Frog Anatomy and Physiology PDF

PHYLUM: CHORDATA EXAMPLE: RANA TIGRINA The systematic position of the common Indian frog. Rana tigrina is; Kingdom: Animalia Phylum: Chordata Group: Craniata Sub phylum: Vertebrata Division: Gnathostomata Super class: Tetrapoda Class: Amphibia Order: Anura Genus: Rana Species:...

PHYLUM: CHORDATA EXAMPLE: RANA TIGRINA The systematic position of the common Indian frog. Rana tigrina is; Kingdom: Animalia Phylum: Chordata Group: Craniata Sub phylum: Vertebrata Division: Gnathostomata Super class: Tetrapoda Class: Amphibia Order: Anura Genus: Rana Species: tigrina Habit and Habitat Indian frog is generally found in fresh-water pond, tank, river, ditches and among green vegetation. In rainy season frogs are found in abundance and can be recognised by their characteristic croaking sound. They can live both in water and on land (amphibious life), so are included within amphibia. They are cold blooded (poikilothermal) i.e. their body temperature varies with environmental temperature.  When the frog is in water it keeps the tip of its snout above the water surface so that air can enter through external nostrils for respiration. The eyes and tympanum are also kept above water surface. When it is below the water surface the nictitating membrane protects the eye from water and injurious particles. In water it swims effectively by its strong webbed hind-limbs.  During winter the body temperature is lowered and it becomes inactive. During the entire severe winter it protects itself by digging the soft earth at the bottom of the pond. This is about 3/4 meter deep. Here it spends the entire winter. This inactive stage is called winter sleep or hibernation. During hibernation mouth and nostrils remain closed. It does not feed and general physiological activities of the body is greatly reduced. In the winter it depends for its nutrition on glycogen stored in the liver.  When winter ends the frog comes out and leads an active life. But after some time during the severe summer when the temperature rises it again goes underground and becomes inactive. This is known as summer sleep or aestivation. After summer when the rainy season begins it ends its summer sleep and comes out to lead an active life. During this time pond is full of water and frog leads an amphibious life.  Frog is a great friend of agriculturists because it feeds on insects found in fields. But snakes, cranes, mongoose and man are its enemies. Frog can change its colour to save itself from its enemies. Whenever an enemy approaches it hides itself in green vegetation the colour of which matches with its skin colour. Thus by matching its own colour with the surrounding, the frog protects itself. The frog breeds during rainy season. The fertilization is external in water. During develop ment a tad-pole larva is formed which metamorphoses into a young frog. This hops, feeds and grows to an adult frog. EXTERNAL FEATURES  The body of frog is covered by a shining moist skin. The ventral surface is faint yellow and the dorsal surface is green with black spots. For evading its enemies it can change its colour according to the surroundings. Body of frog is divided into two parts-head and trunk. There is no neck or tail. I) Head The head of frog is triangular at the anterior end of which is the large and broad mouth which is bounded by upper and lower jaws. The anterior conical end of the head is the snout near the extremity of which are two external nostrils. Behind each nostril is an elevated round eye by which the frog can see things around it without rotating the head. Each eye has three eyelids; 1) Upper eyelid-It is thick and immovable. 2) Lower eyelid-It is small and movable. 3) Third eyelid-It is called nictitating membrane and is a part of lower eyelid. It is transpa rent. During swimming the frog covers the eyes with nictitating membrane to protect it from water and injurious particles present in water.  To some distance behind and slightly downwards of each eye is a round depression covered by tough grey membrane called tympanum or tympanic membrane. Unlike mammal there is no external ear. In male frog on the ventral surface of the throat on each side is a vocal sac which produces the croaking sound specially during breeding season. Vocal sacs are absent in female frog. 2) Trunk It is the main part of the body. It has two pairs of legs. At the anterior part of the trunk is a pair of fore-limbs and at the posterior part is a pair of hind-limbs. Fore-limb has three parts-upper arm, forearm and hand with wrist, palm and 4 digits. The thumb is absent. The hind-limb is stronger and longer than fore-limb. It is divided into thigh, shank and foot with ankle, sole and toes. The toes are connected by a web. At the posterior end of trunk between two hind-limbs is an operture called cloacal aperture.  