7. STRUCTURAL
ORGANISATION IN ANIMALS
MORPHOLOGY & ANATOMY OF FROG
Systematic position
Phylum : Chordata
Class : Amphibia
Genus : Rana
Species : tigrina
·
Rana tigrina is the most common species in India.
·
They
are poikilotherms (cold blooded).
·
They
can change colour to hide them from their enemies (camouflage). This protective coloration is called mimicry.
·
During
summer and winter, they undergo aestivation
(summer sleep) and hibernation
(winter sleep) respectively to protect them from extreme heat and cold.
MORPHOLOGY OF FROG
·
Body
is divisible into head & trunk. Neck and tail absent.
·
Skin
is moist, smooth and slippery due to the mucus.
·
Colour
of dorsal side is olive green with dark irregular spots and ventral side is
pale yellow.
·
The
frog never drinks water but absorb it through the skin.
·
A mouth, paired nostrils and bulged eyes (covered
by nictitating membrane) are present.
· On either side of eyes have a membranous tympanum (ear).
·
The forelimbs (4 digits) and hind limbs (5 digits) help in swimming, walking, leaping and burrowing. The hind limbs are larger
and muscular than fore limbs.
·
Feet
have webbed digits that help in swimming.
·
Frogs
exhibit sexual dimorphism. Male
frogs have sound producing vocal sac
and also a copulatory (nuptial) pad
on the first digit of fore limbs which are absent in female frogs.
ANATOMY OF FROG
Digestive system
-
Consists
of alimentary canal and digestive glands.
-
The
alimentary canal is short because frogs are carnivores and hence the length of
intestine is reduced.
-
Mouth → buccal cavity → pharynx → oesophagus → stomach →
intestine → rectum → cloaca.
-
Liver
secretes bile that is stored in gall bladder. Pancreas produces pancreatic juice containing digestive
enzymes.
-
Food
is captured by the bilobed tongue.
-
Digestion: Gastric juice and HCl secreted from gastric wall digest the food.
Partially digested food (chyme) is
passed from stomach to the duodenum.
Duodenum receives bile and pancreatic juices through a common
bile duct.
Bile emulsifies fat.
Pancreatic juice digests carbohydrates and proteins. Digestion completes in the
intestine.
-
Finger-like villi and microvilli in intestine absorb digested food. The undigested solid
waste moves into the rectum and
passes out through cloaca.
Respiratory system
-
Skin acts
as aquatic respiratory organ (cutaneous
respiration). Dissolved oxygen in the water is exchanged through the skin
by diffusion. During aestivation and hibernation respiration takes place
through skin.
-
On
land, the buccal cavity, skin and lungs (pulmonary respiration) act as the respiratory organs.
-
The
lungs are a pair of elongated, pink coloured sac-like structures present in the
thorax. Air enters through the nostrils into the buccal cavity and then to
lungs.
Circulatory system
-
Closed type. Includes Blood
vascular system (heart, blood vessels &
blood) and lymphatic system (lymph,
lymph channels & lymph nodes).
-
Heart
is 3-chambered, (two atria and one ventricle) and is covered by a
membrane called pericardium.
-
A
triangular structure called sinus
venosus joins the right atrium. It receives blood through major veins (vena cava).
-
The
ventricle opens into a saclike conus
arteriosus on the ventral side of the heart.
-
The
blood pumped from the muscular heart is carried to all parts of the body by the
arteries (arterial system).
-
The veins collect blood from different
parts of body to the heart and form the venous
system.
-
Hepatic portal system (venous connection between liver and intestine) and renal portal system (between kidney and
lower parts of the body) are present
in frogs.
-
Blood
contains plasma and cells (RBC, WBC & platelets). RBCs
are nucleated and contain haemoglobin.
-
Blood
transports nutrients, gases and water to tissues.
Excretory system
- Includes kidneys
(2), ureters (2), cloaca & urinary
bladder.
-
Kidneys are
dark red and bean-shaped. Found posteriorly in the body cavity on both sides of
vertebral column. Each kidney is formed of uriniferous
tubules (nephrons).
-
2
ureters emerge from the kidneys. In male frogs, the ureters act as urinogenital duct which opens into
cloaca.
In females, ureters
& oviduct open separately in cloaca.
-
The
thin-walled urinary bladder is
present ventral to the rectum which also opens in the cloaca.
-
The
frog is a ureotelic animal (excretes urea). Nitrogenous wastes are
carried by blood into the kidney where it is separated and excreted.
Control and co-ordination
Endocrine system
-
The endocrine glands secrete hormones.
- Endocrine glands: pituitary,
thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals &
gonads.
Nervous system
It includes
§ Central
nervous system (brain &
spinal cord),
§ Peripheral
nervous system (cranial & spinal nerves)
§ Autonomic
nervous system (sympathetic &
parasympathetic).
-
There
are 10 pairs of cranial nerves
arising from brain.
-
Brain
is enclosed in a bony brain box
(cranium).
-
The
brain is divided into
§
Fore-brain: Includes olfactory lobes, paired
cerebral hemispheres and unpaired
diencephalon.
§
Mid-brain: Includes a pair of optic lobes.
§ Hind-brain:
Includes cerebellum & medulla oblongata.
-
Medulla oblongata passes out through the foramen magnum and continues into spinal cord, which is enclosed in the vertebral column.
-
Sense
organs include organs of
§ Sensory
papillae: For touch
§ Taste
buds: For taste
§ Nasal
epithelium: For smell
§
Simple eyes: For vision. Paired and situated in orbit
§ Tympanum
with internal ears: For hearing and balancing (equilibrium).
Reproductive system
- Male
reproductive organs consist of a pair of yellowish ovoid testes, which are found adhered to the upper part of kidneys by a
double fold of peritoneum (mesorchium).
- Vasa efferentia (10-12 in number) arise from testes. They enter the
kidneys on their side and open into Bidder’s
canal. It communicates with urinogenital duct that comes out of the kidneys
and opens into cloaca.
-
The cloaca is a small, median chamber that
is used to pass faecal matter, urine and sperms to the exterior.
- The
female reproductive organs include a pair of ovaries. The ovaries are situated near kidneys and there is no
functional connection with kidneys.
-
A
pair of oviduct arising from the ovaries opens into the cloaca separately.
-
A
mature female can lay 2500 to 3000 ova
at a time.
-
Fertilisation
is external and takes place in water.
-
Development
involves a larval stage called tadpole.
-
Tadpole
undergoes metamorphosis to form the adult.
ECONOMIC IMPORTANCE
-
Frogs
are beneficial for mankind because they eat insects and protect the crop.
-
Maintain
ecological balance by serving as an important link of food chain and food web
in the ecosystem.
-
In
some countries the muscular legs of frog are used as food by man.
