Two terrestrial tetrapods

Lectures

Introduction

Additional information: The fetal pig dissection, the frog dissection, Some thought provoking questions, and Handouts and resources.

During this lab you'll be working as a team with one tetrapod, the fetal pig for example, and you'll be matched with another team working on the opposite specimen, in this case the frog. Both teams are responsible for both specimens, but you'll learn about one by dissecting it and the other from being shown the dissection the other team did. You'll use your dissection to show the others the various systems you have observed and how they are organized. They'll do the same for you.

To help understand the specimens we've also provided a set of "probing" questions to ask the other team and that you be prepared to answer yourselves. It would be a good idea to look these over before starting the lab! Here's another piece of advice. Depending on your specimen, it may impossible to show all of one system at once. You may need to remove things to expose underlying organs but do so only after everyone's had a look and a chance to ask their questions. In short don't leave your questions and explanations to the end of the lab. You may want to show something off at any point during the lab and when things look their best. Better still develop a game plan for how you'll approach your dissection and share it with the other team and try and co-ordinate a time budget for the two dissections. Even better, get this done before you start the lab and you'll be surprised how smoothly things will go.

The fetal pig dissection.

The fetal pigs you'll be working with are a by-product of the hog industry. Sows shipped to meat plants are often pregnant and during the butchering process the fetal pigs are usually discarded as waste. By using this specimen we're actually creating a "learning experience" using someone else's garbage! The mammalian nature of the specimen, and its convenient size, makes it easy to see the main systems of the chordate class and at the same time provide some insight into how the internal systems of our own bodies function. On the whole a good example of recycling and a much better use of the animal than would happen normally.

Frog dissection

The frog, in particular the grass frog Rana pipiens, is one of the most often dissected specimens in Biology and Zoology courses. It provides a unique look at an animal that exhibits many of the vertebrate characteristics at a time when vertebrates were making the transition from water to land. It's important to remember that the first amphibians looked nothing like the frogs and salamanders that exist today. These are all that remains of a diverse group of organisms that appeared in huge numbers during the wet and swampy times of the carboniferous. The frogs that we use in the lab have been bred in captivity especially for use by educational institutions. When they were harvested from the wild, there was a time when demand for grass frogs endangered the species - it's no longer the case. Although there are simulations of "virtual frogs" we believe that mastery of dissection techniques itself is essential for all biology students. The detailed observations, and careful dissection required to understand a specimen can't be achieved in these virtual environments where there is no natural variation in the specimens and the skills of those doing the dissection.

Some thought provoking questions.

How does the thin section of your/their specimen's lung compare to the other tetrapod? How is this related to how it functions?
What does the skin of your/their specimen secrete and why?
How are the lungs ventilated in your/their specimen?
How is the skin of your/their specimen related to its function?
How does the pharynx and movement of air and food occur in your/their specimen?
How does the organization of the heart differ in your/their specimen?
How does the tongue work in your/their specimen?
How does the pattern of blood flow differ in your/their specimen?
How does the placenta effect blood flow in the fetal pig?
What is the position of the appendicular skeleton relative to the axial skeleton?
How has the appendicular skeleton of your/their specimen been modified for they way it moves?
Are the digestive systems in the two specimens basically the same or are they radically different?
How well separated are the reproductive system and excretory systems in your/their specimen?
How well does the urogenital system of your/their specimen resemble the ancestral plan of the vertebrates?

Lectures

Objectives

Identify the main external features of each specimen.
Identify and compare the major parts of the axial and appendicular skeletons.
Compare the circulatory, digestive, urogenital, integumental and respiratory systems and how they are modified in each specimen.
Understand unique features of the “almost terrestrial” existence of the frog Understand the fetal modifications of the circulatory system and the role of the placenta.

Lectures

Resources

Online resources

N.B. This handout is in the Adobe Acrobat PDF format. Use this link to get a copy of the Acrobat Reader from Adobe (It's free!)

Resources

Prelab PowerPoint Handout (~750 Kb)
One-line demonstration (Sometimes we only have a few slides that our suppliers don't produce any more. They're usually in Digital Zoology. The Online demonstration gives you a nice big picture to look at):
- Hairy mammal skin - we've been unable to get slides for everyone so this demonstration will take the place of the slides

Resources

The work book contains a handy checklist of everything you can see in the specimens we'll be looking at. Chances are if it's not in the list you won't be able to see it (Of course there may be omissions, I tried to get them all)
Defining differences for the Classes Amphibia and Mammalia
Cladogram of the different Craniate taxa.
Modified lab module of the fetal pig
Modified lab module of the Frog

Evaluation tools

Some evaluation tools will require that you identify yourself using Web Based Student Services (for more info)