Frog: Digestive, Respiratory & Reproductive System | Zoology

In this article we will discuss about:- 1. Digestive System of Frog 2. Respiratory System of Frog 3. Circulatory System 4. Excretory System 5. Nervous System 6. Sense Organs 7. Reproductive System.

Digestive System of Frog:

Frog is a carnivorous animal. It feeds on small insects, spiders, worms, tadpoles and even smaller frogs. Generally only moving animals are caught and eaten by frogs and the motionless are ignored. The prey is captured by the sticky tongue which is attached in front and free behind. When the prey is around, the hind part of the tongue is shot out on the prey, which adheres to it. The tongue is quickly withdrawn with the prey and is swallowed entirely. If the prey is found undesirable it is thrown out.

Digestive system of frog consists of alimentary canal and associated digestive glands.

(a) Alimentary Canal:

The alimentary canal of frog is short and coiled. It starts with mouth and ends in cloaca.

The alimentary canal includes the following parts:

(i) Mouth:

It is wide and terminal.

(ii) Buccopharyngeal Cavity:

The mouth leads into a wide and broad bucco­pharyngeal cavity. It is bounded by two jaws-upper and lower jaws. Teeth are found only on upper jaw. They are of two types-maxillary teeth and vomerine teeth. The maxillary teeth are found on maxilla bone of upper jaw. They are small hook like and are arranged closely in a row.

Lower jaw is devoid of any teeth. The vomerine teeth are found on vomer bones. They are in two patches one on each side of the median line of the roof of the bucco-pharyngeal cavity. The teeth are homodont (similar), acrodont (attached to the bones) and polyphyodont (in many sets).

Teeth are not used for chewing but prevent the escape of prey, so they are prehensile in function. The roof of the bucco-pharyngeal cavity also has a pair of internal nares one on the outer side of vomerine teeth. Through the internal nares, the external nostrils or nares open into the bucco-pharyngeal cavity. The maximum portion of the roof of the buccal cavity is occupie by the in-bulging of the eye-balls.

The region posterior to the in-bulgings is the pharyngeal region. This region has a gullet in the centre, glottis below the gullet and a pair of openings of Eustachian tubes, one on either side near the left and the right jaw angles. Gullet is the larger and transverse aperture which leads the buccopharyngeal cavity into the oesophagus and it opens only at the time of swallowing of the food.

The vertical slit like glottis leads to the laryngo tracheal chamber. The openings of the Eustachian tubes connect the pharynx with the middle ear. In male frogs, a pair of opening of vocal sacs is found on the floor of the buccal cavity near the jaw angles. These act as resonators (increase the pitch of sound) at the time of croaking.

The floor of the bucco-pharyngeal cavity contains a large, thick, fleshy, bifid tongue. It is attached in front and free behind. The tongue can be everted and retracted with the help of protractor and retractor hypoglossal muscles while capturing the prey. The upper surface of the tongue has small papillae (taste buds) and mucous glands.

(iii) Oesophagus:

The gullet leads into a short and wide tubular oesophagus. It is short because of the absence of neck. But it is highly distensible. The oesophagus leads into the stomach but there is no clear demarcation line between oesophagus and stomach to differentiate these two parts externally.

(iv) Stomach:

Stomach is a thick, muscular and ‘J’ shaped part of the alimentary canal. It is differentiated into anterior, broader and larger cardiac stomach and posterior, narrower and smaller pyloric stomach. Internally there are longitudinal folds in the stomach which enable the stomach to expand whenever needed. The stomach leads into the intestine through a pyloric sphincter.

(v) Intestine:

The stomach leads into the longest tubular part of the alimentary canal – the intestine. It consists of two parts – the small intestine and the large intestine. The small intestine is long, narrow and tubular. It is differentiated into proximal duodenum and distal ileum. The duodenum run forward and forms ‘U’ shaped structure with the stomach. The ileum is a coiled part. The duodenum receives the hepatopancreatic duct.

The small intestine has all the four usual layers of the alimentary canal viz., serosa, muscularis, submucosa and mucosa. The mucosa layer of ileum has thrown into finger – like folds called villi which increase the absorptive area of intestine. This layer consists of two types of cells – goblet cells which produce mucus and absorptive cells which absorb digested food material. The ileum leads at its lower end into the large intestine or colon or rectum.

