The external features of Frog is explained under flowing headings;
- Shape and size: - Body is somewhat spindle-shaped,
pointed anteriorly while rounded posteriorly. It is dorso-ventrally
flattened and stream lined. It consists of head, trunk and limbs. Neck and
tail are absent.
Average length is 12 to 18
cm and width is 5-8 cm.
- Skin and colour: – Skin is thin, moist, slimy, and smooth. It fits loosely on the body and subcutaneous spaces are also present.Thus, frog's skin can be removed easily. Skin of dorsal surface is thick and has dorso-lateral folds or thickenings called dermal plicae. Colour of skin is green with black or brown spots dorsally while pale-yellow ventrally. A pale yellow mid-dorsal line runs from tip of snout to cloacal aperture.It's main function is to protect underlying parts while is also responsible for respiration.
The
chromatophores give colour to skin. Three types of chromatophores are present
which are: Melanophores
provide black brown colour, Xanthophores
provide reddish and yellowish colour and Guanophores
provide whitish colour.
- Head: - It is roughly triangular with narrow blunt anterior end and more wide posterior end. Its bluntly pointed anterior end is called as snout. The head consists of following structures:
upper jaw and movable lower jaw.
*External nares: - Two small openings present dorsally above the mouth through
which air passes during respiration.
*Eyes: - Two large, spherical
and protruding eyes are present
dorso-laterally on top of head. Each eye is spherical in shape and protrudes to
some extent. The dorso-lateral position and protuberant nature help to view
side objects in absence of neck. Each eye remains protected by thick, immovable
upper eye lid and thin and movable lower eye lid. A thin, transparent and
freely movable third eye lid or nictitating membrane arises from lower eye lid
which helps in viewing and protects eye inside water.
*Eardrum or tympanum:
- Behind each eye a circular patch of skin called as eardrum or tympanum is present. It receives sound wave.
*Brown spot: - A
light coloured patch present in the middle of head just infront of eyes
representing the third eye of the ancestors.
*vocal sacs: - In male frogs, under
surface of head contains two bluish patches of skin called as vocal sacs, which act as resonator to
intensify sound wave during croaking.
- Trunk: – Head remains joined with flat
trunk at behind. Its back is raised in the middle called as hump which is
generally visible during squatting.
At the posterior end of
trunk, a small circular opening called cloacal aperture is present
through which undigested food metabolic wastage and gametes are passed out.
* Limbs: - Two pairs of limbs are present laterally, fore limbs and hind
limbs.
Fore limbs are short and
arise anteriorly from trunk just behind head. Each fore limb consists of upper
arm, fore arm and hand with 4-digits.
Its thumb is vestigial. In male frog, base of 1st finger get
thickened during breeding season to form a pad like structure called as
copulatory pad or nuptial pad. It helps to clasp female frog during amplexus.
Hind limbs are much
elongated and powerful. They arise posteriorly from trunk. Each hind limb
consists of upper thigh, middle shank and terminal foot with 5 slender toes
connected by broad web which helps in swimming.
Sexual Diamorphism
It refers to some external characters by which male and
female frogs are distinguished.
i.
Males are slimmer while females are stouter.
ii.
Males are generally smaller than females.
iii.
Males possess vocal sacs.
iv.
Males possess copulatory pads.