Frogs are excellent specimens for studying organ systems and for giving your students a hands-on appreciation of the complexity of a living organism. Although they exhibit some specialized amphibian features, frogs are often studied in middle school and high school biology classes as representative vertebrates.

Frog dissection is primarily used to teach anatomy, but it also offers reinforcement in the studies of evolution, species classification, and ecology. Frog dissection allows students the opportunity to learn about some of the adaptations frogs have made to survive in their habitats, as well as some of the traits they still share with their predecessors.

A great way for you to begin an inquiry-based frog dissection activity is through a detailed investigation of the external anatomy of the frog. Carolina’s Teacher Resources Web page On the Cutting Edge: Frog Anatomy provides a thorough, hands-on introduction to the frog’s external structures and their functions:

After they have studied the external anatomy of the frog, it is time to introduce your students to the frog’s internal anatomy. One of our publications, Dissection of the Frog, provides in-depth information, instructions, illustrations, and assessment questions written specifically to help your students see and understand the external and internal anatomy of the frog. Consider the following excerpts, from the units on internal anatomy and the digestive system, as resources for your students’ hands-on dissection activities.

Internal Anatomy
The frog is similar to other adult amphibians in that it has a large central cavity known as the coelom, in which the visceral organs are contained. The coelom is divided into 2 portions by a fold known as the transverse septum. The upper portion, which contains the heart, is known as the pericardial cavity. The lower part is the pleuroperitoneal cavity. This organization is different from reptiles and mammals, although both these groups also possess divided central cavities. In mammals, including humans, the cavities are divided not by a transverse septum, but by a muscle known as the diaphragm. It is thought that the diaphragm is related to the transverse septum in evolutionary origin.

Exposing the coelom
Lay the frog with the ventral side facing upwards. Make a sagittal cut through the body wall just to the side of the midline, beginning at the pectoral girdle and extending to the cloacal opening. Be careful while cutting, so as not to cut too deep and damage organs. This can be accomplished by using scissors and pushing upward on the point of the scissors while cutting, in order to lift up on the body wall. Lift up on the edges of the body wall and gently cut free the ventral abdominal vein. Make a transverse cut along the pectoral girdle and another transverse cut along the pelvic girdle. Pull back the portions of the body wall to expose the coelom.

In the large pleuroperitoneal cavity, lymph typically circulates slowly around the body. This is different than in mammals, in which lymph is circulated through closed vessels known as lymphatic vessels. Without moving organs around in the coelom, identify the lungs, heart, liver, stomach, small intestine, large intestine, urinary bladder, gallbladder, kidneys, fat bodies, ovaries (if frog is female), testes (if frog is male), and spleen.

The lungs are visible in the pleuroperitoneal cavity, lying dorsal to the liver. The heart is located cranial to the lungs and is enclosed in a membrane known as the pericardial sac. The liver is the largest structure visible. It is dark brown in color. The stomach is located beneath the left segment of the liver. The long, coiled tube connected to the stomach and inferior to the liver is the small intestine. Connected, as a continuation of the small intestine, is the large intestine. This may be located underneath the small intestine. The small sac on the caudal floor of the cavity is the urinary bladder. If the frog is female, the partially obstructed ovaries are present as a gelatinous mass. The ovaries may be very large, even larger than the liver, if the frog was collected during mating season. If the frog is male, the testes may be observed as yellow, bean-shaped organs medial to the kidneys. The kidneys are red and are located behind the stomach and small intestine. The gallbladder is a small, swollen sac located on the right liver lobe. Fat bodies can be observed on the kidney as yellow clusters. The spleen can be observed when the stomach and small intestine are moved to the right. It is medial to the kidneys and, if present, the testes.

The Digestive System
Essentially, the frog’s digestive system is a single tube beginning at the mouth and ending at the anus. This tract is known as the gastrointestinal (GI) or alimentary tract. In this tract, various forms of digestion take place. Mechanical and chemical digestion break down food sources into smaller elements. The nutrients released from this breakdown are absorbed, and the waste is stored and eventually eliminated. The tadpole’s digestive system is adapted to handle both plant and animal food sources, but the adult frog is only adapted to handle animal food sources.

Major organs in the GI tract include the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and cloaca. Accessory organs also help during the digestive process; these include the teeth, tongue, liver, gallbladder, and pancreas. The gastrointestinal tract of the frog is similar in structure and function to that of the human.

