Lab: Vertebrates
“Now, with all animals that are supplied with blood and furnished with feet, and are at the same time viviparous,
the bones do not differ greatly one from another, but only in the way of relative hardness, softness, or magnitude. . .
.The dolphin has bones, and not fish-spine.” [Aristotle, The History of Animals (Translated by D'Arcy Wentworth
Thompson), Book III Part 7]
”Whatever animals have milk, have it in their breasts. All animals have breasts that are internally and externally
viviparous, as for instance all animals that have hair, as man and the horse; and the cetaceans, as the dolphin, the
porpoise, and the whale-for these animals have breasts and are supplied with milk. Animals that are oviparous or
only externally viviparous have neither breasts nor milk, as the fish and the bird.” [Aristotle, op. cit. Book III, Part
20]
“The dolphin, the whale, and all the rest of the Cetacea, all, that is to say, that are provided with a blow-hole
instead of gills, are viviparous. That is to say, no one of all these fishes is ever seen to be supplied with eggs, but
directly with an embryo from whose differentiation comes the fish, just as in the case of mankind and the viviparous
quadrupeds. . . . All creatures that have a blow-hole respire and inspire, for they are provided with lungs. The
dolphin has been seen asleep with his nose above water, and when asleep he snores. The dolphin and the porpoise
are provided with milk, and suckle their young. . . It lives for many years, some are known to have lived for more
than twenty-five, and some for thirty years; the fact is fishermen nick their tails sometimes and set them adrift again,
and by this expedient their ages are determined.” [Aristotle, op. cit. Book VI, Part 12]
I
Objectives
Acquire dissection skills, learn basic anatomical terms and the major organs of vertebrates
II.
Introduction
In the taxonomic hierarchy, humans are placed the species Homo sapiens of the class Mammalia
in the subphylum Vertebrata within the phylum Chordata. [Hint: Many biology quiz-makers
believe that a college-educated person should be able to spell his or her name, their planet, and
their species.]. Long before Darwin, biologists had observed that organisms could be categorized
by the structural features that they shared with other creatures. Some three hundred and fifty
years before Christ, for example, Aristotle wrote that porpoises were more similar to land
animals than they were to fish (See the preface to this exercise.). They breathed air, had the same
bones, nursed their young and gave birth to live young. [OK, OK, so there is no species whose
young are born dead. If there ever was one it went extinct, probably after one generation.
Aristotle meant that porpoises do not lay eggs as fish do.]
Aristotle observed that the fins of porpoises were supported by bone and not spines as are fish
fins. Such structures are called analogous structures. Analogous structures perform a similar
task in different species, but have different developmental origins. Certain shapes are far more
efficient than other shapes. Sharp teeth, for example, are effective at holding struggling prey
while flat teeth efficiently crush plant cells. It was Mr. Darwin who pointed out that organisms
that possess the more efficient organs will, to paraphrase Mr. Spock, live longer and prosper
more (i.e. have more offspring) than members of their species with less efficient organs. Thus,
analogous structures approach the most effective shapes over time through a process called
convergent evolution. The wings of birds and insects are classic examples of analogous
structures. The hydrodynamic contours of sharks, cetaceans and swimming penguins are another.
Structures that share the same developmental origins, even if they are dissimilar in shape or
function (porpoise fins and horse legs, for example), are known as homologous structures.
Thus, in addition to the fossil record, DNA sequences, and continental drift evidence, the
evolutionary history of organisms can be discovered through comparative study of anatomy.
III.
Characteristics of Chordates and Vertebrates
Four characteristics appearing at some stage in development distinguish chordates from all other
phyla. These are a dorsal hollow nerve cord, a tail that extends beyond the anus, gill slits, and a
flexible cartilaginous rod or “support beam” called the notochord that extends along the back. In
mammals, the latter two structures are present only during fetal development; they are absorbed
before the young are born. Note that the nerve cord is not the notochord and that neither is the
vertebral column. The latter is a series of bones and/or cartilage that surrounds the nerve cord
and is characteristic of the vertebrate subphylum. To reiterate, in vertebrate chordates, the
notochord is replaced by the vertebral column. Thus, not all chordates have backbones; there are
chordates that are invertebrates. Vertebrates are also characterized by a complex of internal
bones that form an endoskeleton. They have two pairs of appendages and their body organs are
suspended in a body cavity that is lined with connective tissue called peritoneum. Such a body
cavity is called a coelom. The absence of a peritoneum in the body cavity of nematodes is why
their body cavity is called a pseudocoelom. In this exercise, you will be expected to learn the
characteristics of chordates and vertebrates by dissecting specimens of a class of vertebrates
known as amphibians (frogs, toads & salamanders). In addition, study the models of the fish,
frog, and mammals (pig and human) that are on display.
