Comparative Biology: Lab #23: Amphibians

The common mudpuppy, Necturus maculosus, is a member of the Caudata, which together with Anura (frogs and toads) and Gymnophiona (caecilians or apodans) for the Lissamphibia. The Caudata, including the salamanders and newts, are the least specialized amphibians in the body form and locomotion. The body is elongated and stout, with well-developed axial musculature and tail. Salamanders use their limbs in combination with the side to side body undulations characteristic of fish, and thus probably resemble the earliest land vertebrates in locomotion. In contrast, frogs are characterized by a shortened body and specialized salutatory locomotion, while gympnophionans are limbless.

Necturus are neotenic (retaining juvenile features) salamanders that retain their larval, filamentous external gills, two pairs of pharyngeal slits, and caudal fins as adults.

Internal Anatomy:

Enter the pleuroperitoneal cavity by making a longitudinal incision about 0.5 cm to the left of the midventral line. Cut anteriorly until you reach the cloacal opening. Gently spread the abdominal wall. The long elongated dark structure midventrally is the liver. Examine the posteroventral part of the cavity to observe the thin-walled urinary bladder.

Carefully cut through the falciform ligament, and make two transverse cuts through the body wall, one on either side, to create four flaps that can be spread apart. The long, light colored, tubular stomach lies dorsal and slightly to the left of the liver.

The elongated spleen hangs from the posterior left side of the stomach. The stomach ends abruptly at the pyloric sphincter, a marked constriction beyond which the digestive tract continues as the long, coiled small intestine, followed by the short, straight large intestine. The mesentery supports the small intestine. Spread apart the coils of the small intestine to observe it.

The first loop of the small intestine is the duodenum. Several organs and vessels lie in this region, but may be difficult to discern. The pancreas lies along the duodenum. To help identify it, pull the stomach to the left and reflect the liver to the right to expose its dorsal surface. The pancreas is irregular, but note that part of it extends anteriorly toward the spleen.

Observe the long, thin transparent lung lying dorsal to the stomach. The right lung is similar in form. Look between the stomach and liver. You should discern the left and right lungs. The liver is supported anterodorsally by the hepatocavoplumonary ligament. The posterior vena cava is the large vessel passing through the posterior end of this ligament.

Next, reflect the stomach to the right, so it lies on the liver’s dorsal surface. Note the relationship among the liver, stomach, spleen, and pancreas, as well as their associated mesenteries. Let the viscera fall back in place, and then reflect the liver to the left. Examine the posterodorsal part of the liver for the gall bladder, a thin, translucent greenish sac. Gently lift it and examine the region where it attaches to the liver.

Urinary and reproductive structures should be examined next. Lift the coils of the small intestine in the posterior part of the peritoneal cavity to locate one of the paired gonads. In males the testis is an elongated organ posteriorly in the pleuroperitoneal cavity lying ventral to the kidney. The kidney, longer than the testis, is considerably wider posteriorly than anteriorly. Its narrow anterior part is genital in function, while the wider posterior portion is urinary.

In females the elongated ovary may be quite large. The presence of numerous eggs within follicles gives the ovary a lobulated or granular appearance, in contrast to the more regular surface of the testes. The follicles and eggs vary in size depending on their stage of maturity, being quite large in some specimens and smaller in others.

Heart

The pericardial cavity lies just anterior to the liver. It is enclosed by the pericardial sac and contains the heart. Continue the midventral incision of the abdominal wall anteriorly, cutting through the coracoid cartilage, to expose the pericardial cavity. In doing so you will also cut through the transverse septum, the partition separating the pericardial and pleuroperitoneal cavity. Do not injure the posterior vena cava, which passes through the septum to reach the pericardial cavity. Carefully remove the musculature ventral cavity. The largest and most conspicuous part of the heart is the ventricle, which occupies the posteroventral part of the pericardial cavity. Lift the posterior end of the ventricle to observe the sinus venosus. The atrium lies anterodorsal to the ventricle and is partially divided into left and right atria. The conus arteriosus is the narrow tube extending anteriorly between the atria from the right side to the ventricle.

Conclusion:

Draw your conclusions.
A detailed comparison of the different organisms you observed in the amphibian lab.
A detailed comparison of amphibian to past organisms you have observed.
A hypothesis as to how the organisms you have observed evolved into each other (past and current lab).
What went well while dissecting and observing? Why?
What didn’t go so well while dissecting and observing? Why?
Questions:

Why does a necturus look the way it does? Why is it colored the way it is? Why is it shaped the way it is? Why are the structures located where they are? Refer to specific structures and use the language of the theories of evolution and natural selection to answer the questions.
What are the defense mechanisms found in each of the specimens studied? Why did they develop the way they did? Frame your answer using the language of natural selection.
Choose two features from the structures listed in the procedure and explain why they are in the necturus. Frame your answer using the language of natural selection.