Chapter-7 Structural Organisation in Animals: Frog
Morphology of Frog
Morphology refers to the study of external form and features of an organism. In this section, we examine the physical structure of the common Indian bullfrog, Rana tigrina.
Body Design and Skin
The body of a frog is divisible into two regions: Head and Trunk. A neck and tail are absent. The skin is smooth, moist, and slippery due to the presence of mucus. It is always maintained in a moist condition because it serves as a major site for cutaneous respiration. The color of the dorsal side of the body is generally olive green with dark irregular spots, while the ventral side is uniformly pale yellow. The skin never has scales.
External Features
- Eyes: A pair of bulging eyes is present, which are covered by a nictitating membrane that protects them while in water.
- Tympanum: On either side of the eyes, a membranous tympanum (ear drum) receives sound signals.
- Forelimbs and Hindlimbs: The forelimbs are shorter and help in walking and holding, whereas the hindlimbs are larger, muscular, and webbed, specialized for leaping, swimming, and burrowing.
- Sexual Dimorphism: Male frogs can be distinguished by the presence of sound-producing vocal sacs and a first digit on the forelimb that bears copulatory pads (nuptial pads) during the breeding season.
Definition: Sexual dimorphism is the condition where the two sexes of the same species exhibit distinct characteristic differences beyond the sexual organs themselves.
Digestive System
Anatomy refers to the study of internal organs. The digestive system of a frog consists of an alimentary canal and digestive glands.
Alimentary Canal
The alimentary canal is short because frogs are carnivores, and plant matter (cellulose) requires a longer tract for digestion. Meat-containing diets are easier and faster to break down.
- Mouth: Opens into the buccal cavity that leads to the esophagus.
- Pharynx and Esophagus: The esophagus is a short tube that opens into the stomach.
- Stomach: Digests food enzymatically. It leads to the intestine.
- Intestine: Highly coiled and responsible for the absorption of digested nutrients.
- Rectum: The terminal part of the alimentary canal that opens into a common chamber called the cloaca.
- Cloaca: A common chamber for the exit of fecal matter, urine, and reproductive cells. It opens to the exterior through the cloacal aperture.
Digestive Glands
- Liver: Secretes bile, which is stored in the gall bladder. Bile aids in the emulsification of fats.
- Pancreas: Produces digestive enzymes that break down carbohydrates, proteins, and fats.
Respiratory System
Frogs respire on land and in water by two different methods, making their respiratory mechanism highly adaptable.
| Type of Respiration | Site / Organ | Condition |
|---|---|---|
| Cutaneous Respiration | Skin | Takes place in water and on land. Dissolved oxygen in water is exchanged directly through the moist skin. |
| Pulmonary Respiration | Lungs | Takes place on land. Air enters through the nostrils into the buccal cavity and then flows into the lungs. |
During aestivation (summer sleep) and hibernation (winter sleep), gaseous exchange takes place exclusively through the skin (cutaneous respiration).
Circulatory System
The circulatory system of a frog is a closed type, possessing a lymphatic system and a blood vascular system.
Blood Vascular System
- Heart: A muscular, three-chambered heart is situated in the upper part of the body cavity. It has two atria and one ventricle.
- Covering: The heart is covered by a protective membrane called the pericardium.
- Sinus Venosus: A triangular structure joins the right atrium on the dorsal side of the heart and receives blood from the major veins (vena cava).
- Truncus Arteriosus: A ventral structure arising from the ventricle that pumps oxygenated and mixed blood away from the heart.
- Blood Composition: Blood consists of plasma and blood cells (RBCs, WBCs, and platelets). RBCs are nucleated and oval-shaped.
Nervous System
The nervous system is organized into the central nervous system, peripheral nervous system, and autonomic nervous system.
- Central Nervous System (CNS): Consists of the brain and the spinal cord.
- Brain: Enclosed in a bony brain box (cranium). Divided into the forebrain (olfactory lobes, cerebrum, diencephalon), midbrain (optic lobes), and hindbrain (cerebellum, medulla oblongata).
- Spinal Cord: Enclosed in the vertebral column, originating from the medulla oblongata.
- Peripheral Nervous System (PNS): Comprises cranial nerves arising from the brain and spinal nerves arising from the spinal cord.
- Autonomic Nervous System (ANS): Comprises the sympathetic and parasympathetic nervous systems regulating involuntary visceral functions.
Frogs possess well-organized sense organs including organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes), and hearing (tympanum with internal ears).
Reproductive System
Frogs have highly developed male and female reproductive systems. Fertilization is external.
Male Reproductive System
- Consists of a pair of yellowish, ovoid testes adhered to the upper part of kidneys by a double fold of peritoneum (mesorchium).
- Vasa efferentia (10-12 in number) arise from testes and enter the kidneys on their side, opening into Bidder's canal.
- Finally, urine and sperms pass through a common urinogenital duct that opens into the cloaca.
Female Reproductive System
- Consists of a pair of ovaries situated near the kidneys. Ovaries have no functional physical connection with the kidneys.
- A pair of oviducts originating from the ovaries opens separately into the cloaca.
- A mature female can lay 2,500 to 3,000 ova at a time.
Exam Note: Fertilization in frogs is strictly external and takes place in water. Development involves a larval stage called a tadpole, which undergoes metamorphosis to transform into an adult frog.