IV.
Dissection
Directional Terms: You will need to know some basic anatomical terms to help orient you in
the dissection. The terms will be presented in opposite pairs in the same manner that up is the
opposite of down. Dorsal refers to the “back” region and ventral is the “belly” or underside
region in those organisms that walk on all fours. Anterior means “forward” or the direction the
eyes are facing. Posterior means “behind.” Note that in humans (who stand upright) “ventral” is
also “anterior.” This is not the case in fish, cats and frogs. Cranial means toward the head and
caudal is towards the tail. Thus, in a frog “cranial” is also “anterior.” Medial means toward the
mid-line or middle of the animal. Lateral is toward the side. Superficial refers to being closer to
the surface while profound means deep.
Before you dissect the frog, your lab instructor will show the relevant structures on the human
model to which you can compare with those present on the frog.
External Anatomy of the frog: Working in groups and using Figure 1A as a guide, examine the
external features of the frog before dissecting. Note the obvious, but important, presence of four
appendages. Ventral and posterior to the eyes is a pair of large circular tympanic membranes
that detect sound vibrations in the air and water. The tympanic membrane in males of this
species is larger than the eye. Mammals also have tympanic membranes or “eardrums’ but they
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are recessed into the temporal bones of the head. Find the two external nares (= nasal openings)
on the anterior portion of the face.
Forcibly pry open the jaw to examine the structures of the mouth. Once the mouth is open, cut
the mandible bone on each side so that the jaw can be pulled fully down (See Figure 1B). Find
the two internal nares. Note that amphibians do not have a nasal cavity because they do not
have a palate which forms the “roof” of the mammalian mouth. Unlike mammals, the air that
amphibians inhale through their nasal openings enters the same cavity occupied by the tongue.
Notice that the tongue has a very different shape than that of typical mammals. The tongue of a
frog facilitates catching food while in mammals it serves as surface for taste receptors and
participates in swallowing. Observe that the eyes protrude or “bulge” into the mouth cavity.
Figure 1 Anterior structures of the male bullfrog, Rana catesbeiana A. Exterior of the head. B.
Mouth cavity. Es: Esophagus, EP: eye pocket; IN: Internal nares; T: opening to the trachea
(glottis); VP: vocal pouch in the male (Photos by Joel Borden, 2004).
When a frog swallows, the eyes are pulled in and the protruding eye pockets help push the prey
item down the esophagus, a muscular tube leading to the stomach. On the lower jaw, anterior to
the esophageal opening are two swollen ridges that surround the opening to the trachea, a
cartilaginous tube that leads to the lungs. In male frogs, between the tongue and the opening to
the trachea is a swollen vocal pouch used by males to make their distinctive mating calls.
Internal Anatomy of the frog: Lay the frog on its back with the ventral side facing upward.
Using a sturdy pair of dissecting scissors or a sharp scalpel, open the abdominal cavity with an
incision along the midline from the junction of the hind legs to the head. The skin will have
separated from the underlying muscle layer and you will have to cut through both layers. Take
care not to damage the internal organs. Use a forceps (“tweezers”) to hold the skin and
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superficial abdominal muscles from the underlying organs. If you are using a scalpel, be careful
to keep the sharp edge of the blade up (Do not cut by “stabbing” downward into the abdominal
cavity.) and lift the blade while cutting away from you toward the head. If using dissecting
scissors, twist the tool such that the blade inside the abdominal cavity lies against the abdominal
muscles while cutting. This will minimize the chances of cutting internal organs before you can
see them. Once the abdominal cavity has been opened, cut laterally under the lower jaw and
along the legs. Use dissecting pins to fasten the flaps of abdominal muscles to the dissecting pan
and expose the internal organs. If your animal is an adult female, her ovaries and developing
eggs may fill the cavity while juvenile males and females will be filled with yellow and red
finger-like lobes of the fat body. These structures are absorbed during mating season when the
animal’s lipid reserves are sequestered in eggs and sperm. Remove any of the fat bodies that
obscure other organs. The large, dark red, hard organ in the anterior of the abdomen is the liver.
