LABORATORY
EXERCISE #7
KINGDOM
ANIMALIA - PART V
THE LOWER CHORDATES
Introduction
The phylum Chordata
includes the animals with which we are most familiar (fish, amphibians,
reptiles, birds and mammals), as well as the less-familiar tunicates and lancelets. In this lab we will consider several taxa,
which we have arbitrarily designated as the lower chordates. This group includes all those chordates
termed "cold-blooded".
This phylum is indeed a diverse group, assembled on
the basis of three major characteristics which all possess at some time in their life cycles. Please note that all three characteristics need not be present
in the adult organism, in fact, they seldom are! These characteristics include:
A. A hollow, dorsal
nerve cord. In most of the creatures studied thus far,
any nerve cord has been ventral. (Do
you know the difference between dorsal and ventral?)
B. A notochord. This is a thin rod of cartilage, running anterior to posterior,
which supports the axis of the body.
C. Gill pouches
or slits. These are located in the area
of the pharynx, just behind the mouth.
For convenience we divide the chordates into two
categories:
The Invertebrate Chordates
The invertebrate chordates are mostly small, marine organisms,
which lack a "backbone” or
vertebral column. These simple
organisms are primarily filter-feeders,
which live buried in the sand or attached to submerged rocks, wharf pilings,
shells, etc. We will consider two
groups.
1. The
tunicates or sea squirts (Figure
21.3, page 355 of Starr) constitute a forgettable taxon of sea creatures, which
seem totally unrelated to humans.
Although they may reach one foot in diameter, most are far smaller. These humble animals have the nerve cord and
notochord only as larvae and are
named for the self-secreted, sack-like "tunic” or body covering. Their common name is derived from the habit
of suddenly squirting water from body openings when touched.
2.
The lancelets (Amphioxus) are small, fish-like organisms, which inhabit the
shifting sands of shallow, temperate to tropical seas. These animals are of particular interest to
biologists because they retain all three
primary chordate characteristics throughout their lives.
ACTIVITIES
a.
Look at the preserved lancelet,
then select a prepared slide of Amphioxus. Examine it on the lowest power of your microscope. Using the photograph in Figure 108b in your Photo Atlas and the diagram on page 355
of your textbook, locate the following parts: tentacles, gill slits and arches, nerve
cord, notochord, segmental muscles and tail.
Know common name, phylum and kingdom.
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 have three- or four-chambered hearts and are more or less adapted to a
terrestrial existence.
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 (with the possible exception of students).
ACTIVITIES
b.
Consider the model of the sea
lamprey. Note the suctorial
mouth, the gill openings, the median location of the fins, and the long, eel-like body. Now consider the anterior end of the lamprey
embedded in plastic. Note the teeth. These are parasitic animals, which attach themselves to the side
of a fish, use the teeth to bore into their host, and suck the blood out. The host usually dies from the attack or
subsequent infection. See Figure 21.7, page 357 of your
textbook. Know the common name, the class,
the phylum and the kingdom.
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.
ACTIVITIES
c. Examine all available specimens of cartilaginous fishes. This should include shark and ray. If present,
locate the following structures: gill slits, mouth, dorsal fins, pectoral fins, pelvic fins and caudal fin. Know common name, class, phylum and kingdom.
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.
ACTIVITIES
d.
The bony fishes will be illustrated by the perch. Select a preserved
specimen of the yellow perch, rinse it in water and place it in a dissecting
pan. Consult the illustration provided
in Figure 110b of Perry and Morton and identify the following parts: Operculum, gills, dorsal fins, caudal fin, anal fin, pectoral fin, pelvic fin, anus, lateral line, nostrils, and mouth. Note the
scaly skin and compare it with the fishes seen thus far. Be able to give the common name, class,
phylum and kingdom of these animals.
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.
ACTIVITIES
e.
Obtain a specimen of Rana, the bullfrog. Rinse under running water and place in a dissecting pan. Using the illustration in Figure 111a, identify
the following parts of the frog's external anatomy: Eyes, nostrils, tympanic membranes, and vent. Note the frog’s smooth, slick skin. When
alive, the skin is kept moist by many mucous glands. Look at the frog’s feet to see the thin webbing between the toes, an adaptation to its aquatic
life-style. Do you see any claws or
nails on the toes? Now pull the frog's
mouth open and find the parts labeled on Figure 112a. Compare your preserved specimen with the life-like Biocast of the
bullfrog.
f.
Select a specimen of Necturus,
the mud puppy. This is a giant salamander which is
permanently aquatic. Note the large, flat head with the prominent gills.
Compare this animal with the collection of smaller salamanders
provided. Could you recognize these
animals as amphibians?
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.
ACTIVITIES
g.
As an example of a lizard, examine the Biocast of the Gila monster. This large, slow-moving lizard of the
southwestern deserts is the only poisonous lizard in the world! Note the general shape of the organism. The fat
tail is in contrast to the long, slender tail of most lizards, such as the
small chameleons provided by your
instructor. Examine the scaly skin and compare it to the
amphibians you have seen. Look at the
feet. Do you find claws? Note the prominent eyes.
In lizards there are distinct upper
and lower eyelids. Can you find the opening for the ear?
h.
Consider the Biocast and the skeleton of the turtle. This animal is
enclosed in a shell made of bone and covered with a horny skin. Look at the dorsal shell. Which bones
have been modified to form it? How
about the ventral shell? Now consider the mouth. The jaws lack teeth, but note the tough covering,
which allows the turtle to cut, tear and crush its food. The toes typically bear claws, but in sea turtles the legs are modified into flippers.
i.
The alligator is a
crocodilian. These Native American
reptiles live in swamps or rivers. They
burrow in the banks for shelter and eat a variety of animals. The female guards the nest and protects the
young after they hatch. Examine the Biocast
model of the alligator and note the prominent head on its short neck. The tail is long and powerful and the legs
are more lateral than ventral in their placement.
j.
Pity the poor snake, so much
maligned and undeservedly so! This
Judeo-Christian personification of evil is, in fact, a beautifully designed and
highly specialized organism. To
illustrate this interesting taxon is a Biocast model of the copperhead. Examine it, noting the well-defined head and the slender, legless
body. Touch the model and feel the
dry skin. A live snake feels much the
same way. Now look at the heart-shaped
head and locate the eyes and nostrils. On the copperhead and other pit vipers you can find small
depressions or pits, which are heat
detectors used to locate prey. The pit
vipers also have a pair of specialized teeth or fangs, which inject poison into their victims.
k.
Examine the snake skeleton or study
Photo Atlas Figure 114a. Note an absence of any limb bones, the lack of a sternum or breastbone, and
the greatly increased number of
vertebrae and ribs. The bones of
the head are also highly modified to permit the swallowing of large prey.
l. Since living,
as well as preserved, dinosaurs are in short supply, we will satisfy ourselves
with scale models. The larger of the
two is Brachiosaurus, one of the largest land animals to ever
live. These giant sauropods weighed in
at about 77 tons and were up to 27 meters long. These animals could have looked over the campus center without
even tiptoeing! The other model is Tyranosaurus
rex, the largest terrestrial
carnivore to ever live. T. rex was about 14 meters in length,
weighed 7 tons, and had 6-inch teeth!
These giant vertebrates became extinct after ruling the earth for
approximately 180 million years.