Comparative Vertebrate Structure Lab Exercise 4
Comparative Vertebrate Structure Lab Exercise 4
Lab Exercise 4 – Comparative Vertebrate Structures 1
Exercise 4
COMPARATIVE VERTEBRATE STRUCTURE
Student Learning Outcomes
At the completion of this exercise you should:
(1) Be able to define and contrast the anatomical terms Homologous and Analogous.
(2) Be able to state if the forelimb of a human is Homologous and/or Analogous to the forelimb of a
cat, whale, or frog.
(3) Be able to describe what a mammalian “dental formula” is and be able to interpret the Human
dental formula: 2:1:2:3 / 2:1:2:3
(4) Be able to identify basic organs in the digestive, circulatory, respiratory, and excretory systems of a
rat.
Introduction
Over the past 100 years a great deal of evidence has been gathered to substantiate the theory that the
vertebrates we see today, including the human, had their origin millions of years ago in an aquatic
environment. Through long periods of time the basic form of a common ancestor was modified until a
successful invasion of the land was achieved. With the capacity to live on land, many new
environmental situations were available. Today we see the results of this exploitation in that vertebrates
now have occupied virtually every basic kind of ecological habitat. They have mastered the intricacies
of flight, have successfully occupied all climatic zones, including the poles, have burrowed within the
earth, and have even returned to their ancestral place of origin, the water. Biologists account for this
remarkable divergence on the basis of characteristics which aid the animal in meeting the problems
posed by the different environments. These characteristics confer a selective advantage on the organism
possessing them and may be structural, physiological, behavioral, or a combination of these types. Such
characteristics which affect survival in a positive way are termed adaptations. They are believed to
have evolved from a generalized condition to a specific one. An example of a highly specialized
adaptation would be the canine teeth of a saber tooth tiger as compared to the relatively generalized
structure of human canine teeth. The more highly adapted an organism is to a specific environment, the
greater will be the number of specialized structures. In each of the different environments are found
vertebrates who demonstrate adaptive characteristics, both specialized and generalized which assist them
in their respective habitats.
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Lab Exercise 4 – Comparative Vertebrate Structures 2
Homology and Analogy
If we examine structural features of vertebrates, a conceptual generalization becomes apparent. The
form of an organism and its parts are related to function. Since efficient function is essential to
survival in a specific habitat, form is related to the habitat in which the organism lives. For example,
a terrestrial mammal shows legs of one sort or another, a structural adaptive feature which makes
locomotion possible. A bird possesses wings, an adaptation which allows flight.
In addition, if one compares the origin and development of the front appendages of a terrestrial mammal
(foreleg of a cat) and the wing of a bird it is found that they have a common origin and parallel
embryonic development, being secondarily modified to perform different functions. Such structures are
said to be homologous and biologists believe homologous structures indicate a common ancestor
between two forms. (Again, notice that homologous structures do not necessarily have the same
function.)
If two structures perform the same function, even when structure and embryonic development are vastly
different, they are said to be analogous. Thus, the wing of a bird and the wing of a butterfly are
analogous, having a similar function, but they are not homologous since their structural origin is
dissimilar.
Question 1. Place the letter A for analogous or the letter H for homologous next to the pairs of
structures below.
Homologous/ Analogous?
human arm cat forelimb
bat wing frog forelimb
bird wing butterfly wing
horse forelimb bird wing
paired whale flippers paired fish fins
Many examples of structural characteristics which are both homologous and analogous are seen in
nature. From such observations biologists have developed another generalization which says, “the more
closely two organisms resemble each other, the more similar must be their genetic makeup and therefore
the closer their relationship.” By comparing a variety of organisms, the results of evolutionary
adaptation become apparent.
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Lab Exercise 4 – Comparative Vertebrate Structures 3
I. Skeletal Adaptation in Vertebrates
Procedure:
You will find on the laboratory tables the skeletons of various vertebrates. As you proceed with this part
of the lab exercise, keep in mind the kinds of environments in which these vertebrates live. Constantly
look for features which have survival value and which give the animal a greater capacity to succeed in
its particular environment. These skeletal adaptive features are primarily those which enable the animal
to adapt its activities to its environment: e.g., locomotion (limbs), feeding behaviors (teeth), etc.
