Digestive System of Frog

The digestive system of frogs comprises the alimentary canal or digestive tract along with the related digestive glands. The system chiefly includes the digestive glands and the alimentary canal. In the alimentary canal, processes such as digestion, mastication, absorption occur while the digestive glands produce some enzymes which cause digestion of the food that is ingested.

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The different parts of the digestive system of frogs and their glands are discussed briefly in this article, read on.

Alimentary Canal

In frogs, the alimentary canal is said to be complete. It is a coiled and long tube having different diameters extending from the mouth to the cloaca. It comprises –

• Pharynx
• Buccal cavity
• Oesophagus
• Stomach
• Small intestine
• Large intestine
• Cloaca

Mouth

The alimentary canal starts with the mouth. It is a very wide gap extending from one side of the snout to the other. The two bony jaws are found in the mouth, and the jaws are covered by the immovable lips. The upper jaw is fixed, while the lower jaw is flexible – it can move up and down to open or close the mouth.

Buccal Cavity

The mouth opens into the buccal cavity which is wide, large and shallow. It comprises a ciliated columnar epithelial lining, which has the mucous glands and these glands secrete mucus which aids in food lubrication. The frogs do not have salivary glands.

Teeth

The lower jaw does not have teeth. But it occurs in a row of either side on the maxillae and premaxillae bones of the upper jaw. Teeth are pointed backwards. There are two more patches of teeth seen on one either side of the median line of the roof of the buccal cavity called vomerine teeth. The vomers also comprise two groups of vomerine teeth. These teeth are not utilized to chew but check the escape of captured prey. The upper jaw has a row of closely set, uniform, small and hook-like pointed teeth.

Teeth are similar in shape, they are homodont. It is curved backwards and joined to the bones rather than being fixed to the sockets. Each of the teeth is conical in shape and comprises two parts – the crown (front part of the tooth) and the base. The base is fixed to the jaw bone and comprises bone-like matter.

Consequently, teeth are not meant to chew, instead are functional in simply holding prey and preventing it from slipping out. The dentine is traversed by many fine canals while the enamel covers the tip of the crown. The enamel is a tough, resistant and glistening particle. The tooth comprises a central pulp cavity opening at the sides. These are filled with soft nourishing pulp and contain blood vessels, connective tissues, nerves and odontoblast cells producing new substances for tooth’s growth. Frogs are polyphyodont – teeth get replaced many times in their life span.

Internal Nostrils

In its roof near the vomerine teeth, the buccal cavity comprises two openings – the posterior or internal nares associated with the nasal cavities through which respiratory gases move to and from the buccal cavity at the time of respiration.

Tongue

The tongue in frogs is large, sticky, muscular and protrusible. It is found at the base of the mouth cavity. The anterior end of it is attached to the inner border of the lower jaw while the posterior end is bifid and free.

The upper surface has taste buds forming small papillae and mucous glands of which the secretions cause the tongue to be sticky. Digestive enzymes are neither produced by the mucous glands nor the taste buds.

The tongue can be protruded and suddenly retracted to engulf and capture insects. It is said that the throwing out of the tongue is caused by a sudden flow of the squeezed lymph contained in the lymph sac to the other as a result of muscular contraction. However, swallowing is attained by elevating the buccal cavity’s floor wherein flat, hyoid cartilage is implanted.

Orbit-Bulging

Behind the vomerine teeth, the roof of the buccal cavity has two oval and large pale areas, the bulgings of the eyeballs. At the time of swallowing food, eyes are pressed down into the buccal cavity, pushing the food into the pharynx.

Pharynx

The buccal cavity tapers behind the pharynx. This, in turn, opens through the gullet into the oesophagus. The pharynx and the buccal cavity at times are referred to as the bucco­pharyngeal cavity. At the roof of the pharynx on each of the lateral sides, a wide Eustachian tube can be found with an opening that communicates with the middle ear.

The glottis, which is a median slit in the pharynx behind the tongue, shields the entry into the lungs. During breathing, it is always open and closes at the time of swallowing. In the angle of the lower jaw on the floor of the pharynx in male frogs, two openings of the vocal sacs are formed too. They serve as resonators during croaking.

