LABORATORY # 9
VERTEBRATE ANIMALS
LABORATORY OBJECTIVES
Upon completion of this laboratory the student will:
REFERENCE
Textbook: chapter 22
Photo Atlas: chapter 7, pp 178 – 188, and chapter 8, pp. 189, 195, 197, 203, 205, 211 – 215
INTRODUCTION
Most of the animals that we recognize are the ones known as Vertebrates. Vertebrates are animals with backbones, that is, vertebrae forming a spine. The phylum Chordata is divided into two subphyla: invertebrate chordates and vertebrate chordates. The vast majority of chordates are vertebrates. Let’s look at an outline of the taxonomy.
Phylum: Chordata – Animals with a (1) notochord, (2) gill pouch with slits, and (3) dorsal, hollow nerve cord. All are present at some time in the development of all chordates.
Subphylum: Invertebrates – tunicates and lancelets; they are simple and use their gill pouch with slits to filter tiny food particles from the water.
Subphylum: Vertebrates
Lower Chordates – those that are cold-blooded (do not maintain a constant body temperature, but, instead, assume the temperature of their environment.)
Class: Jawless fishes
Class: Cartilaginous fishes
Class: Bony fishes
Class: Amphibia
Class: Reptilia
Higher Chordates – those that are warm-blooded
Class: Aves – Birds
Class: Mammalia – Mammals
The Lower Vertebrate Chordates
The several classes of vertebrates make up the majority of phylum Chordata. In addition to the three primary chordate characteristics, all have an enlarged brain, enclosed in a cranium or brain case, and a vertebral column of bone or cartilage, which replaces the notochord as the primary axial support structure. Five classes are included in this group. The first three are fishes, animals with a two-chambered heart that is adapted for life in the water. The last two (amphibians and mammals) have three- or four-chambered hearts and are more or less adapted to a terrestrial existence.
Classes:
1. The jawless fishes are the only vertebrates without jaws. These primitive vertebrates have sucker-like mouths, which are armed with a circle of horny "teeth". These are slender, eel-like organisms with a persistent notochord. They lack the paired fins found in other fishes and the eyes, when present, are without lids. Included in this group are the sea lampreys and the hagfishes. Some of the jawless fishes are parasitic; the only vertebrate group with this lifestyle.
2. The cartilaginous fishes are the first vertebrates with moveable jaws, a spine of separate vertebrae and paired fins. These animals typically have a spindle-shaped or dorsoventrally flattened body with an endoskeleton made of cartilage. The skin is tough and covered with tiny, tooth-like scales. No swim bladder is present and the gills, like those of the jawless fish, are uncovered. This class includes the sharks and rays.
3. The bony fishes, as the name would indicate, have a skeleton, which is more or less bony. The gills are covered with a bony operculum, which is used to pump water over the respiratory surfaces. These fishes all possess a gas-filled swim bladder, which allows them to maintain their position in the water with a minimum of effort. The class contains the most common game and commercial fishes, including bass, perch, trout, salmon and tuna, as well as most of the colorful tropical fishes.
4. The amphibians, though partially adapted to life on land, have not fully divorced themselves from the watery habitat of their ancestors. While some amphibians have functional lungs, many retain gills and all depend at least partially on their moist skin for respiration; consequently, all must inhabit a moist environment. The amphibians are not equipped for internal fertilization, thus they must return to the water to lay their eggs and it is there that the young develop. The variety of amphibians was much greater in the past, with only such creatures as frogs, toads and salamanders surviving.
5. The reptiles are a group well adapted to life on land. The body is covered with dry, scaly skin and respiration is via lungs. A leathery and/or limy shell protects these animals engage in internal fertilization and the eggs, when laid. In some taxa of lizards and snakes, the eggs are retained within the female and the young born alive. The heart of most reptiles is nearly four-chambered and totally so in crocodiles, where the ventricle is completely divided. The reptiles dominated the earth for nearly 200 million years, reaching their evolutionary zenith in the dinosaurs. Four groups remain today, these being the turtles, snakes, lizards and crocodilians.
