The digestive system of frogs consists of the digestive tract (alimentary canal) and the associated digestive glands. The digestive tract is responsible for processes such as digestion, mastication, and absorption, while the digestive glands produce enzymes to aid in digestion of the ingested food.

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This article provides a brief overview of the digestive system and glands of frogs. Read on to learn more!

Alimentary Canal

The alimentary canal of frogs is said to be complete, and is a long, coiled tube with varying diameters that extends from the mouth to the cloaca. It consists of:

Pharynx

Buccal Cavity

Oesophagus

Stomach

Small Intestine

Large Intestine

Cloaca

Mouth

The alimentary canal begins with the mouth. It is a wide opening extending from one side of the snout to the other. The two bony jaws are located 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 buccal cavity is wide, large, and shallow and opens into the mouth. It is lined with ciliated columnar epithelial cells that contain mucous glands, which secrete mucus to help lubricate food. Frogs do not have salivary glands.

Teeth

The lower jaw does not have teeth, however it is bordered on either side by the maxillae and premaxillae bones of the upper jaw. This row of teeth is pointed backwards and two more patches of teeth, called vomerine teeth, can be found on either side of the median line of the roof of the buccal cavity. These vomerine teeth are not used for chewing, but rather to prevent the escape of captured prey. The upper jaw has a row of closely set, uniform, small and hook-like pointed teeth which are used for chewing.

Teeth have a similar shape and are homodont, with a curved backward orientation that is joined to the bones rather than being fixed to the sockets. Each tooth is conical in shape and consists of two parts: the crown (front part) and the base. The base is attached to the jawbone and is composed of bone-like material.

Consequently, teeth are not meant to chew, but instead are functional in simply holding prey and preventing it from slipping out. The dentine is traversed by many fine canals, and the enamel covers the tip of the crown. The enamel is tough, resistant, and glistening. The tooth is composed of a central pulp cavity which opens at the sides, filled with soft nourishing pulp and containing blood vessels, connective tissues, nerves, and odontoblast cells which produce new substances for tooth growth. Frogs are polyphyodont, meaning their teeth get replaced multiple times throughout their life span.

Internal Nostrils

The buccal cavity has two openings in its roof near the vomerine teeth: the posterior or internal nares, which are connected to the nasal cavities and through which respiratory gases move to and from the buccal cavity during 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

The roof of the buccal cavity has two oval and large pale areas behind the vomerine teeth. These areas are the bulgings of the eyeballs, which are pressed down into the buccal cavity when swallowing food, pushing it 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, a median slit located in the pharynx behind the tongue, serves to protect the entry into the lungs. During respiration, the glottis is always open and closes when swallowing. In male frogs, the angle of the lower jaw on the floor of the pharynx also hosts two openings of the vocal sacs. These sacs act as resonators during croaking.

Oesophagus

The oesophagus, which is a broad, short, and muscular section of the alimentary canal, is directed by the gullet. Despite its lack of a neck, the oesophagus is highly distensible due to its inner lining of numerous longitudinal folds, allowing it to expand when food passes through it to the stomach. There is no demarcation line between the oesophagus and the stomach when they open into each other.

Stomach

The stomach is located on the left side of the body cavity and is connected to the dorsal body wall by a mesogaster. Digestive enzymes, secreted by the digestive glands found in the walls of the stomach, aid in the digestion of ingested food. The stomach is composed of two parts: the short, narrow posterior pyloric stomach and the large, wider anterior cardiac stomach. These two parts are connected by a curved and wide tube between the intestine and oesophagus.

The inner lining of the stomach has many longitudinal folds which allow it to expand when needed. This lining is composed of a mucous epithelium containing multicellular gastric glands which secrete pepsinogen enzymes. Additionally, hydrochloric acid is secreted by unicellular oxyntic glands. The pyloric end of the stomach is narrowed, and the opening into the small intestine is protected by a circular ring-like muscle called the sphincter, which controls the passage of food from the stomach to the intestine.

Intestine

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

Small Intestine

Large Intestine

Small Intestine

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

The mucosal lining of the small intestine, in addition to the intestinal glands, consists of two types of cells.

Goblet cells are large cells with granular substances and oval vacuoles that produce mucus. The nucleus of these cells is located near the base.

Absorbing Cells - These are small cells with nuclei that are typically 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

The openings of the anus and the urinogenital apertures are received by tiny sac-like structures, which lead to the exterior through the cloacal opening or vent located at the posterior end of the body.

