STRUCTRUAL
ORGANISATION IN ANIMALS
BODY ORGANISATION
FROG:
Probably the best example of an amphibian that you remember
right from your childhood is the frog. Did you know that just like the
butterfly, a frog also undergoes complete metamorphosis?
From a tadpole that lives in water, breathes through gills
and has a tail. In the period of time, that same organism develops an organ
system which helps them to survive in terrestrial habitat.
The frog is selected as a type of animal to introduce the
student to vertebrates due to the following reasons:
(1) It is easily available.
(2) Its anatomy and physiology can be conveniently
demonstrated to students because of the larger size of the body organs.
(3) The study of frog reveals as to how fish-like animals
got evolved into amphibia, the first land vertebrates.
Systematic Position of Frog
Phylum- Chordata
Sub-phylum - Vertebrata
Class - Amphibia
Order- Anura
Family – Ranidae
Genus- Rana
Species- tigrina
Common name - Indian frog
Frogs belong to Class Amphibia of Phylum Chordata. Common
Indian species of frog is Rana tigrina. They are poikilotherms/cold-blooded,
Le. their body temperature varies according to the environment.
Camouflage/Mimicry is the feature in which they have a
protective colouration to keep their predators away from them They play a
crucial role in the ecosystem's food chain and food web.
Rana tigrina is the most widely distributed species in North
India. Generally, frogs are found in ponds, tanks, pools, ditches, etc. However,
they may leave their habitat and come on land to hunt for their prey. The frog
not only breeds in water but also passes their early life there. The frog is
called an amphibious animal because it lives both in water and on land.
Frogs are not seen during extreme heat and cold conditions
as they hide in deep burrows to protect themselves. This phenomenon is termed
as aestivation (summer sleep) and hibernation (winter sleep).
Important
Frog never drinks water but absorb it through the skin.
Frogs mainly respire through their skin, they occasionally go into the water to
moisten their skin, which is a very essential factor for such respiration.
Morphology of Frog
(1) Texture of skin is smooth and slippery because of the
presence of mucus.
(2) Body is divided into a head and a trunk Neck and tail
are absent A pair of nostrils is present above the mouth.
(3) Nictitating membrane is the membrane that covers bulged
eyes and protects them in water.
(4) Membranous tympanum (ear) receives sound signals.
(5) Swimming is made easy with the presence of webbed digits
in feet Hind limbs have five digits and are larger and more muscular as
compared to forelimbs. Forelimbs have four digits and have a copulatory pad or
nuptial pad on the first digit in males only. Function of forelimbs and hind
limbs is to help in swimming, walking, leaping and burrowing.
(6) Frogs exhibit sexual dimorphism. During the breeding
season, the male frog makes a peculiar sound with the help of their vocal sacs
(known as croaking) to call the female.
Important
The frog shed off almost once a month its skin during Its
active life in the form of small castings This phenomenon is known as moulting.
The frog is capable of changing its body colour, gradually
with the change in its surroundings and climatic conditions. The phenomenon is
known as protective colouration or metachrosis.
Anatomy of Frog
Digestive system
The digestive system consists of a short alimentary canal
(reduced intestine as frogs are carnivores) and digestive glands. Following are
the parts of the alimentary canal:
(1) Mouth: It is
a wide anterior most opening leading to a spacious buccopharyngeal cavity.
(2) Buccopharyngeal
cavity: It is called so because the
buccal cavity and pharynx have been merged into one cavity. (3) Oesophagus: A short tube opening into
the stomach, leading to intestine and rectum, and finally opens outside through
cloaca.
(4) Bilobed tongue: For capturing food.
(5) Stomach: The
stomach is divided into two parts:
(i) Cardiac Stomach: The anterior larger part is
present near the heart.
(ii) Pyloric Stomach: It is the posterior narrow
tapering part, which is separated from the duodenum by a muscular constriction.
It secretes HCl and gastric juice for digestion.
(6) Small intestine: It is the longest part of
the alimentary canal suspended by mesentery, and divisible into two parts, Le
Duodenum and Ileum Duodenum receives chyme (partially digested food from
stomach), bile from gall bladder and pancreatic juices from pancreas through
common bile duct.
(7) Rectum (Large
intestine): The ileum leads into a broad, thin-walled short but wider tube,
known as rectum or large intestine. The rectum stores the faecal matter and
water is absorbed by its wall.
(8) Cloaca: It is
the last part of alimentary canal. The urinary bladder opens into cloaca. The
cloaca opens out through a cloacal aperture, situated at the posterior most end
of abdomen.
Following are the digestive glands of digestive system:
(1) Liver: It is
the largest gland of the body. It secretes bile stored by gall bladder. Bile
emulsifies fat.
(2) Pancreas: It
is a branched flat elongated pale yellow gland lying in mesentery extended in
the loop between stomach and duodenum. The pancreas is made up of lobules,
connective tissues and inner core of islets of Langerhans. It produces
digestive juice containing digestive enzymes. Pancreatic juice digest
carbohydrates and proteins.
Important
Villi and microvilli: Finger-like folds In Inner walls of
intestine which increase absorption area.