Activitiy:
Classification of Lower Vertebrate Chordates
You will be given a large group of models and preserved lower chordates. Using the differences characteristics that are noted above, arrange them into classes. Arrange the classes in the order of their complexity and in which they evolved.
Higher Vertebrate Chordates
Like all chordates, these higher chordates possess the three primary characteristics of the phylum - hollow, dorsal nerve cord, notochord, and pharyngeal gill slits - plus two additional ones:
A. They are "warm-blooded", that is, they maintain a nearly constant internal body temperature, regardless of the external environment.
B. They have a four-chambered heart. The heart is divided by muscular walls into two atria and two ventricles, thus allowing separation of the circulatory system into distinct pulmonary and systemic circuits.
Both taxa included here (birds and mammals) descended from reptiles early in the Mesozoic Era and coexisted with the dinosaurs throughout most of the era. With the extinction of the dinosaurs, both groups expanded in variety and importance throughout the earth.
Class: Aves; the Birds
The birds are perhaps the most easily recognized of all categories of animals, since they are large enough to be seen, tend to be active in daylight, usually have the ability to fly and often are brightly-colored. They are unique in their covering of feathers, a feature seen nowhere else in the animal kingdom. Many of the unusual anatomical features of birds are adaptations, which make flight possible. In general, birds are characterized as follows:
A. The body is covered with feathers.
B. The anterior limbs are modified for flight.
C. The skeleton is strong and light, the mouth a toothless, horny bill, the neck long and flexible, and the sternum keeled.
D. Respiration is by compact lungs attached to numerous air sacs, which extend between internal organs.
E. Fertilization is internal, with the hard-shelled eggs incubated externally.
Birds seem to have evolved from reptiles during the Jurassic Period, just under 200 million years ago. An interesting fossil species named Archaeopteryx shows features, which are both reptilian and avian (bird-like). Birds have a major impact on human society. They provide us with food and clothing, consume hordes of pestiferous insects, and brighten our lives with their beauty and song.
Activities:
a. Fossil of Archaeopteryx.
Examine the reproduction of the fossil Archaeopteryx. Note the long, bony tail, the toothed mouth and the structure of the front limbs. Were it not for the possession of feathers, this organism would more closely resemble a small dinosaur, a clear indication of the bird’s reptilian ancestry.
b. Pigeon Skeleton
Examine the pigeon skeleton and notice the long flexible neck. Note the fusion of vertebrae with the pelvis and the very short tail. Now find the sternum or breastbone. See the keel extending from it? This is where the large flight muscles are attached. Compare the bones of the bird’s wing with those of your arm. Some bones have become reduced or lost, while others are fused. Finally, turn your attention to the skull and note the prominent beak and the large orbits (eye holes).
Class: Mammalia – the Mammals
The mammals probably evolved from mammal-like reptiles late in the Triassic Period, over 200 million years ago. They remained as small, primitive beasts throughout the remainder of the Mesozoic Era, living in the "cracks" of a biotic community dominated by the dinosaurs. With the demise of the dinosaurs in the great episode of extinction at the end of the era, the mammals ascended to roles of prominence in habitats all over the world.
In addition to the characteristics shared with other higher vertebrates, the mammals usually have:
A. The body covered with hair (though scant on some groups);
B. In the females, mammary glands secrete a fluid which nourishes the young;
C. Respiration by lungs, with a muscular diaphragm separating the thorax and abdomen;
D. In the male, a copulatory organ (penis) and testes, the latter typically held outside the body cavity in a scrotum;
E. Internal fertilization of a minute egg, with the embryo retained in the uterus of the female and nourished through a placenta; and
F. A large brain which provides for a high degree of coordination, memory and learning.
There is considerable diversity among the species of Class Mammalia. They range in size from tiny shrews scarcely larger than the terminal joint of your little finger, to the giant blue whale which, at 100 feet and 220 tons, is the largest animal ever to live on the earth. Mammals inhabit mountains and plains, jungles and deserts, forests and seas. Their life styles include flying insectivore, swift grazer, fierce predator, nocturnal scavenger and intelligent tool-user. The three major groups are:
1. The egg-laying mammals or monotremes - the most primitive group. Only two modern taxa belong to this group, these being the duck-billed platypus and the spiny anteater.