A. Skull Bones
1. Comparative Skull Structure
Using Figure 1 as a reference, generally compare the skulls of the human, the cat, and the horse. Look
for individual bones, the general shape of the skull, and especially the position of the large hole
(foramen magnum) through which the spinal cord enters the skull.
Question 2. List some general similarities in the skulls of the cat, horse, and man.
Replace this text with your answer.
Question 3. Examine the sutures joining the several bones of the skulls. How are the skulls generally
comparable in terms of the number and types of bones?
Replace this text with your answer.
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Lab Exercise 4 – Comparative Vertebrate Structures 4
Figure 1. Lateral View of the Human Skull
(Diagram shows the principal bones and the plane of the foramen magnum
with a pencil placed at a right angle to that plane.)
a. Placement of the Foramen Magnum:
Notice the large “hole” in the base of the human skull. This is the foramen magnum, through
which the spinal cord enters the skull to connect to the brain. Notice that an imaginary line drawn
straight through the foramen magnum, then through the skull, would emerge through the top of the
skull. (See Figure l.)
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Lab Exercise 4 – Comparative Vertebrate Structures 5
Question 4. Examine the skull of the separate cat skull, as well as that of the mounted cat skeleton.
Imagine a line, similar to that described for the human skull, drawn straight through the foramen
magnum into the skull. Where would such a line emerge from the skull of the cat?
Replace this text with your answer.
What about the horse?
Replace this text with your answer.
Question 5. Examine the mounted cat skeleton. If the cat’s foramen magnum were on its skull in the
same position as on the human skull, in what direction would the cat’s eyes be looking?
Replace this text with your answer.
Question 6. What basic difference between locomotion of the cat (or horse) and the human accounts for
the difference in placement of the foramen magnum?
Replace this text with your answer.
b. Zygomatic Arch
Using Figure 1 as a reference, locate the zygomatic arch (cheekbone) in the human and in the cat. Find
your own zygomatic arch. Then close your mouth and place your flat hand on your cheek. Tightly
clench and unclench your jaw as though chewing. Notice the position of the active muscle in your
cheek.
Question 7. Generally, bones provide structure to the organism, acting as points of attachment for soft
tissues. What do you suppose to be the specific function of the zygomatic arch?
Replace this text with your answer.
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Lab Exercise 4 – Comparative Vertebrate Structures 6
B. Teeth
1. Tooth Types and Dental Formulas
The teeth of mammals usually indicate the type of food that constitutes their main diet. Except for
baleen whales, anteaters, and pangolins, all adult mammals have teeth. The primitive dental formula
that has been modified in different placental mammal groups is:
3 : 1 : 4 : 3 / 3 : 1 : 4 : 3 (times 2 = 44 teeth.)
In other words, on each side of each jaw of a primitive placental mammal there were three incisors, one
canine, four premolars, and three molars making a total of 44 teeth. In the course of evolution most
mammals have changed from this primitive number to fewer teeth. Rodents and lagomorphs (rabbits)
have large chisel-shaped ever-growing incisors, which are sharpened as the animal rubs the cutting
edges together. The tusks of an elephant are the enormously enlarged second incisor teeth. Canine teeth
are absent in rodents and lagomorphs and on the upper jaws of most hoofed mammals.
Genus Dental Formula
1. Pig or Wild boar (Sus)* 3 : 1 : 4 : 3 / 3 : 1 : 4 : 3
2. Deer (Odocoileus) 0 : 0 : 3 : 3 / 3 : 1 : 3 : 3
3. Wild burro (Equus) 3 : 1 : 3 : 3 / 3 : 1 : 3 : 3
4. Bobcat (Lynx) 3 : 1 : 2 : 1 / 3 : 1 : 2 : 1
5. Beaver (Aplodontia) 1 : 0 : 2 : 3 / 1 : 0 : 1 : 3
6. Human (Homo) 2 : 1 : 2 : 3 / 2 : 1 : 2 : 3
*Some variability occurs in the genus Sus
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Lab Exercise 4 – Comparative Vertebrate Structures 7
2. Comparisons
a. Human: Examine a skull using Figure 1 as a reference. Note the following kinds of teeth:
Incisors
● Four in the front of each jaw. These are the cutting teeth which have crowns shaped like
a chisel.
