The Common Grass Frog
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As the first vertebrate group to successfully colonize land, the amphibians must, of course, possess lungs and pulmonary circulation, although these differ significantly from the more advanced mammal. As amphibians, frogs are not emancipated from water, as it is a necessary medium in which to reproduce. Therefore, the reproductive system differs from the mammal, and more closely resembles the condition of their aquatic fish ancestors. As you examine the frog, keep in mind its phylo-genetic position between the fish and the more advanced tetrapods, and also keep in mind that many of the differences between the frog and the rat reflect important differences in the environments to which they are adapted.
Probably the most obvious modifications of the basic chordate plan is the absence of a tail in the adult (hence the term Anura). There is no clear distinction between head and trunk. The mouth is terminal with external nares above it. The eyes are prominent and very mobile. Nictitating membranes are located along the lower border of each eye. The tympanic membranes (ear drums) connect interiorly to the stapes (columella) of the middle ear, and are located posterior to the eyes.
The trunk has the usual four appendages characteristic of tetrapods, with fore-limbs and hind-limbs on the pentadactyl plan. Note the webbed feet. Since there is no tail, the cloaca is terminal. Frog skin is soft and moist. It lacks scales, but is well supplied with pigment and mucus glands. In males thickened thumb pads known as nuptial pads form during the mating season.
Examine the following external images of the frog:
Prop open the mouth and locate the internal nares, located on the roof of the buccal cavity. Medial to the internal nares are the vomerine teeth. Maxillary teeth (so called because they are located on the maxilla bone) are located around the inner margin of the upper jaw. At the posterior portion of the roof of the buccal cavity are a pair of apertures, one on each side, which lead into the eustachian tubes.
Behind the bi-lobed tongue on the floor of the mouth is a cartilaginous elevation, the larynx (voice box) with its longitudinal slit, the glottis. Notice that the tongue attaches at the front of the mouth rather than at the back. The tongue is important in capturing prey for frogs.
Examine the frog buccal cavity
Mid-ventrally, note the longitudinal line of connective tissue, the linea alba in which runs the ventral abdominal vein. To avoid mutilating this vessel, make incisions parallel to the linea alba on each side and cut transversely at about the mid-point. Then fold back the two portions to expose viscera. Further transverse cuts in the pectoral and pelvic regions can be made to fold back the remaining body wall.
Examine the ventral abdominal vein
Push the tri-lobed liver to the left and expose the esophagus running back from the mouth to the large J-shaped stomach. The stomach consists of a large anterior cardiac portion and smaller posterior pyloric portion which terminates at the pyloric sphincter. The first portion of the small intestine, the duodenum follows directly after the phyloric sphincter. Posterior to the duodenum lies the elongated and coiled ileum, which in turn, connects posteriorly with the large intestine. The large intestine is easily identifiable as a marked expansion of the alimentary canal in the posterior region of the body cavity.
Locate the pancreas, a distinct gland lying between the stomach and duodenum. The pancreas is best observed by lifting the stomach at the pyloric sphincter and viewing the dorsal aspect in the space between the stomach and the duodenum. The gall bladder is located on the dorsal surface of the right lobe of the liver (lift up the liver). The bile duct carries bile from the liver to the duodenum. This duct unites with a duct from the pancreas, forming the common bile duct which enters the duodenum. Note the hepatic portal system (same location and function as in the rat). The spleen is a roughly spherical dark organ which lies in the intestinal mesentery ventral to the kidneys. Note the spongy lungs. You should also note that there is no diaphragm dividing the coelom of the frog as there is in the rat.