The mouth of the frog is easily observed with the frog lying on its back with the ventral side facing upward. Inside the mouth are several structures that help with mechanical digestion. Teeth are present in 2 different forms. The maxillary teeth are small, conical, sharp teeth that line the edge of the mouth. The vomerine teeth are located in 2 patches on the roof of the mouth. These unique structures help the frog grip and hold live prey. Rostral to the vomerine teeth is the palatine ridge, which lines the mouth directly inferior to the maxillary teeth. The internal nares are also located rostrally to the vomerine teeth. They allow for the passage of air from the external nares into the mouth. The large tongue should be easy to identify, as it is attached to the lower jaw and forms part of the floor of the mouth. The frog’s tongue is an extremely strong, muscular structure that extends and retracts rapidly, allowing the frog to catch insects for food.

Immediately inferior to the mouth is the pharynx. Air passes through the pharynx during respiration, but food also passes through the pharynx on its way to the esophagus. There are 2 larger openings on the sides of the pharynx, known as the Eustachian tubes. These tubes pass through to the middle ear on each side of the head, and are directly linked to the tympanums. As the food leaves the pharynx, it passes through a short tube known as the esophagus via muscular peristaltic contractions, and from there into the stomach. Cilia line the esophagus and aid passing food through to the stomach. The stomach is J-shaped and located behind the liver, an accessory organ in the digestive system. The stomach wall is thick and muscular, and is responsible for most of the mechanical digestion of food. Numerous folds in the mucosa (internal lining) of the stomach allow it to expand during digestion.

Extending from the stomach is a long, coiled tube known as the small intestine. The small intestine is divided into 3 sections: the duodenum, the most cranial section; the jejunum, the middle section; and the ileum. The ileum is the most inferior section of the small intestine, and connects to the large intestine. The inner surface of the small intestine is relatively thin and contains many folded projections, known as villi. The villi increase the surface area of the small intestine and allow for more efficient nutrient absorption, which is the primary function of the small intestine. This is similar to the small intestine in humans.

Nutrients are absorbed in the small intestine, but some types of food require further chemical digestion as they pass through the coiled tube. Fats are often digested in the small intestine when they come into contact with a substance known as bile. Bile is produced in the liver. The liver is located superficially to the stomach and small intestine, and is dark brown in color. The liver is divided into 3 lobes, right, middle, and left. In addition to bile production, the liver also functions for the storage of fats and glycogen, the detoxification of the blood, and the formation of urea. The bile that is produced in the liver is transported to the gallbladder, a small, greenish sac located deep under the right and middle lobes of the liver. Here, bile is stored until it is released into the small intestine via the common bile duct.

Other elements that require chemical digestion in the small intestine include certain proteins, various fats, and some sugars. The chemical digestion of these elements is moderated by the pancreas, which secretes pancreatic enzymes in a fluid known as pancreatic juice via the common bile duct into the duodenum. Not only does the pancreas secrete enzymes used in digestion, it also secretes hormones including insulin and glucagon that affect blood sugar levels. The pancreas is small and is located deep to the middle lobe of the liver. It can be partially hidden by the peritoneum.

After digestion is complete and nutrients are absorbed in the small intestine, food passes into the large intestine, which ends at the cloaca at an opening known as the anus. The large intestine is primarily responsible for the reabsorption of water from waste. As the water is removed from the waste, it solidifies and becomes feces. The anal opening allows waste to pass through the cloaca, a tube that extends to the exterior of the frog. The cloaca can be examined by pushing aside the urinary bladder. The opening to the external environment is known as the cloacal opening or cloacal aperture.

Lining the pleuroperitoneal cavity is a membrane known as the peritoneum. Not only does the peritoneum line the cavity, it also lines various organs. The portion of the peritoneum lining the cavity itself is known as the parietal peritoneum, and the portion that lines the organs is known as the visceral peritoneum. Folds within the peritoneum help suspend and anchor the organs in the pleuroperitoneal cavity.

Questions

1. What major organs are involved in digestion? How does this compare with the human digestive tract?
2. What accessory organs are involved in digestion? How does this compare with the human digestive tract?
3. What is the function of the vomerine teeth? Where are they located?
4. Which portions of the digestive tract are involved in chemical digestion?
5. In which portions of the digestive tract are nutrients absorbed?

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Additional resources

• NEW! Dissection of the Frog Manual
• Carolina’s Perfect Solution® Frogs
• Frog Dissection BioKit®
• Frog Anatomy Kit with Dissecting Set