LABORATORY EXERCISE #8

KINGDOM ANIMALIA - PART VI

THE HIGHER VERTEBRATE CHORDATES

Introduction

Like all chordates, the two groups included in this lab 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.

Birds

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. 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 possession of feathers (not evident on this specimen), this organism would more closely resemble a small dinosaur; a clear indication of the birds' reptilian ancestry.

b. Aided by Figure 114d in your Photo Atlas, 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).

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.

ACTIVITIES

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.

c. Select a fetal pig, rinse it under running water and place it in a dissecting pan. Using the photograph in Figure 115a of Perry and Morton, 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. Assisted by Figure 115b, determine the sex of your pig.

d. 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.

e. Referring to Figure 116a in the Photo Atlas, 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. Figure 119a may be useful here.

f. Again using Figure 116a, 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.

g. Carefully pull the stomach and intestines aside and locate the kidneys, using Figure 118d as a guide. In Exercise c. above, you determined the sex of your pig. If you have a female, use Figure 119c to 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.

h. 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. Figure 118c is helpful in this endeavor. The veins have thinner walls than the arteries, but the blue latex will help identify them. With the aid of Figures 118a and b, locate the following veins: Posterior vena cava, internal jugular, renal, common iliac, umbilical.

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