Through this aperture faeces, urine and genital products (sperms and ova) are passed out. Sexes are separate. Male Rana tigrina differs from female in the following characters (i) Male is larger than female and fore-limbs of male are more muscular than fore-limbs of female. (ii) Vocal sacs are present in male; these are absent in female frog. (iii) During breeding season the skin of the first finger of male becomes thickened & swollen to form copulatory pad. (iv) During copulation male holds the female by copulatory pad. Small horny pad like structures are found on the ventral surface of each digit of both male and female. These are called articular pads. FOOD AND METHOD OF CAPTURING PREY Frog is a carnivorous and feeds on insects, fishes and worms. The tongue, teeth of upper jaw and vomerine teeth help to capture prey. Tongue is in buccal-cavity, the posterior end of which is free and bifid and it is slimy in nature. When a prey comes near it the frog throws out its tongue and the prey sticks to the slimy tongue which is then withdrawn in the buccal cavity and the prey is immediately swallowed. The teeth prevent the escape of prey.  LETS WATCH A VIDEO DIGESTIVE SYSTEM Digestive system of frog includes the alimentary canal and associated glands. 1) ALIMENTARY CANAL It extends from mouth to cloacal aperture consisting of the following parts : (i) Mouth and bucco-pharyngeal or buccal cavity (ii) Oesophagus (iii) Stomach (iv) Small intestine including duodenum and ileum (v) Rectum (vi) Cloacal aperture. I) Buccal cavity. Mouth opens behind into a cavity called buccal cavity or bucco pharyngeal cavity. If the jaw are separated the cavity will be visible. The following structures will be visible in the buccal cavity. - Upper jaw - Lower jaw - Internal nares - Impression of the eyeball - Vomerine teeth - Opening of gullet - Glottis - Openings of the vocal sacs - Tongue Structures in buccal cavity (a) Upper jaw. It is provided with premaxillary and maxillary teeth. All the maxillary teeth are alike. So these are homodont. These can be replaced by new teeth if anyone is broken. So these are called polyphyodont. All the teeth of frog are attached on upper jaw bone, so these are also called acrodont. Each teeth has a base which is fused with maxilla and the upper part is called crown. Each tooth has a pulp cavity inside it. The tooth is made mostly of dentine, the crown is covered by enamel (Toad has no maxillary tooth). (b) Lower jaw. It is toothless and can be moved upwards and downwards.  (c) Internal nares. These are two small openings at the anterior end of the roof of buccal cavity. (d) Impression of the eye ball. Behind each internal nostril is a rounded elevation of the eye ball. (e) Vomerine teeth. Behind each internal nostril is a group of small vomerine teeth. In toad vomerine teeth are absent.  (f) Opening of eustachian tube. At the junction of the jaws an opening is present on each jaw. (g) Sub-rostral fossae. There are three such depressions situated near the tip of the upper jaw. (h) Opening of gullet. It is a transverse opening at the centre of the buccal cavity which opens into oesophagus. (i) Glottis. Below the opening of the gullet is this longitudinal fissure which opens into laryngeotracheal chamber. (j) Openings of the vocal sacs. In the male frog only at each angle of the lower jaw is an opening of vocal sac (Absent in male toad). (k) Tongue. Tongue is muscular and sticky helping to catch the prey. The free end is bifid (round in toad).  (II) Oesophagus. The gullet opens into a short knarrow tube called oesophagus which passes from below the liver lobes to open into the stomach. (III) Stomach. It is a thick walled bag like structure divided into two parts an anterior cardiac and a posterior pyloric part. At the junctions of stomach and small intestine is the pyloric constriction or pylorus which allows food to pass from stomach to deodenum and not vice versa. Stomach contains gastric glands and cardiac part secretes gastrin hormone. Stomach opens into the duodenum which is the first part of small intestine.  (iv) Small intestine. Stomach opens by pylorus into small intestine. The first part of small intestine lies parallel to stomach. It is called duodenum. Duodenum and stomach together form an 'U' shaped configuration. Duodenum opens into the next part which is much coiled and called ileum. (v) Large intestine. Physiology of Digestion Digestion is the breaking down of large food molecules by enzymes into such small molecules that can be absorbed. These digested food materials are carried by blood to different parts of the body for utilisation. The process by which these absorbed digested food are converted into protoplasm, heat and energy is called assimilation. The process of eliminating undigested food is called egestion. DIGESTION BUCCAL CAVITY AND OESOPHAGUS In mouth in frog is bounded by upper and lower jaws. Upper jaw is with maxillary teeth and lower jaw is edentatous. Teeth simply prevents the escape of food from mouth cavity to outside. In frog as there is no salivary gland food is not digested in buccal cavity. Food passes through the oesophagus into stomach. Thus oesophagus acts only to conduct food from buccal cavity to stomach and no digestion takes place in oesophagus also.  