The large intestine is a short and wide tube. Its hinder end leads to a cavity called cloaca. The cloaca also receives the openings of ureters, urinary bladder and the oviducts in female frog. The cloaca stores the faeces temporarily and absorbs water from them. It opens to the outside by the cloacal aperture through which the undigested material (faeces), sex cells and excretory products are removed from time to time.

(b) Digestive Glands:

The digestive glands of frog include:

i. Liver.

ii. Pancreas.

iii. Gastric glands.

iv. Intestinal glands.

i. Liver:

Liver of frog is a solid, reddish-brown, bilobed largest gland. It consists of two lobes – right and left. The left lobe is larger than the right and is again subdivided into two lobes. Between the right and left lobes there is a thin walled, round, greenish sac called gall-bladder. The liver secretes bile which is temporarily stored in the gall bladder. The bile helps in the emulsification of fats, changes pH of food from acidic to alkaline and checks the growth of bacteria. Liver does not secrete any enzymes.

ii. Pancreas:

The pancreas is situated between the stomach and the duodenum. It is an irregularly shaped gland. The pancreas secretes pancreatic juice which contains many digestive enzymes such as trypsin, amylase, lipase, etc. The pancreatic ducts from the pancreas join the bile duct and form a common hepatopancreatic duct which opens into the duodenum.

iii. Gastric Glands:

The gastric glands present in the mucosa of stomach secrete gastric juice containing HCl and pepsin.

iv. Intestinal Glands:

The intestinal glands secrete intestinal juice, which along with pancreatic juice complete the digestion of proteins, carbohydrates and fats in the intestine.

Respiratory System of Frog:

Frog exhibits three types of respiration due to its amphibious mode of life:

(a) Cutaneous Respiration:

This type of respiration takes place through thin, moist, highly vascular and naked skin. This is the most important mode of respiration where skin acts as a respiratory organ. It is an amphibious organ of respiration as the skin helps in respiration both on land and in water.

Skin serves as the only organ of respiration when the frog undergoes aestivation and hibernation. Since the skin is devoid of any structures that may prevent the diffusion, the exchange of oxygen between the water or air and the blood; and exchange of carbon dioxide between the blood and the water or air takes place very easily.

(b) Buccopharyngeal Respiration:

This type of respiration takes place when the frog is on land through thin, moist, vascular lining of the buccopharyngeal cavity. The lining of the buccal cavity which remains moist by the mucus is richly supplied with blood capillaries. In the buccopharyngeal respiration, the floor of the buccal cavity is lowered and raised continuously during which the air is continuously sucked in and forced out through the external and internal nares.

When the floor of the buccal cavity is lowered the air enters the buccal cavity through the nares. The oxygen of air dissolves in the mucus and then goes into the blood with the help of haemoglobin. At the same time carbon dioxide is given out from the blood into the buccal cavity which is expelled through the nostrils when the floor of the buccal cavity is raised. During buccal respiration the glottis remains closed.

(c) Pulmonary Respiration:

Pulmonary respiration takes place by lungs when the frog is on land. The lungs are a pair of elongated or oval, thin walled, pinkish elastic sacs. Their inner lining is raised to form many chambers called alveoli which increase the inner surface for exchange of gases. The alveoli are separated from each other by septa. They are lined by blood capillaries and remain moist with mucus secreted by mucous glands situated in the wall.

The lungs open in front into the small laryngotracheal chamber through the short bronchi. The larynx is supported by a ring like cricoid cartilage and two crescentic arytenoid cartilages. These prevent the larynx from collapsing. The laryngo tracheal chamber communicates with the pharynx by glottis. Here also the exchange of gases between the alveoli and blood capillaries takes place by diffusion.

Circulatory System of Frog:

The circulatory system of frog is of closed type. It comprises of blood vascular system and lymphatic system which are interconnected.

Blood Vascular System:

The blood vascular system of frog consists of the following:

(i) Heart:

Heart is a dark-red coloured, conical, highly muscular, pulsatile organ situated in the thorax between the lungs. It is enclosed in a two layered membranous sac called pericardium with pericardial fluid in it. This protects the heart from mechanical shocks. The heart comprises of 3 chambers – two auricles and one ventricle. The auricles are thin walled chambers and are separated by an interauricular septum while the ventricle is an undivided chamber.

