FROG ANATOMY AND DISSECTION TECHINQUES

The purpose of this guide is to introduce students to animal anatomy and structure by using basic dissecting tools, techniques and vocabulary. Please note: If it becomes necessary to interrupt the dissection process of the specimen, the animal should be carefully returned to the preserving solution or wrapped in a damp paper towel and placed in a plastic bag. Once the specimen has dried out, it will become unusable.

A Little About The Frog

Frogs are members of the zoological class called Amphibia. Amphibians are cold blooded vertebrate animals, meaning that they have backbones. When on land they breathe through lungs and when in the water breathe with gills. They differ from reptiles like snakes and lizards in that they lack scales and generally return to water to breed. Frogs and toads are one of three types of Amphibians and are called Anuran. Salamanders and newts are called Caudate and the Caecilians are primitive, worm like amphibians found mainly in the tropics.

The frog begins life in the water as a fertilized egg and after hatching becomes a tadpole, feeding almost exclusively on small green aquatic plants. After a period of growth the tadpole undergoes a developmental process called metamorphosis and is changed into an adult frog. Insects are the primary food of the adult and in many circumstances; frogs are significant factors in insect control.

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External Anatomy

Like most vertebrates, the frog has a body divided into three regions: the head, neck and trunk. However, the frogs’ neck is so small that it is nearly useless. This allows only restricted head movement. The body of the frog is covered with a smooth protective skin which contains mucous glands to keep it moist and pigment cells to regulate its color. Two pairs of legs are attached to the trunk. Each front leg is made up of an upper arm, wrist, hand and digits. Each back leg is made up of a thigh, shank, ankle, foot and digits. Note the close similarity of the frogs’ limbs to those of humans.

By examining the head and mouth you will notice two External Nares or nostrils that open near the anterior end of the head. Posterior to the external nares are the Eyes. Each eye has an upper and lower eyelid. A transparent third lid, the Nictating Membrane, covers each eye when the frog is underwater. A Tympanic Membrane or eardrum is located just behind each eye.

Open the frogs mouth and break the lower jaw at the hinges. The frog has two sets of Teeth. One set is located around the margin of the upper jaw and the other set is attached to the roof of the mouth. Rub your finger around these surfaces to feel the rough edges of the teeth. The frog uses its teeth for holding prey, not for chewing. The Internal Nares are located in the roof of the mouth. Probe to find the air passages which connect them with the external nares. Anteriorly attached to the lower jaw is the large flat Tongue.

The mouth cavity narrows posteriorly to form the Pharynx. The Esophagus leads from the pharynx to the stomach. Just ventral to the opening of the esophagus is the Glottis. This structure guards the air passage to the lungs. As the frog swallows food, the vertical slit in the glottis is closed. When the frog breathes, the slit is open so air exchange is possible. Internal to each tympanic membrane is a space or middle ear which is connected to the mouth cavity by an air passage called the Eustachian Tube. This tube establishes an open system so that pressure within the ear equalizes with the external pressure. The Eustachian tube openings are located lateral and just anterior to the glottis.

Internal Anatomy

Place the frog with the ventral

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or stomach, bottom side up, on the dissecting pad. With dissecting scissors, carefully cut through the skin along the midline from the back legs to the throat. At each end of this cut, make lateral or sideways cuts so the skin may be pulled back. Observe the exposed abdominal muscles and then cut them in a similar manner being careful not to damage the structures immediately below. Pin the skin and muscle layers to the dissecting pad so the body cavity is fully exposed.

Digestive System

Frogs do not have salivary glands but after food is taken into the mouth it is saturated with mucus. The food then passes through the short, muscular esophagus into the stomach. Food is fragmented and partially digested in the stomach before it passes into the small intestine where digestion is completed. The large, three lobed liver lies immediately anterior to the stomach. A main function of the liver is to secrete bile. Bile is stored in the gallbladder (the greenish, saclike structure located just under the left lobe of the liver.) From, the gallbladder, bile flows through a duct in the small intestine where it helps break down fats and neutralize stomach acids. Digestive enzymes are secreted into the small intestine by the pancreas. The pancreas is the tan colored tissue supported by the membrane connecting the stomach and the first portion of the small intestine. Blood absorbs nutrients from the small intestine. Some of the nutrients are stored in the liver and others are converted into fat bodies (the yellow, fingerlike lobes in the abdominal cavity). These food reserves supply nutrients during hibernation. Undigested materials pass into the large intestine where water is reabsorbed.

Circulatory System

The triangular shaped heart is located anterior to the liver. It has three chambers: two dark, thin walled auricles and one light colored, thick walled ventricle. Oxygen deficient blood from the body and oxygenated blood from the skin enter the right auricle. Oxygenated blood from the lungs enters the left auricle. Note the saclike lungs under and the each side of the heart. The auricles contract to force blood into the ventricle. As the ventricle contracts, it pumps blood into the arteries. Branches of the arteries become gradually smaller as they supply blood to all parts of the body. The capillaries are minute vessels which cp0nnect the tiny branches of the arteries to the tiny branches of the veins. As they carry blood back to the heart, the veins merge to form large and larger vessels. In the capillaries, oxygen carbon dioxide and nutrient waste exchanges are made between the blood and the body cells. New blood cells are formed in the bone marrow and in the spleen. The spleen is the small dark body located on the membrane just under the large intestine and is also responsible for disposing of old, worn out blood cells.

Excretory System

The two dark, elongated structures on the dorsal side of the body cavity are the kidneys. They extract liquid wastes in the form of urine from the blood. Small tubes or ureters lead from the kidneys and empty urine into the terminal portion of the large intestine or cloaca. Urine is forced from the cloaca into the urinary bladder which is the membranous sac attached to the cloaca. When the urinary bladder is full, the liquid wastes and solid wastes from the large intestine enter the cloaca and are excreted through the anus. The skin and lungs remove gaseous wastes from the blood.

Reproductive System

In the male frog, the testes are the white, ovoid structures attached ventrally to the anterior portion of the kidneys. Sperm pass through very small ducts to the ureter and are stored in the seminal vesicle or enlarged posterior portion of the ureter. During mating, sperm pass into the cloaca and out the anus.

In the female, the ovaries are just ventral to the kidneys. Undeveloped ovaries appear as small gray masses. In most cases, however, the ovaries are enlarged with thousands of small black and white eggs. During the breeding season, mature eggs are released into the body cavity where ciliary action moves them anteriorly into the openings of the oviducts. The eggs are coated with a gelatinous substance as they pass through the oviducts. Eggs are stored in the uterus, the swollen posterior region of the oviducts until mating, when they are released. They pass through the cloaca and out the anus. The gelatinous substance then swells, causing the eggs to stick to vegetation.

Fertilization takes place immediately as the eggs and sperm combine externally. The embryos develop quickly and emerge as tadpoles. Through the process of metamorphosis, tadpoles are transformed into adult frogs.

Nervous System

Turn the frog dorsal side up and remove the skin from the top of the head. With a scalpel, carefully make an incision along the midline of the skull. To expose the brain, remove the top portion of the skull with scissors.

The cerebrum has two lobes. These are the elongate structures between the eyes. The small bulbous structures at the anterior end of the cerebrum are olfactory lobes. The large bulbous structures at the posterior end of the cerebrum are the optic lobes. The cerebellum lies just posterior to the optic lobes. The elongate posterior portion of the brain is the medulla oblongata. The medulla connects to the spinal cord which is in turn surrounded by the backbone. Nerve branches from the spinal cord connect to all parts of the body.

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