STOMACH The cardiac part of the stomach secrete gastrin hormone which stimulates the gastric glands. The gastric gland consists of mucous secreting cells mucine, oxyntic cells which secrete HCI and peptic cells which secrete pepsin. Digestion starts here. Gastric glands of stomach secrete gastric juice which contains 0.4% HCI and digestive enzyme pepsinogen. HCI (i) kills the insects and bacteria present in food, (ii) makes the food acidic and converts pepsinogen to pepsin. Pepsin converts protein of the food into peptones and proteoses. Protein + Pepsin Peptones + Proteoses Water present in gastric juice makes the food pulpy (soft) which is now called chyme. Carbohydrate and fat are not digested in stomach. The chyme now passes the pylorus and enter the duodenum.  DUODENUM As soon as food reaches duodenum the acidic food stimulates the duodenum to secrete four kinds of hormones. These are secretin, cholecystokinin, enterokinin and enteroges terone. Cholecystokinin stimulates the gall-bladder so that its forces the bile into the duodenum. Secretin activates pancreas so that its secretion reaches the lumen of the duodenum. Enterokinin activates the glands of small intestine and enterogesterone stops the activity of gastric glands of stomach. (I) Sodium bicarbonate (NaHCO3) present in bile neutralises the acidity of the food and makes it alkaline. (ii) Water of bile mixes with food and helps to make it further thinner. (iii) Bile contains no digestive enzyme so it does not directly take part in digestion. Bile salts break fat into smaller particles. This process is called emulsification of fat.  Pancreatic juice acts on food in the following way- (i) Sodium and potassium ions present in it make the food more alkaline as pancreatic enzymes can act only in alkaline medium. Pancreatic juice contains the following digestive enzymes-trypsinogen, amylase and lipase. Trypsinogen is changed to trypsin by enterokinase secreted by duodenal mucosa. It acts as an activator. Trypsin acts on unaffected proteins, peptones and proteoses and change them to peptides. Pancreatic amylase converts starch into glucose and lipase changes fat into fatty acids and glycerol. (i) Trypsin + Proteins, Peptones and Proteoses Peptides (ii) Amylase + Starch Glucose (sugar) (iii) Lipase + Fat Fatty acids + glycerol. ILEUM (SMALL INTESTINE) Food reaching the ileum is not completely digested. The mucous layer of duodenum and ileum secrete the intestinal juice called succus entericus. It is watery and alkaline containing different kinds of enzymes like peptidase, erepsin, lipase, invertase, maltase and lactase. Peptidase acts on peptones and peptides, which are converted to amino acids. Lipase converts fat into fatty acids and glycerol. Starch digesting enzymes act as follows- (i) Sucrase (Invertase) + Sucrose =Glucose + Fructose (ii) Maltase + Maltose Glucose (2 Molecules) (iii) Lactase + Lactose Glucose + Galactose  RECTUM It absorb water from the food and undigested food is expelled out through cloacal aperture. ABSORPTION On the innerside of duodenum and ileum are found many finger like projections called villi (sing. villus). Each villus is covered by a layer of mucosa and contains inside capillaries of blood vessels and lymph vessels called lacteal. Glucose, fructose and amino acids enter the blood capillaries through the mucosa of villus. From here these reach the veins of duodenum and ileum and finally enter the hepatic portal vein and reach the liver.  Liver controls excess of sugar and amino acids. The excess sugar (glu cose) is converted into glycogen and stored in liver cells. This is used during hibernation or whenever needed. Amino acids from liver enter blood and reach different parts of the body and are converted into required proteins. Excess amino acid is not stored like glycogen as it is harmful. From amino acid urea is formed which is filtered in kidney and expelled out. This process by which excess amino-acid is expelled out in called deamination. Glycerol and fatty acids are absorbed in a different way. These reach the lacteal vessels of villus and are changed to fat droplets. These now reach the liver through blood. Liver according to need changes fat to car bohydrate and carbohydrate to fat and protein into carbohydrate. ASSIMILATION The end products of digestion reach different tissues of the body and help in protoplasm formation, providing temperature and energy to the body. At the end undigested food reaches large intestine, in the wall of which excess water is absorbed. Thus water loss from body is checked. Egestion