Oesophagus

Gullet directs into a broad, short and muscular section of the alimentary canal referred to as the oesophagus. This section of the alimentary canal is extremely short as a result of the absence of the neck however, highly distensible as their inner lining into numerous longitudinal folds that enables enough expansion of the oesophagus at the time of passage of ingested food via it to the stomach. They open into the stomach, wherein no demarcation line forms between the stomach and the oesophagus.

Stomach

It is present on the left side of the body cavity which is attached to the dorsal body wall by a mesogaster. Digestion of the ingested food occurs with the assistance of some digestive enzymes which are secreted by the digestive glands found in their walls. They are in the form of curved and wide tubes found between the intestine and oesophagus. The stomach can be split into two parts – the short narrow, posterior pyloric stomach and the large, wider anterior cardiac stomach.

The inner lining of the stomach has many longitudinal folds that enable expansion of the stomach when required. The mucous epithelium in them comprises multicellular gastric glands that secrete pepsinogen enzymes. Hydrochloric acid is secreted by the unicellular oxyntic glands. The pyloric end of the stomach is narrowed. Their opening into the small intestine is shielded by a circular ring-like muscle – the sphincter which regulates the food passage from the stomach into the intestine.

Intestine

The stomach directs into the tubular, long and coiled structure – the intestine which also is attached to the dorsal body wall by the mesentery. It comprises two parts –

• Small intestine
• Large intestine

Small intestine

It is found in many loops braced by the mesentery – a fan-like membrane. The anterior part of the small intestine curves upwards forming a U with the stomach – that is the duodenum. The other part of it continues as the ileum which is coiled. A common hepatopancreatic duct opens into the duodenum from the liver and the pancreas which brings the pancreatic and the bile juice. The inner mucous lining is thrown into the low transverse folds.

The mucosal lining of the small intestine apart from the intestinal glands comprise two types of cells –

• Goblet cells – large cells possessing granular substances and oval vacuoles producing mucus. The nucleus is found in them near the base of the cell
• Absorbing cells – these are distinguished as small cells with nuclei found near the base

Large intestine

The ileum forms a narrow, long coiled tube where its lower end directs into the structure of the large intestine – the rectum. The internal mucous lining of the ileum forms many longitudinal folds. The glands, true villi and crypts of the higher vertebrates are not seen. It is in this part that absorption and digestion of food occurs. The lower terminal leads to the cloaca by the sphinctered anus. Their mucosal lining goes on to form the low longitudinal fold.

Cloaca

These are tiny sac-like structures receiving the openings of the anus and the urinogenital apertures. The cloaca leads to the exterior through the cloacal opening or vent located at the posterior end of the body.

Physiology of Digestion In Frogs

Food of Frogs

Frogs are carnivorous, mainly feeding on spiders, earthworms, fishes, snails, small frogs and small insects that capture and swallow fully into the stomach with the aid of the protractible tongue.

Food Ingestion in Frogs

Frogs occupy an appropriate spot that is frequented by insects when preying. When an insect is in proximity, they open their mouths and suddenly flick their sticky tongue out and strike its prey. The moment the prey is in contact with the tongue, it adheres to it immediately. The tongue now is withdrawn into the buccal cavity.

After the prey is trapped into the buccal cavity, it will not be able to escape because of the presence of the hook-like inwardly directed maxillary and the vomerine teeth. Then, from the buccal cavity, it is pushed into the oesophagus by the contraction of the pharyngeal wall. Then by the peristalsis due to the contraction and dilation of the muscular walls of the oesophagus, it is pushed into the stomach.

Food Digestion In Frogs

At the time of feeding, the food that is ingested comprises complex organic particles which cannot be used immediately due to their nature of being insoluble, they cannot diffuse through the mucous membrane lining of the alimentary canal. Hence, it must be treated with chemical and physical changes of digestion for it to be transformed into soluble forms for immediate use by the body.

Peristaltic movements of the alimentary canal cause physical changes while the enzymes acting as organic catalysts bring about chemical changes that only accelerate the chemical reactions without causing any change to themselves.

These are complex proteins and specific in their actions which are produced by the exocrine glands as they always function at the optimum temperature of the body. They can reverse reactions too, that is to say, that substances altered could be reformed. These are various kinds depending on the type of food on which they act. Hence, proteins are digested by proteolytic enzymes, fats by lipolytic enzymes and carbohydrates by diastatic enzymes.

