Morphology, Anatomy, and Functions of Different Systems of the Frog

Frogs, belonging to the class Amphibia, are fascinating vertebrates that can live both on land and in water. The Indian bullfrog, Rana tigrina, is a typical example. Frogs have a remarkable body plan that showcases adaptations for their dual lifestyle. They are poikilotherms (cold-blooded), meaning their body temperature fluctuates with the environment. They undergo periods of inactivity called hibernation and aestivation to survive extreme cold and heat.

1.0Morphology of a Frog

A frog's body is streamlined and divided into two central regions: the head and the trunk. Unlike many other vertebrates, they lack a neck and tail.

• Skin: The skin is smooth, moist, and slippery due to mucus secretion. It is highly permeable, acting as an accessory respiratory organ. The colouration of the skin provides camouflage.
• External Structures: A pair of nostrils is present above the large mouth. Prominent, bulging eyes are located on the top of the head. Behind each eye, a membranous tympanum (eardrum) is visible, which receives sound vibrations.
• Limbs: Frogs have two pairs of limbs. The shorter forelimbs have four digits, while the longer, muscular hindlimbs have five digits with webbing between them, which aids in swimming.
• Sexual Dimorphism: Male and female frogs can be distinguished by specific features. Male frogs have vocal sacs and a copulatory pad on the first digit of the forelimbs, which are absent in females.

2.0Anatomy and Functions of Different Systems

A frog's body cavity accommodates several well-developed organ systems.

Digestive System

Frogs are carnivorous. Their digestive system consists of a short alimentary canal and associated digestive glands.

• Alimentary Canal: The mouth opens into the buccal cavity, which leads to the pharynx, a short oesophagus, and a J-shaped stomach. The stomach continues into the small intestine, which then leads to the rectum and finally opens into the cloaca.
• Tongue and Teeth: The frog's tongue is long, muscular, and protrusible. It is attached to the front of the jaw and is sticky at the tip, used to capture prey. The upper jaw has teeth (maxillary and vomerine teeth) to hold the prey, but they are not used for chewing.
• Digestive Glands: The liver secretes bile, and the pancreas produces pancreatic juices containing digestive enzymes. These secretions aid in the digestion of food in the small intestine.

Respiratory System

Frogs use three modes of respiration:

• Cutaneous Respiration: This is the primary mode of respiration in water and during hibernation/aestivation. The moist, highly vascularized skin allows for the diffusion of gases (oxygen and carbon dioxide) directly with the environment.
• Buccal Respiration: On land, the lining of the buccal cavity can also facilitate gas exchange.
• Pulmonary Respiration: This occurs on land using a pair of small, sac-like lungs. Air is drawn into the lungs through the nostrils and buccal cavity, and gas exchange takes place.

Circulatory System

The frog has a well-developed, closed circulatory system.

• Heart: It has a unique three-chambered heart with two atria and one ventricle. The right atrium receives deoxygenated blood from the body via the sinus venosus, and the left atrium receives oxygenated blood from the lungs and skin.
• Circulation: Both oxygenated and deoxygenated blood enter the single ventricle, where some mixing occurs. From the ventricle, the mixed blood is pumped out to the body and lungs via a conical structure called the conus arteriosus. This type of circulation is known as incomplete double circulation.
• Blood: Blood contains plasma and three types of cells: red blood cells (nucleated), white blood cells, and platelets. Frogs also have a lymphatic system.

Excretory System

Frogs are ureotelic animals, meaning they excrete urea as their main nitrogenous waste product.

• Kidneys: A pair of dark red, bean-shaped kidneys are the main excretory organs. They filter blood to remove nitrogenous waste and form urine.
• Ureters and Urinary Bladder: Urine passes from the kidneys through the ureters into the urinary bladder, where it is temporarily stored. The bladder opens into the cloaca.

Nervous System

The frog's nervous system is well-organized and includes the central, peripheral, and autonomic nervous systems.

1. Central Nervous System (CNS): Composed of the brain and the spinal cord. The brain is divided into three parts:

• Forebrain: Includes the olfactory lobes and cerebrum.
• Midbrain: Composed of a pair of optic lobes.
• Hindbrain: Consists of the cerebellum and medulla oblongata.

2. Peripheral Nervous System (PNS): Composed of cranial nerves (10 pairs) and spinal nerves.
3. Sense Organs: Frogs have well-developed sense organs for touch, taste, smell, hearing, and sight. The eyes are simple in structure, and the tympanum is the organ of hearing.

Reproductive System

Frogs are dioecious, with separate male and female individuals.

• Male System: Consists of a pair of yellowish, ovoid testes attached to the kidneys. From each testis, vasa efferentia emerge and join the Bidder's canal, which connects to the urinogenital duct. The urinogenital duct opens into the cloaca.
• Female System: Includes a pair of ovaries located near the kidneys. Oviducts arise from the ovaries and open into the cloaca.
• Fertilisation: Fertilisation is external and occurs in water. The female lays eggs, and the male releases sperm over them, leading to the formation of a tadpole larva that undergoes metamorphosis.