The liver performs numerous functions associated with processing nutrients that have just been
absorbed from the small intestine. Cut and set aside the two largest liver lobes of to expose the
underlying organs. (See Figure 2.) When you have done this, you should be able to see a
relatively large sac, the gall bladder, between the lobes. (Not shown in Figure 2). The gall
bladder, stores a green liquid made in the liver called bile. Bile helps break fat into small
droplets for easier digestion and it is delivered to the small intestine when food leaves
Figure 2. Internal organs of a juvenile male bullfrog, Rana catesbeiana FB: fat body; H: heart;
LI: large intestine; Lg: lung: Lv: liver; P: peritoneum; SI: small intestine; Sp: spleen; St:
stomach; UB: urinary bladder. (Photo by Joel Borden, 2004)
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the stomach. Observe that the muscles and organs are covered with a shiny, thin membrane of
connective tissue called the peritoneum. It is the presence of the peritoneum that defines
vertebrate body cavities as coelomic peritoneal cavities. The triangular or pointed organ in the
anterior medial region is the heart which pumps blood to the lungs and body organs. Adjacent
on either side and dorsal to the heart are the usually pink (Sometimes the fixative turn them
purple) lobes of the lungs. If you tease the lungs apart under a dissecting microscope you will
see small tubes of cartilage, the bronchioles. These look like small vacuum cleaner hoses and
extend from the trachea. You may also find parasitic trematodes (= lung flukes) of the genus
Haematoloechus. (Frogs become infected when they eat infected dragonfly larvae.) In the center
of the abdominal cavity is the muscular stomach which serves as a storage organ for ingested
prey. The esophagus is attached to the stomach at the cranial end. Insert a blunt probe down the
opening of the esophagus to the stomach and the trachea to the lungs to confirm that you have
located these organs. Leading caudally from the stomach is the much narrower small intestine
where most of the digestion and absorption of food occurs. At the caudal end of the abdominal
cavity, the small intestine joins the wider large intestine. It is in the large intestine that water is
absorbed and feces are formed. Next to the small intestine in the middle of the abdominal cavity
is the dark red, bean-shaped spleen. This organ removes old red blood cells and is an important
component of the immune system. Remove the digestive tract intact by cutting through the
esophagus where it joins the stomach as well as through the caudal section of the large intestine.
(See Figure 3). Adjacent to the stomach and small intestine is a relatively thin, flat organ called
the pancreas. This organ produces enzymes that digest food in the small intestine as well as the
hormone, insulin, which helps control the concentration of glucose in the blood.
Figure 3. Digestive tract of the bullfrog, Rana catesbeiana P: pancreas; other symbols as in
Figure 2 (Photo by Joel Borden, 2004).
Once you have removed the digestive tract, you can see the kidneys, two elongated organs on
either side of the vertebral column. The kidneys remove nitrogenous wastes from the blood and
control the concentration of salts in the blood. Fluid processed by the kidneys is stored in the
urinary bladder prior to its discharge from the body. The urinary bladder is superficial and
caudal to the large intestine. You will have to carefully separate the bladder from the surrounding
peritoneum to see it well. Both the urinary bladder and the large intestine open into a cloacal
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cavity. A cloaca is a reservoir (organ) that receives the products of the digestive, reproductive
and excretory systems. Adult placental mammals do not have cloacas, but they are found in
amphibians, reptiles, birds, and some non-chordate phyla. You will not be responsible for finding
the cloaca, but recognize the term. The gamete generating organs (ovaries in females and testes
in males) are difficult to find in juvenile animals. In reproductive adults, especially mature
females, the reproductive organs may fill most of the abdomen. As noted by Aristotle,
amphibians are oviparous; that is they lay eggs.
V.
Terms
Your lab instructor will give you more information concerning which of the terms listed below
he or she considers important enough to be included on quizzes.
amphibian
analogous structures
anterior
bile
bronchiole
caudal
chordate
coelom
convergent evolution
cranial
dissecting scissors
dorsal
endoskeleton
esophagus
external nares
eye pocket
fat bodies
forceps
gall bladder
gill slits
heart
homologous structures
internal nares
kidneys
large intestine
lateral
liver
lungs
mammal
medial
mouth
nasal cavity
nerve cord
notochord
oviparous
ovaries
palate
pancreas
peritoneum
posterior
Written by Jack O’Brien, Ph.D., 2004
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probe
profound
pseudocoelom
small intestine
scalpel
spleen
stomach
superficial
tail
testes
tongue
trachea
tympanic membrane
urinary bladder
uterus
ventral
vertebral column
vertebrate
viviparous
vocal pouch