Associated with the heart, there are two accessory chambers – a tubular truncus arteriosus which is situated on ventral side of the right auricle and a triangular sinus venosus which is present on the dorsal side of the heart. Sinus venosus receives the deoxygenated blood from the body parts by three large veins – right and left venae cavae (precavals), bringing the blood from the anterior region and a posterior vena cava (postcaval), bringing the blood from the posterior region.

Sinus venosus opens into the right auricle. The left auricle receives oxygenated blood from lungs. Both auricles send their blood into ventricle where the blood gets mixed up. The ventricle pumps out the mixed blood to all parts of the body through truncus arteriosus and arches.

(ii) Blood Vessels:

The blood vessels found in frog include arteries, veins and capillaries. Arteries distribute the blood all over the body, veins collect the blood and return it to the heart and fine capillaries connect arteries and veins. The various arteries and veins collectively form the arterial system and venous system respectively.

Arteries:

Arteries carry blood away from the heart to the different parts of the body.

In frog there are 3 pairs of aortic arches which supply blood to various body parts. They are:

a. Carotid Arches:

These arches supply blood to head region by internal and external carotids.

b. Systemic Arches:

Systemic arches supply blood to most parts of the body. Each systemic arch before meeting its counter-part gives rise to following arteries – oesophageal (to oesophagus), occipito- vertebral (to the jaws, eye orbit, nasal chamber, vertebral column, spinal cord and skin) and subclavian arteries (to fore limbs). The two systemics join to form dorsal aorta.

The arteries arising from the dorsal aorta are – coeliaco-mesenteric (supplies to liver and stomach by coeliac artery, duodenum, pancreas, spleen and small intestine by anterior mesenteric artery), gonadials (to gonads), renals (to the kidneys), lumbar (to the dorsal body wall), posterior mesenteric (to the rectum) and iliac arteries (to the urinary bladder, ventral abdominal muscles and hind limbs).

c. Pulmocutaneous Arches:

They supply blood to lungs and skin.

Veins:

Blood from the fine capillaries passes into thin venules and then into veins. Blood is collected by the veins and returned to sinus venosus by three venae cavae – anterior or superior venae cavae (precavals) collect the blood from the head by external jugulars (from lower jaw and tongue), innominate (from brain and shoulder) and subclavian (from fore limbs). The posterior or inferior vena cava or postcaval collects the blood by renals (from kidneys), gonadial (from gonads) and hepatic veins (from liver).

In frog, there are two well-developed portal systems:

A portal system is defined as “a network of blood vessels that begin in the capillaries of one organ and end in the capillaries of another”.

i. Renal Portal System:

Renal portal system is the one in which the renal portal vein collecting the blood from hind-limbs by femoral and sciatic veins, divides into blood capillaries inside the kidneys, where the nitrogenous wastes are removed.

ii. Hepatic Portal System:

Hepatic portal system is the one in which the hepatic portal vein collecting blood from stomach, pancreas, intestine, etc., divides into capillaries inside the liver. Here the excess of nutrients and some waste substances are removed.

(iii) Blood:

Blood of frog is formed of fluid plasma and the blood cells called corpuscles.

The corpuscles are of three types:

i. Erythrocytes.

ii. Leucocytes.

iii. Thrombocytes.

i. Erythrocytes:

The erythrocytes or red blood corpuscles are oval, biconvex and nucleated. They have red pigment, haemoglobin in their cytoplasm.

ii. Leucocytes:

The leucocytes also called white blood corpuscles are amoeboid or circular, whitish and nucleated. These are of five types – neutrophils, basophils, eosinophils, lymphocytes and monocytes.

iii. Thrombocytes:

The thrombocytes are spindle shaped and nucleated. The blood helps in transportation of digested food, respiratory gases, hormones and nitrogenous wastes from one part of the body to another. It also plays important role in the immune system.

