External Anatomy of Frog with Specific Organs and Functions

Frogs are amphibians found near water and damp places. The commonly found frog in the world belongs to the genus “Rana.” These animals prefer to stay in water or near water for two main reasons

• Keep their skin moist for cutaneous respiration 
• To quickly escape from predators by jumping into the water.

Frog External Anatomy

In the frog picture below, notice the labeled external anatomical parts

The picture above shows the major external anatomical features of the frog, like:

1. Head region – containing eyes, nostrils, and tympanum
2. Trunk – housing internal organs
3. Forelimbs – adapted for landing and support
4. Hindlimbs – specialized for powerful jumping

Size of Frog

• The size of an average frog is around 12 to 18 cm in length and 5 to 8 cm in width. However, this varies considerably across various species. For example

This remarkable size variation demonstrates the adaptive diversity of frogs across different ecological niches, from tiny soil-dwelling species to massive pond predators.

Shape

• The frog’s body is slightly spindle-shaped, rounded posteriorly, and pointed anteriorly.

• Sometimes, the body is slightly flattened dorsoventrally and streamlined to swim through the water. 

• The frog’s body can be divided into a head, trunk, and limbs with no neck and tail. 

• However, the frog has a tail at birth, disappearing as the baby frog transforms by metamorphosis.

Frog Skin and Color

• The skin is loosely fitted to the frog’s body, which remains like loose skin layers.

• The back part of the frog skin has dorsolateral folds and thickenings called dermal plicae. 

• It is generally seen that the color of the frog’s skin is mainly green with black and yellow spots over it. 

• In some cases, brown spots are also found in the central part. 

• A light yellow mid-dorsal line is seen on the frog’s body, which runs from the tip to the snout and then to the cloacal opening at the back of the body. 

• Changes in skin color are also seen in frogs due to the expansion of melanophores pigment.

Frog skin and respiration

• The frog’s skin is thin and slimy and kept constantly moist by mucous secretions.

• This moisture is critical for cutaneous respiration, wherein frogs absorb oxygen directly through their skin.

• The membranous tissue in the skin is permeable to water and has a large network of blood vessels, which help the diffusion of respiratory gases.

• Unlike animals relying only on the lungs to breathe, this skin route provides the frog with up to 30 to 60% of the required oxygen.

👉 This allows the frog to

1. Remain in water for prolonged periods
2. Survive in oxygen-poor environments and
3. Undergo hibernation in winter months.

• This specialized skin respiration is complemented by the lung respiration in frogs, making them highly adaptable to both aquatic and terrestrial environments.

Head

• The head of the frog is somewhat demarcated clearly from the trunk region. 

• The region is a flat and rough triangle in outline with a blunt anterior snout that terminates in a wide transverse mouth. 

• Two small openings are found on the head region, which lies dorsally above the mouth tip of the snout and serves in respiration.

• In the head region, the two protruding eyes are situated dorsolateral to each other on the top.

• When the eyes are pressed downwards, they are imminent in the buccal cavity.

• Each eye has a thick and pigmented, immovable lower eyelid.

• From this place arises a thin, semi-transparent eyelid called a nictitating membrane. 

• This membrane covers the eye and protects it during swimming, keeping it always moist.

• Behind each of the eyes, there is a pigmented circular patch of skin called the eardrum, which is also known as the tympanum.

• This is responsible for receiving the sound waves for hearing.

Trunk

• The head is broadly connected with a flat, ovoid-shaped trunk (the body).  

• Its back part is raised in the middle region in a characteristic sacral prominence of the hump.

• This hump is especially conspicuous when the frog starts to squat. 

• The trunk region encases the lungs, heart, digestive tract, liver, genitals, and kidneys.

Cloacal System: A Multifunctional Adaptation

At the posterior end of the frog’s trunk, there is a small opening called the cloacal aperture. This single opening serves multiple critical functions like:

• Elimination of waste (feces and urine)
• Release of reproductive gametes like sperm or ova.
• Water regulation and balance

Did you know?

• Unlike mammals with separate openings for digestive, urinary, and reproductive systems, frogs exemplify evolutionary efficiency with this single multipurpose opening.

• This adaptation is common among amphibians, reptiles, and birds, representing an interesting evolutionary difference from mammals.

Why this matters:

Understanding the cloaca helps scientists track evolutionary relationships between vertebrate groups and provides insights into how different body systems became specialized over millions of years of evolution.

• A small aperture called the cloacal aperture at the frog’s trunk’s posterior end.

• This aperture is the opening that facilitates the disposal of feces, urine, and reproductive gametes like the sperms and ova.

Limbs

• Laterally, the trunk of the frog has two pairs of limbs.

• The short limbs arise from the anterior part of the trunk, from just behind the head, while the hind limbs arise from the posterior of the trunk. 

Each of the forelimbs shows the following parts.

1. The upper arm called Brachium
2. Forearm called Antebrachium
3. Wrist called Carpus
4. Hand called Manus

• The hand or manus bears 4 digits (fingers) without webbing.

• Of these 4 digits, the thumb or pollex is vestigial.

• In the case of a male frog, the base of the first finger is thickened, forming the nuptial pad in breeding seasons that helps clasp the female amplexus.

The hind limbs are powerful and arise from the trunk region.

Each of the hind limbs consists of:

• The thigh
• Shank called crus
• Ankle called tarsus
• Large foot called pes (distal portion of the hind limb)

These hind limbs support swimming and walking on damp lands.

Sexual Dimorphism

Sexual dimorphism is strongly seen in the frog population.

Several imminent identifying features help in distinguishing male and female frogs, which are as follows:

• The males are smaller than the females and have darker shades than the males.
• Male frogs are slimmer, whereas female frogs are stouter when they carry eggs.
• The male frogs croak louder.
• The female frogs lack the eardrum present in males.
• The male frogs have swollen copulatory and nuptial pads on the inner fingers, which are lacking in female frogs.

External anatomy of Frog quiz

References

1. Duellman, W. E., & Trueb, L. (1994). *Biology of Amphibians*. Johns Hopkins University Press.
2. Wells, K. D. (2007). *The Ecology and Behavior of Amphibians*. University of Chicago Press.
3. Vitt, L. J., & Caldwell, J. P. (2013). *Herpetology: An Introductory Biology of Amphibians and Reptiles* (4th ed.). Academic Press.
4. Kardong, K. V. (2018). *Vertebrates: Comparative Anatomy, Function, Evolution* (8th ed.). McGraw-Hill Education.
5. AmphibiaWeb. (2023). University of California, Berkeley. Retrieved from https://amphibiaweb.org
6. MAURICIO & JOSE (1970). The Structure and Membrane Properties of the Frog Nictitans. Investigative Ophthalmology & Visual Science. 
7. Goliath Frog