Canines
● Two in each jaw, lateral next to the incisors. The crown terminates in a single, relatively
sharp point.
Premolars
● Four in each jaw and lateral to the canines, they are characterized by a crown with a
grinding surface with two projections (cusps) and are frequently referred to as bicuspids.
Molars
● Six in each jaw. These are larger than the premolars and have a broad grinding surface
with four cusps.
Deviations from the above are not uncommon, with people occasionally showing varying
numbers of incisors and/or large wisdom teeth behind the molars.
Question 8. Locate and count the above tooth types in your own mouth. Which ones are you missing,
compared to the formula?
Replace this text with your answer.
b. Comparisons with other species:
In our examination of skulls the numbers are useful mostly in helping to identify the different kinds
of teeth. The basic morphology (form) is significant because it permits the observer to relate form to
function and therefore to the feeding habits of the organism. After having identified the different
kinds of teeth, and with some reflection on how you use your own teeth, examine the teeth in the
skulls of a cat and a horse.
Question 9. How are the canines of a cat different from those of the human?
Replace this text with your answer.
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Lab Exercise 4 – Comparative Vertebrate Structures 8
Question 10. Compare the shapes and numbers of premolars and molars between the cat and the
human:
Cat Human
Premolars: Number
Premolars: Shape
Molars: Number
Molars: Shape
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Question 11. Do any of the teeth (cat or human) appear to be specialized? If so, which ones and for
what?
Incisors Canines Premolars Molars
Cat: Description:
Specialized? (yes
or no)
Purpose?
Human:
Description:
Specialized? (yes
or no)
Purpose?
Question 12. On the basis of your observations and on your knowledge of the diet of a cat, write a
general statement summing up the adaptive features observed in the teeth of the cat.
Replace this text with your answer.
Question 13. How do the teeth of the rodent (e.g., beaver) differ from those of the cat? Consider both
number and type of teeth.
Replace this text with your answer.
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Question 14. Do the differences between the rodent (beaver) teeth and cat teeth represent a specialized
or generalized feature? Explain your answer.
Replace this text with your answer.
Question 15. How do the teeth of the horse differ in number and form from the cat?
Replace this text with your answer.
Question 16. How are the teeth of the horse adapted to its particular environment and way of life?
Replace this text with your answer.
Question 17. Which basic tooth type is exaggerated in the dentition of the pig?
Replace this text with your answer.
Question 18. Of what adaptive significance are tusks as illustrated in the pig?
Replace this text with your answer.
Question 19. In a sentence, indicate the type of teeth you might expect to find in an animal which
swallows its prey whole?
Replace this text with your answer.
Question 20. What common animal employs the type of teeth described in Question 19?
Replace this text with your answer.
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C. Appendicular Skeleton (appendages) in vertebrates:
1. Human
Locate the limb bones shown in Figures 2 and 3 using the human skeleton provided in the laboratory.
Figure 2. Right Arm Figure 3. Left Leg
of Human of Human
(from front) (from front)
Question 21. List some similarities between the bones in the human arm and leg.
Replace this text with your answer.
Question 22. List some differences between the bones in the human arm and leg.
Replace this text with your answer.
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Question 23. The human skeleton consists of 206 bones. There are 106 bones in the wrists, hands,
ankles and feet. What is the advantage of having this many bones? Explain.
Replace this text with your answer.
2. Comparisons with Other Species
Comparative forelimb bones are diagrammed in Figure 4 (frog, bird, cat and human).
Question 24. Observe the diagrams of the forelimbs. On the list of the six kinds of bones below, place
the respective numbers indicating a specific bone type in the blanks below after the named skeletal bone.
As an example, this has been done for the humerus. (See also Figure 2.)
