

In Water: Skin acts as an aquatic respiratory organ (cutaneous respiration).
On Land: The buccal cavity, skin, and lungs act as the respiratory organs.
- Lungs: A pair of elongated, pink-colored sac-like structures present in the upper part of the trunk region (thorax). Air enters through the nostrils into the buccal cavity and then into the lungs (pulmonary respiration).

Type: Closed type.
Heart: A three-chambered heart with two atria and one ventricle.
Blood Vessels: Arteries and veins.
Blood: Composed of plasma and blood cells (RBCs, WBCs, platelets). RBCs are nucleated.
Circulation: The heart receives deoxygenated blood from the body in the right atrium and oxygenated blood from the lungs and skin in the left atrium. Both types of blood get mixed in the single ventricle, which pumps out this mixed blood (incomplete double circulation).
Lymphatic System: Consists of lymph, lymph channels, and lymph nodes.

Organs: A pair of kidneys, ureters, cloaca, and a urinary bladder.
Kidneys: Compact, dark red, and bean-like structures situated a little posteriorly in the body cavity on both sides of the vertebral column.
Urine Formation: Nitrogenous wastes are carried by blood into the kidney where it is separated and excreted. Frog is a ureotelic animal (excretes urea).
Excretory Pathway: Ureters emerge from the kidneys and open into the cloaca. In females, the ureters and oviducts open separately into the cloaca. In males, the ureters act as a urinogenital duct and open into the cloaca.

Central Nervous System (CNS): Brain and spinal cord.
Peripheral Nervous System (PNS): Cranial and spinal nerves.
Autonomic Nervous System (ANS): Sympathetic and parasympathetic.
Brain: Divided into fore-brain (olfactory lobes, paired cerebral hemispheres, and an unpaired diencephalon), mid-brain (a pair of optic lobes), and hind-brain (cerebellum and medulla oblongata).

Male: A pair of yellowish, ovoid testes, which are attached to the upper part of the kidneys by a double fold of peritoneum called mesorchium. Vasa efferentia arise from the testes, enter the kidneys, and open into the Bidder's canal, which finally communicates with the urinogenital duct that opens into the cloaca.
Female: A pair of ovaries situated near the kidneys. A pair of oviducts arising from the ovaries opens into the cloaca separately. A mature female can lay 2500 to 3000 ova at a time.
Fertilization: External, takes place in water.
Development: Indirect, involving a larval stage called a tadpole, which undergoes metamorphosis to form the adult.