Digestive System of Frog
The alimentary canal of the frog is complete. The alimentary canal is a long, coiled tube of varying diameter that extends from mouth to cloacal aperture of the frog. Alimentary canal comprises of:
- Buccal cavity
- The pharynx or windpipe
- Small intestine
- Large intestine
- Cloaca apperture
The alimentary canal of the frog starts from the mouth opening, which is very wide, extending from one side of the snout to the other. The mouth of the frog is bounded with two bony jaws that are covered with immovable lips. The upper jaw is fixed and immovable like in higher animals. But, the lower jaw is movable and moves up and down to close and open the mouth.
The mouth of the frog opens into a large, shallow oral cavity, which is called a buccal cavity. The buccal cavity is lined with ciliated columnar epithelial cells that contain mucus. This mucus is helpful in lubricating the food and inside the buccal cavity. Frog has no salivary glands.
The lower jaw lacks teeth, but some conical and backwardly pointed teeth occur in a row on either side on the premaxillae and maxillae bones in the upper jaw. The two small bones are found in the roof of the mouth, called vomers, which also bears two groups of vomerine teeth. The teeth in the frog are not meant for the chewing; they are simply required for the catching of their prey, holding it firmly, and prevent its prey from slipping from its capture.
The teeth nature in the frog is homodont (similar), acrodont (not having teeth socket). But the tooth is attached to the jaw bone. In the life cycle of the frog, teeth are replaced several times (polyphyodont) throughout their life by the growth of new teeth when old ones are worn out or lost.
On the floor of the buccal cavity lies a large, protrusible muscular sticky tongue. The tongue’s anterior end is attached to the inner border of the lower jaw, and the posterior end is free and bifid which can be flicked out and retracted suddenly after the capture of the prey. The surface of the tongue is slimy in nature to stick its prey.
Posteriorly, the buccal cavity of frog passes without demarcation into a short pharynx. hence, the two sometimes are defined together as the Bucco-pharyngeal cavity.
Many apertures open in the pharynx. In a male frog, on the floor of pharynx on either side near the angle of two jaws, is present the small opening of a vocal sac. And these vocal sacs act as resonators during croaking or to call upon their mate for mating. Pharynx abruptly tapers behind to lead into the esophagus through a wide opening, which is called as the gullet.
The esophagus is the food pipe that traverses from pharynx to the stomach of the alimentary canal. It is a short, wide, muscular and highly distensible tube that has a mucous epithelial lining that contains some mucous glands. The longitudinal foldings of the esophagus allow its expansion during the passage of food into the stomach. The esophagus’s glandular lining secretes an alkaline digestive fluid. In the peritoneal cavity, the food pipe or esophagus enlarges to merge with the stomach.
The stomach of frog lies in the left side in the body cavity, attached to the body wall by a mesentery called mesogastor.mesogastor is the large (4cm), broad, and slightly curved bag or tube with thick muscular walls. The large broader anterior part is called a cardiac stomach, while the short narrower posterior part is called the pyloric stomach. The inner surface of the stomach contains many prominent longitudinal folds which allow its distension when food is consumed. The stomach’s mucous epithelium secretes an enzyme called pepsinogen, and unicellular oxyntic glands secrete hydrochloric acid. The posterior or the pyloric stomach is minutely constricted, and the opening is guarded by a circular ring-like sphincter muscle, called the pyloric valve.
The small intestine of a frog is a long, coiled narrow tube , about 30cm long, and it attaches mid-dorsally to the body wall by mesenteries. The small intestine is composed of two parts : a small anterior duodenum and longer posterior ileum.
The mucosal lining of the intestine consists of two types of cells besides intestinal glands, large goblet cells, and small absorbing cells. the goblet cells contain oval vacuoles and granular substances, which secretes mucus. The absorbing cells have oval nuclei near the base.
The duodenum runs parallel to the stomach forming a U shaped structure. the duodenum receives hepatopancreatic duct from the liver and pancreas, which brings bile and pancreatic juice. The internal mucous forms the low transverse folds.
The ileum is the longest part of the digestive system of the frog, which makes several loops before dilating into the large intestine. The internal mucous lining of the ileum forms many longitudinal folds, but there are no true villi seen in frogs . digestion of food and the absorption of the same is done here in the small intestine.
Large intestine or Rectum
The large intestine or rectum is a short, wide tube, about 4 cm long, which runs straight behind to open into the cloaca by the anus . the anus is guarded by anal sphincter. the inner lining has many longitudinal folds . the main function is re-absorption of water and preparation and storage of feces.
The cloaca is the small terminal sac-like part which opens to the exterior via the anus and the urinogenital apertures. Cloaca lies in the hind of the body.
Associated digestive glands
Besides the gastric glands and intestinal glands, two large glands namely the liver and pancreas also play a vital role in the digestion of food.
The liver is the largest gland found in the organism’s body. It is reddish-brown in color, the multilobed gland, which is situated close to the heart and lungs.
The liver of frogs consists of 3 lobes –right, left, and median. The polygonal cells of the liver secrete a greenish alkaline fluid called bile. Bile gets stored in a large sac-like structure called gall bladder, which lies between the lobes of the liver. Bile has no digestive purpose; it only emulsifies fats for proper digestion.
The pancreas of frogs is much-branched than that of any vertebrate. The pancreas is branched, an irregular, flattened, and yellow-colored gland that is lying in the mesentery between stomach and duodenum. The pancreas has dual nature, i.e., both exocrine, as well as endocrine.the endocrine part consists of islets of Langerhans, which manufactures insulin hormone. The exocrine part secretes pancreatic juice, which contains many digestive enzymes for the digestion of food. The pancreas doesn’t have any separate duct; the juice is conveyed through the hepatopancreatic duct.