Respiratory
system
Being amphibious in nature, the frog can utilise free oxygen
and also the dissolved oxygen in water. The adult frog respires in three
different manners:
(1) Cutaneous
respiration: The skin of a frog provides an extensive surface for the
exchange of gases. It is thin, richly supplied with blood and kept moist by the
mucus and water. The cutaneous respiration is always carried out. During
hibernation and aestivation, it is the only method of respiration in a frog.
(2) Buccopharyngeal
respiration: This type of respiration occurs when the animal is on the land
or partially immersed in water. There are two external nares situated at the
tips of the snout. Each naris (nostril) leads into a nasal chamber that opens
into the buccopharyngeal cavity by an internal naris.
(3) Pulmonary
respiration: It is less frequent than the cutaneous and buccopharyngeal
respiration. It occurs when need for oxygen is increased. It occurs by lungs.
Lungs: Elongated
sac-like structures, air enters through nostrils into the buccal cavity and
then to lungs.
Blood
vascular (Circulatory) system
(1) Blood vascular system is of closed type as the blood
flows in the blood vessels. It represents single circulation. It means both
oxygenated and the deoxygenated blood enters the heart and gets mixed in the
ventricle.
(2) Blood vascular system comprises blood. heart and blood
vessels. The blood is a mobile connective tissue, composed of a fluid, the
plasma and the cells and the blood corpuscles.
(3) Heart is three-chambered consisting of two atria and one
ventricle. It is covered by pericardium (membrane).
(4) Sinus venosus (triangular structure) joins the right
atrium which receives blood through vena cava Ventricle joins conus arteriosus
(sac-like).
(5) Arterial system carries the blood to all parts of the
body by arteries.
(6) Venous system carries blood from different parts of the
body to heart through veins.
(7) Blood contains plasma, RBCs (red blood cells) or
erythrocytes, WBCs (white blood cells) or leucocytes and platelets. RBCs are
nucleated and contain haemoglobin. Blood transports nutrients, gases and water.
(8) Venous connection between the liver and intestine is
called hepatic portal system and that between the kidney and lower parts of the
body is called renal portal system.
Blood circulation: It can be represented by the following
graphical flow chart:
Control and
coordination system
Control and coordination system of frogs is highly evolved.
It includes both neural system and endocrine glands. The pituitary, thyroid,
parathyroid, thymus, pineal body, pancreatic islets, adrenals, and gonads are
the most significant endocrine glands in frogs.
Nervous
system
Nervous system of a frog consists of
(1) Central nervous
system
(i) Brain:
Enclosed in cranium (bony brain box), composed of three parts, Le. Forebrain
which consists of olfactory lobe, cerebral hemisphere and diencephalon.
Midbrain consists of optic lobes and Hindbrain consists of cerebellum and
medulla oblongata.
(ii) Spinal cord:
Located in the vertebral column and joins the medulla oblongata via foramen
magnum of the cranium.
(2) Peripheral
nervous system: It consists of cranial nerves (10 pairs, arise from brain)
and spinal nerves (9 pairs).
(3) Autonomic nervous
system: It includes the nerves and ganglia that control and coordinate such
organs which are not under voluntary control It comprises sympathetic and
parasympathetic nervous systems.
Sense
Organs
Sense organs receive stimuli (changes in environment) from
the outside or inside of the animals and pass impulses to the nervous system.
Frog has five types of sense organs:
(1) Tangoreceptor:
Organs of touch - Sensory papillae
(2) Olfactoreceptors:
Organs of smell - Nasal epithelium
(3) Gustatoreceptors:
Organs of taste - Taste buds.
(4) Photoreceptors:
Organs of vision or sight - Eyes
(5) Statoacoustic
receptors: Organs of hearing and balancing - Tympanum with internal ears.
Reproductive
system
Frogs have well-organised male and female reproductive
systems and exhibit dimorphism. In male frogs, vocal sacs and copulatory pads
or nuptial pads are present. In female frogs, vocal sacs and copulatory pads
are absent Male reproductive system of a frog consists of:
(1) Ovoid testes
(2) Mesorchium:
Testis attached to the upper part of the kidneys by a double fold of peritoneum
known as the Mesorchium.
(3) Vasa efferentia:
Rise from testes and are 10 to 12 in number.
(4) Bidder's canal:
Vasa efferentia arrive on the side of the kidneys and open into Bidder's Canal.
Female reproductive system of a frog consist of.
(1) A pair of ovaries are located near the kidneys but have
no functional relationship with them
(2) A pair of oviducts open into the cloaca separately.
At any given time, a mature female can lay between 2,500 and
3,000 ova. Fertilization occurs in water and is done externally. Development is
indirect, it involves a larval stage called tadpole. Tadpole undergoes
metamorphosis to form an adult.
Importance of Frogs
They eat insects and protect the crop, maintain ecological
balance as they serve as an important link of food chain and food web in the
ecosystem. The muscular legs of the frog are consume as food by men in some
countries.
Frogs were exported in large quantities but it is now
legally banned. Froglets (very small frogs) are used as fish bait. Due to
habitat loss and large scale use of this animal, it has become a threatened
species. Since the frog is an important animal in the food chain, it helps to
maintain our ecosystem. So, it should be protected. Use of frogs for dissection
purposes is banned by the animal welfare organisation.