Physiology of Digestion in Frogs

Frogs’ Food

Frogs are carnivorous, mainly preying on spiders, earthworms, fishes, snails, small frogs, and small insects that they capture and swallow whole with the help of their protractible tongues.

The Ingestion of Food in Frogs

Frogs take advantage of their environment when preying, positioning themselves near insects. Upon detecting an insect, they open their mouths and rapidly extend their sticky tongue, which quickly adheres to the prey. The tongue is then retracted back into the frog’s mouth.

After the prey is trapped in the buccal cavity, it is unable to escape due to the presence of the inwardly directed maxillary and vomerine teeth, which are hook-like in shape. The prey is then pushed down the oesophagus by the contraction of the pharyngeal wall. From there, the prey is transported to the stomach via peristalsis, which is caused by the contraction and dilation of the muscular walls of the oesophagus.

Digestion of Food in Frogs

The food ingested during feeding is composed of complex organic particles that cannot be absorbed straight away, as they are insoluble and cannot diffuse through the mucous membrane lining of the alimentary canal. Therefore, it must undergo the chemical and physical processes of digestion to be changed into soluble forms that can be used by the body immediately.

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

Proteins are complex molecules with specific functions and are produced by exocrine glands, which function optimally at the body’s temperature. Furthermore, they can also reverse reactions, meaning substances altered can be reformed. There are various types of proteins depending on the type of food they act on; for example, proteins are digested by proteolytic enzymes, fats by lipolytic enzymes and carbohydrates by diastatic enzymes.

Buccal Digestion

The buccal epithelium of frogs lacks digestive glands, so the prey is neither chewed nor subjected to any chemical action in the buccal cavity. Instead, it is pushed directly into the oesophagus, where physical changes occur due to the peristaltic movement of the walls. The glands of the oesophagus secrete the pepsin enzyme, however, digestion does not occur until it reaches the stomach. The mucus present in the oesophagus makes the food inactive and soft, allowing for an easier passage.

Gastric Digestion

The oesophageal relaxations and contractions bring the food to the stomach, which performs three main functions:

Chemical Alterations

Mechanical Mixing

Storage

When the food reaches the stomach, the peristaltic movements occur, allowing it to be pushed down and broken into small pieces. These fragments are then mixed thoroughly with the help of gastric juices secreted by the gastric glands found in the internal lining of the stomach. These glands produce their secretions when they are activated by the hormone gastrin, which is produced by the stomach wall in response to the presence of food.

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.

The muscular contractions of the wall of the stomach cause the food, which has been in the stomach for 2-3 hours, to be churned thoroughly and mixed. This forms a thick, creamy, acidic fluid known as chyme. The contractions of the stomach wall then force the chyme to pass in small quantities through the pylorus and into the duodenum.

Intestinal Digestion

Once food enters the duodenum, it is exposed to HCl which is secreted by the oxyntic cells of the gastric glands of the stomach. This acid triggers the production of cholecystokinin and secretin hormones, which pass through the blood to the pancreas and liver. Secretin stimulates the pancreas to produce pancreatic juice, and cholecystokinin triggers the gallbladder to produce bile juice.

The pancreatic and bile juice are 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.

Absorption of Food 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 through 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 necessary amount of amino acids and sugars into the blood. Excess sugar is stored as glycogen, but excess amino acids cannot be stored and are instead converted into urea by the liver cells and then discarded as urine from the kidneys.

The liver cells alter the reserve glycogen into glucose which is then sent to the blood. From there, cells take up the necessary amino acids to synthesize proteins and form protoplasm if there is a decrease in the normal level of sugar concentration in the blood.

The fatty acids and glycerol pass into the lymph vessels referred to as lacteals. Glycerol is easily absorbed due to its water-solubility, however, fatty acids cannot be absorbed in their insoluble form. Therefore, they must be mixed with bile salts to make them soluble and be absorbed. Once absorption occurs in the lacteals, the fatty acids and glycerol are recombined into fat globules with smaller molecules. As a result, the fats enter the blood through the lymph vessels, with glycerol and fatty acids separately.

Assimilation of Food in Frogs

The process of taking up digested food by different cells from the blood, either to be used as energy or to build new protoplasm, is known as assimilation. Vitamins play an important role in the conversion of the digested food into new protoplasm, while mineral salts also form parts of the protoplasm.

Egestion of Undigested Food in Frogs

The ingested food enters the small intestine where absorption and digestion are completed. From there, peristalsis transports the food to the rectum for the formation and storage of faeces. The faeces is composed of leukocytes, old epithelial cells, bile pigments, and a huge number of bacteria, and is discarded through the cloacal opening in a timely manner.