2. The pouched mammals or marsupials - a group with limited geographic distribution. Most of these creatures live in the Australian region or in South America. The opossum is our only local marsupial.
3. The placental mammals - most of the mammals we know and see, including humans. They all nourish their fetuses through a structure called the placenta and give live birth to well-developed infants.
Activity:
Dissection of the Fetal Pig
As a representative mammal, the fetal pig will be examined in some detail. This mammal was chosen for two reasons. First, the pig's internal anatomy is quite similar to that of humans, both being omnivores. Second, fetal pigs are by-products of the commercial meat industry. They are taken from the bodies of pregnant sows when they are slaughtered; consequently, no animals are sacrificed for our dissection.
a. Select a fetal pig, rinse it under running water and place it in a dissecting pan. Locate all parts labeled. Note that the pig has a cut in its neck region. The pig's blood was drained at this site and replaced by latex. This is a rubbery material which will assist you in identifying the arteries, colored red, and the blue veins.
On the ventral surface of the abdomen, locate the umbilical cord. At the cut end of the cord you can see the blood vessels, which connected the fetus with the placenta of the sow. Determine the sex of your pig.
b. Place the pig on its back in the dissecting pan. Cut a piece of string long enough to reach from one leg, under the pan, to the other leg, leaving enough extra for two knots. Tie the front legs apart, then repeat with the hind legs. This should position the pig securely on its back for dissection, most of which will be done with the scissors and dull probe. Use extreme caution when cutting with scalpel, razor blade or sharp probe.
Locate the pig's diaphragm, just posterior to the front legs. Using your scissors, cut through the body wall just anterior to the diaphragm. Continue the cut to the hairs on the chin. Next make two lateral cuts, just anterior to the pig’s front legs. Make two more lateral cuts just anterior to the diaphragm. Pull back the two flaps you have made to expose the thoracic cavity.
Now make another mid-ventral cut from just behind the diaphragm to just in front of the umbilical cord. Cut a half-circle around the umbilicus, and then proceed posteriorly on each side as shown in the drawing. Make two additional lateral cuts just posterior to the diaphragm, then two more on each side of the umbilicus. Expose the abdominal cavity by pulling back the flaps you have created. Carefully lift the umbilical cord with its skin flap and clip the umbilical vein underneath.
c. Locate the following organs in the neck region and the thoracic cavity: Larynx, thymus gland, heart, lungs and diaphragm. It will probably be necessary to pick away some tissue with the forceps to expose the organs in the neck. Now carefully open the sac covering the heart and identify an atrium and the ventricles.
d. Locate the following organs in the abdominal cavity: Liver, spleen, small intestine, colon, gall bladder, and stomach. Feel free to use your dull probe and forceps to move the various organs aside as necessary. Do not, however, remove organs from your pig.
e. Carefully pull the stomach and intestines aside and locate the kidneys. If you have a female, locate the ovaries, the body and horns of the uterus, the vagina and the urinary bladder. If you have a male pig, use Figure 119b to find the testes, the scrotum and the urinary bladder. Be sure you see both sexes.
f. Use the dull probe and forceps to clear material from the area around the heart, thus exposing the arteries and veins of the region. Keeping in mind that arteries are red (or pink), locate the following arteries: Pulmonary, aortic arch, common carotid, aorta, renal, umbilical. The veins have thinner walls than the arteries, but the blue latex will help identify them. Locate the following veins: Posterior vena cava, internal jugular, renal, common iliac, umbilical.