Excretory System of Frog:

The excretory system of frog comprises of the following:

(i) Kidneys:

Kidneys are the chief excretory organs of frog. These are paired, dark-red coloured flattened and elongated structures found in the abdominal cavity one on either side of the vertebral column. The kidneys are of mesonephric type. Internally each kidney is formed of around 2000 tubules called kidney tubules or uriniferous tubules or nephrons. The nephrons are the structural and functional units of kidney.

(ii) Ureters:

Arising from the outer side and posterior end of each kidney there is a ureter or mesonephric duct or Wolffian duct. The two ureters run backward and open on the roof of the cloaca. In male frog the upper end of ureter is swollen to form seminal vesicle and it helps to store the sperms. The ureters of male frog transport both urine and sperms and hence they are called urinogenital ducts, while ureters of female frog carry only urine.

(iii) Urinary Bladder:

Opening into the cloaca on its ventral side there is a bilobed thin walled membranous sac called urinary bladder. The opening of the urinary bladder is situated just beneath the openings of the ureters. The excretory product in the form of urine is stored in the urinary bladder from where it is expelled out through the cloacal aperture from time to time.

(iv) Cloaca:

Cloaca is a thin walled small common chamber into which the rectum, ureters and reproductive ducts open. Cloaca opens out by cloacal aperture.

Nervous System of Frog:

The nervous system of frog is differentiated into the central nervous system consisting of brain and spinal cord, the peripheral nervous system consisting of the nerves coming out from the brain and the spinal cord, and the autonomic nervous system consisting of sympathetic and parasympathetic nervous systems. The autonomic nervous system is often considered as a part of the peripheral nervous system because they are closely related to each other.

Central Nervous System (CNS):

It is formed of brain and spinal cord.

Brain:

The brain of frog is a soft, bilaterally symmetrical white coloured structure situated in the cranial cavity of the skull.

The brain is divisible into three regions:

(a) Fore-Brain (Prosencephalon):

This largest part of brain consists of a pair of olfactory lobes (for smell), a pair of cerebral hemispheres (control voluntary functions, intelligence, will, etc.) and a short unpaired diencephalon. The diencephalon supports the pineal body and anterior choroid plexus dorsally and the optic chaisma and pituitary body ventrally.

The diencephalon acts as a relay centre. The cavity present inside the olfactory lobe is called rhinocoel, inside the cerebral hemisphere, lateral ventricle or paracoel and inside the diencephalon is called third ventricle or ditocoel. The lateral ventricles communicate with each other and also with the ventricle of diencephalon by an opening called foramen of Monro.

(b) Mid-Brain (Mesencephalon):

Mid-brain consists of two optic lobes (control vision) and two crura cerebri (acts as coordination centres between fore-brain and hind-brain). The cavity of the optic lobe is called optocoel which communicates with the fourth ventricle behind through a narrow cavity called iter or aqueduct of Sylvius.

(c) Hind-Brain (Rhombencephalon):

Hind-brain consists of cerebellum or metencephalon (coordinates; voluntary muscles and controls equilibrium) and medulla oblongata or myelencephalon (controls involuntary functions). The cavity of the medulla oblongata is called metacoel or fourth ventricle which is joined infront with the iter and posteriorly with central cavity of spinal cord. On the dorsal surface of the medulla oblongata lies the posterior choroid plexus. Medulla oblongata continues as spinal cord through an aperture at the posterior end of skull called foramen magnum.

Sense Organs of Frog:

Frog has five types of sense organs. They are:

(i) Tangoreceptors (Organs of Touch):

The entire surface of the skin is supplied with sensory nerve endings and thus the entire skin serves as an organ of touch. Here and there the epidermal cells form groups which are supplied with sensory nerve endings.

(ii) Olfactoreceptors (Organs of Smell):

These include a pair of nasal sacs or olfactory sacs which are located in the olfactory capsules of the skull. Each nasal sac communicates with the buccal cavity through the internal nares and with the outside by the external nares. The olfactory sacs consist of special modified cells called neurosensory or olfactory cells.

(iii) Gustatoreceptors (Organs of Taste):

The gustatoreceptors occur in the form of taste buds on the tongue and palate. Each taste bud is somewhat spherical in shape and consists of columnar neurosensory and supporting cells.