Buccal Digestion

Since the buccal epithelium lacks any digestive gland, the captured prey in frogs is neither subjected to any mastication nor any chemical action in the buccal cavity. Prey from the buccal cavity is pushed directly into the oesophagus wherein physical changes are experienced as a result of the constant peristaltic movement of the wall. Apart from the mucus, glands of the oesophagus too secrete pepsin enzyme; however, digestion does not occur as they are not active until they reach the stomach. The mucus causes the active food to become inactive and soft and hence makes its passage easier.

Gastric Digestion

The food reaches the stomach as a result of the oesophageal relaxations and contractions. The stomach is involved in performing three main functions –

• Chemical alterations
• Mechanical mixing
• Storage

When the food arrives at the stomach, the so-called peristaltic movements allows food to be pushed down and disintegrates it into small fragments and mixes thoroughly with the help of gastric juices secreted by the gastric glands found in the internal lining of the stomach. The glands produce their secretions when they are activated by the hormone gastrin produced by the stomach wall when food comes to the stomach.

Gastric glands secrete gastric juice which comprises large quantities of water, the inactive pepsinogen enzyme and free hydrochloric acid. Inactive pepsinogen, upon being mixed with hydrochloric acid, changes into active pepsin. The acid is functional in preventing bacterial decomposition and dissolving inorganic salts, making the food softer. Pepsin of the stomach and the oesophagus act on the proteins of food and change into peptones and proteoses.

Food after being in the stomach for 2-3 hours is churned thoroughly and mixed by the contractions of the muscles of the stomach wall, forms a thick creamy acid fluid referred to as chyme. The muscular contractions of the wall of the stomach force the chyme to pass in small quantities via the pylorus into the duodenum.

Intestinal Digestion

Once food enters the duodenum, it gets acidic because of HCl which is secreted by the oxyntic cells of the gastric glands of the stomach. The acid found in the food triggers the duodenum to produce hormones cholecystokinin and secretin that pass through the blood reaching the pancreas and liver. The secretin triggers the pancreas to produce pancreatic juice, the cholecystokinin triggers the gallbladder to produce bile juice.

The pancreatic and bile juice is secreted side by side into the duodenum via the hepatopancreatic duct which is common. Simultaneously, the intestinal mucosa too secretes intestinal juice referred to as succus entericus using the hormone enterocrinin. Consequently, three substances in the intestine mix up with the food – succus entericus, pancreatic juice and bile which act on the food to thoroughly digest it.

Food Absorption In Frogs

The final products through the walls of the small intestine are absorbed. The absorptive surface seen internally is fold-wise increased with the presence of villi-like processes. Each villus is supplied richly with blood capillaries and lacteal or lymph vessels. Mineral salts, water and other nutrients are absorbed directly through the mucosa.

The glucose, amino acids and fructose pass by diffusion from the mucosa into the blood capillaries of the intestine reaching the liver through the hepatic portal vein. The liver maintains a steady supply of the needed amount of amino acids and sugars into the blood. Excess sugar as glycogen is stored but excess amino acids cannot be reserved hence are converted into urea by the liver cells discarded as urine from the kidneys.

If there is a drop in the level of sugar concentration from the normal in blood, the reserve glycogen is altered into glucose by the liver cells and sent to the blood. From blood, cells take up the needed amino acids for the synthesis of proteins forming protoplasm.

The fatty acids and glycerol pass into the lymph vessels referred to as lacteals. Glycerol can be easily absorbed as they are water-soluble, however, fatty acids cannot be absorbed as they are water-insoluble. Hence, before fatty acids are absorbed, they are mixed with bile salts to make them soluble so that they can be absorbed. Once absorption occurs in the lacteals, the fatty acids and glycerol are converted again into fat globules with smaller molecules. As a result, the fats enter as glycerol and the fatty acids into the blood through the lymph vessels.

Food Assimilation In Frogs

Different cells from the blood take up the amount of different digested food required either to be used to provide energy or to build new protoplasm; this is assimilation. The conversion of the digested food into new protoplasm is brought about by vitamins, the mineral salts too go on forming parts of the protoplasm.

Egestion of Undigested Food In Frogs

It is in the small intestine that the absorption and digestion are completed. The ingested food enters the rectum through peristalsis for the formation and storage of faeces. The leukocytes, old epithelial cells, bile pigments, and a huge number of bacteria form the faeces which are discarded timely through the cloacal opening.

This was about the digestive system of frogs. Explore related concepts on NEET, at BYJU’S.

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