Station #1 - The Skin

The body of the frog is divided into 2 regions - the head and the trunk. The anterior (front) region of the head contains a wide mouth. Above the mouth, you should notice the paired external nares (nostrils), which connect to the internal nares inside the mouth. Each eye has three eyelids. There are upper and lower eyelids as well as the nictitating membrane, which is transparent to allow the frog to see when it is under water. The eyes can be pulled in to help push food down the frog’s gullet during swallowing.

Behind the eyes is a pair of external ear drums or tympanum. These will vibrate when sound waves hit them. Nerves will send impulses to the brain for the interpretation of sound. The frog’s limbs have four digits on the forelimbs and five on the much larger hind limbs. A large thumb pad on the forelimb is an indicator that your specimen is male.

The anus is found at the end of the trunk and opens internally into the cloaca. You will note the color difference between the ventral (belly) side and dorsal (back) side of the frog. You should also note that the skin is loosely attached to the musculature underneath. There are only a few regions where the frog’s skin is tightly attached to the underlying muscles.

Questions:

1. How is a frog adapted to see underwater?2. What are frog’s ears called?3. How can you tell the sex of a frog by looking at its thumbs?

Station #2 – The Mouth

The posterior part of the mouth cavity is the pharynx, which connects to the gullet and then the esophagus. The following structures can be found inside the mouth: vomerine teeth (two teeth found on the roof of the mouth), maxillary teeth (found along the edge of the mouth), internal nares, eye socket, eustachian tube openings (these tubes go to the middle ear), gullet opening (leads to the esophagus), glottis opening (leads to the lungs), opening to vocal sac, tongue (note the anterior attachment of tongue).

Questions:1. What are a frog’s nostrils called?2. Where is a frog’s tongue attached (front or back of the mouth)?

Station #3 – The Body Cavity

The type of muscle you will see is called skeletal muscle. Remember, cardiac muscle is found only in the heart and smooth muscle is muscle of the blood vessels and internal organs. The thin membrane that you see lining the body cavity and all its organs is called the peritoneum. The part of the peritoneum which holds together and supports the digestive organs is the mesentery (the pancreas is found here). Blood vessels flow through the mesentery to supply the digestive organs. Note the fat bodies, which are large, yellowish, lobed organs attached near the kidneys. They store fat during hibernation and aid in the production of eggs and sperm. They are much larger in the fall than in the spring. This would be an indication of when your frog was preserved.

Questions:1. What type of muscle tissue will you observe?2. How can you tell when a frog was preserved?

Station #4 - The Digestive System

Food is pushed down the gullet by the eyes being pulled in. This forces food into the esophagus, a short tube extending from the gullet to the large, curved stomach. Food leaves the stomach through the pyloric valve and enters the small intestine. Here the final digestion of food and the absorption of nutrients take place. The large intestine is the enlarged part of the digestive tract, which follows the small intestine and empties into the cloaca. It’s function is as a storage chamber where solid fecal waste and liquid metabolic wastes collect. Reproductive cells (sperm or eggs) will pass through the cloaca during reproduction.

Below the heart and lungs lies a dark three-lobed liver, which stores glycogen and detoxifies the blood. Between the lobes of the liver, note the small greenish gall bladder, which stores bile produced in the liver. The yellowish pancreas is found in the connective tissue (mesentery) between the stomach and the small intestine. This organ supplies digestive enzymes into the intestine and produces insulin.

Questions:1. Through which structures does food pass as it moves through a

frog’s digestive tract? 2. How does food travel down the gullet and into the esophagus of

a frog?

Station #5 - The Circulatory System

The three-chambered heart of the frog is enclosed within a sac called the pericardium. You should be able to identify the two atria (receiving chambers) by their thin walls. Identify the single muscular ventricle (pumping chamber). The spleen is an organ of the circulatory system that produces blood cells. It is reddish-brown in appearance and is found behind the stomach. It is normally about the size of a pea.

Questions:1. How is a frog’s heart different from a human heart?2. Why is the ventricle so muscular?

Station #6 - The Respiratory System

Gas exchange in the frog occurs mainly in the lungs, but also through the highly vascular skin and through the lining of the mouth. Air enters the respiratory system through the external nares and enters the mouth through the internal nares. Air is forced down through the glottis at the back of the mouth cavity by contractions and relaxations of muscles in the floor of the mouth. The glottis opens into a short, cartilagenous larynx. The larynx divides into bronchi, which lead to the lungs.

The lungs are elastic sacs contract to expel air. While air passes through the larynx, the air passes over vocal cords. This air may flow back and forth between the lungs and vocal sacs on the floor of the mouth. The sounds created by the frog are used primarily to attract mates. Note the frog lacks a diaphragm and ribs.

Questions:1. How does a frog breath?2. What is the purpose of frog sounds?3. How is frog respiration different from human respiration?

Station #7 - The Nervous System

At the base of the frog’s neck you will see the spinal column. The skull protects the fragile brain tissue. Olfactory lobes - two fairly large lobes which are used to interpret the frog’s sense of smell (one of its major senses), cerebrum - the two largest lobes found in the frog’s brain where memory and intelligence are found (not nearly as developed as more complex animals), optic lobes - two large lobes used to interpret the frog’s vision (its most important sense), cerebellum - the ridge going across the brain behind the optic lobes (controls coordination of movements), medulla oblongata - last region of the brain which extends to form into the spinal cord (involuntary control of mechanisms such as heartbeat, reflexes, swallowing, and breathing).

Questions:1. What are the frog’s two major senses and where is this information interpreted?