(iv) Photoreceptors (Organs of Sight):

These include a pair of prominent spherical eyes situated in the orbits of the skull. Eyes of frog are bulged out, which enables it to see in almost all directions.

(v) Statoacoustic Receptors (Organs of Hearing):

Ear forms the statoacoustic receptors of the frog. It consists of only tympanum and external ear is absent.

Reproductive System in Frogs:

In frogs sexes are separate and the reproductive system can be described under two sub-headings:

(A) Male Reproductive System:

The male reproductive system consists of:

(a) Testes:

An ovoid yellow-coloured testis is found attached to the antero-ventral surface of each kidney by a double fold of peritoneum called mesorchium. Internally each testis is formed of a large number of much coiled tubules called seminiferous tubules. The epithelial lining of which produces spermatozoa by spermatogenesis. Near the anterior end of each testis there are several branched finger like processes called fat bodies. These bodies provide reserve food to the spermatozoa and the frog during hibernation.

(b) Vasa Efferentia:

From each testis several thin, ciliated tubules called vasa efferentia emerge through mesorchium run inward and open into a longitudinal duct called Bidder’s canal which is situated inside the kidney along its inner border. The Bidder’s canal is connected to the urinogenital duct through the transverse and longitudinal collecting tubules. Thus the vasa efferentia transport mature spermatozoa from testes to the ureter of kidney.

(c) Urinogenital Ducts:

These are the two ducts which arise from the posterior ends of kidneys carrying both urine and sperms. Each urinogenital duct swells immediately after coming out of kidney to form seminal vesicles, which serve to store sperms. The urinogenital ducts open into cloaca.

(B) Female Reproductive System:

Female reproductive system consists of two ovaries and two oviducts:

(a) Ovaries:

These are a pair of yellow coloured irregular structures found attached on the ventral side of outer border of the kidneys. Each ovary is suspended by a fold of peritoneum called mesovarium. Each ovary is a lobulated sac-like structure composed of a number of ovarian follicles with a countless number of black and white ova. In breeding season the ovaries become greatly enlarged.

Each ovary is lined by germinal epithelium, where in the germ cells produce ova by the process of oogenesis. When ripe, the ova are released into the body cavity by the rupturing of the ovarian wall, from where they are moved by the cilia of the peritoneum into the ostia of oviducts. Just like male frog, the female frog also has fat bodies anterior to each ovary.

(b) Oviducts:

On each side of the ovary there is a long much coiled whitish glandular tube called oviduct or Mullerian duct. At the anterior end of each oviduct there is an oviducal funnel with a wide aperture called ostium. The ostia are placed quite anteriorly in the body cavity, one on either side of the oesophagus. Next to the oviducal funnel, the oviduct part is much coiled, thick walled, glandular and is situated outer to the kidney.

This part secretes three jelly coats around the ova. The posterior part of the oviduct dilates to form a thin walled ciliated ovisac (sometimes erroneously called uterus), where the ova are stored and agglutinated. The ovisac opens on the roof of cloaca near the openings of the ureter. The oviducts become much enlarged and highly coiled just before the breeding season.

Breeding:

Most of the frogs breed in rainy season, i.e., during July to September months. They navigate their way to the nearby water source with the help of special sensory organs situated in the mouth called osmoreceptors. Generally quiet, shallow water is selected for mating purpose. During mating season, the male and female frogs gather in large numbers. The males usually reach the mating ground much earlier than females. The males attract the females by croaking call or mating call usually at night.

At the time of croaking call, the air from the lungs is forced from the vocal cords into the mouth- cavity and back again. The males possess a pair of special sacs called vocal sacs for this purpose. The vocal sacs which are distensible bloat up like balloons act as resonators and increase the pitch of croaking by the male.

Once paired, the paired male and female frogs remain just like that for several days with male mounted on the back of female. Ultimately the female spawns and the male sheds sperms on them, after which they separate. This sexual embrace of male and female is called amplexus.

Fertilization is external. From the fertilized egg, a larva called tadpole hatches out in about a fort-night. The tadpole is aquatic. It breathes with gills and swims with tail. The herbivorous, gill breathing, aquatic larva gets transformed into a carnivorous, lung breathing terrestrial adult by metamorphosis and R are true but R is not an explanation to A.