Humerus 11, 17, 5, 1
Radius 7, 4, 9, 12
Ulna
Carpals
Metacarpals 15
Phalanges
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Lab Exercise 4 – Comparative Vertebrate Structures 14
II. Soft Tissue Structure (Anatomy) in Mammals and Other Vertebrates
A. Comparisons between Vertebrate Groups
1. Brain
Comparative Study of Three Divisions of the Brain
Brain models are provided for your study. Note the size and shape of the optic lobes (colored yellow,
function: sight), cerebellum (colored green, function: regulation of muscular coordination), and
cerebrum (cream color function: sense of smell in lower vertebrates; however, is concerned with
coordination, voluntary movements, and consciousness in higher vertebrates)
Question 25. Compare the size and shape of the three brain divisions as you observe the models: for
each part, estimate the percent of the total brain mass it occupies.
(Note: The human model is not color-coded the same as the rest. Use the chart provided to locate the parts of the human
brain.)
Estimated Percent of Total Brain Mass
Species Optic Lobes Cerebellum Cerebrum
Trout (Salmo)
Frog (Rana)
Pigeon (Columba)
Dog (Canis)
Human (Homo) *
The ratio between the weight of the brain and spinal cord can serve as a general criterion for an animal’s
relative intelligence. Fish and amphibian brains have a ratio of 1:1, while the human brain has a ratio of
55:1 (or 55 times heavier than the spinal cord).
*Not found in the human brain—optic lobe function taken over by occipital lobes of cerebrum.
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Lab Exercise 4 – Comparative Vertebrate Structures 15
2. Digestive
Examine the animals set up as demonstrations. (You may need to refer to your text in order to answer
some questions.) All vertebrates have a tubular digestive tract with a mouth and an anus for the entrance
of food materials and the exit of waste. The details may vary but the basic needs and, therefore, the plan
remains very much the same.
Question 26. Do all of the following organisms have teeth to grind up their food? If not, attempt to
explain why they don’t need teeth.
Species Teeth? If not, explain
a. pig
b. seed-eating bird
c. frog
d. turtle
Question 27. With what structure does the frog capture its food?
Replace this text with your answer.
Question 28. Some food types are more difficult to digest and therefore require more time in the
digestive tract. Suggest an anatomical adaptation that would increase the length of time the food remains
in the digestive tract, as well the surface area available for absorption of food molecules.
Replace this text with your answer.
Question 29. Do the liver and pancreas appear in all of our study animals?
Replace this text with your answer.
What are some important functions of these organs (related to digestion)?
Replace this text with your answer.
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3. Circulatory and Respiratory Systems
Examine the preserved demonstration specimens.
Question 30. In an animal such as the fish, oxygen enters the blood through the tissues of the gills with
their rich capillary supply by diffusion from the oxygen in the water. How many chambers are present
in the heart of the perch?
Replace this text with your answer.
One of the chambers of the fish heart (atrium) receives blood and the other (ventricle) pumps it to the
gills and then on to the rest of the body.
Question 31. How many chambers are seen in the heart of the frog?
How many chambers are seen in the heart of the turtle?
How many chambers are seen in the hearts of the mammals?
Animal Heart chambers
Frog
Turtle
Mammal
Question 32. Observe the lungs of the frog and turtle and compare with those of the fetal pig, bird, and
cat. Do you suppose that frogs and turtles require as much oxygen (per gram body weight) as pigs, birds
and cats? Explain
Replace this text with your answer.
Can you also relate this to the presence or absence of thermoregulation in these animals?
Replace this text with your answer.
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Question 33. The four-chambered heart, as seen in mammals, is regarded by biologists as more efficient
than the three-chambered heart. With respect to the role of circulation in helping the respiratory system,
explain why the four-chambered heart might be more efficient than the three-chambered heart.
Replace this text with your answer.
B. Rat Dissection
In this activity we will examine the structures that make up the major body systems of mammals, using a
model organism, the rat. The rat’s general internal anatomy is similar to human anatomy with two
exceptions: 1) they lack a gallbladder, and 2) they have an enlarged cecum compared to the vestigial
appendix of humans. This anatomical similarity, as well as ease of care and a rapid reproductive rate, has
made them the most common animal used in medical experiments.
As you work your way through the activities, refer to the Rat Dissection Guide laminated color photo
cards.
Objectives
You should be able to do the following after today’s activities:
1. Determine the sex of an adult rat;
2. Identify major external features of the rat (listed in bold) on a diagram or specimen;
3. Be able to identify major internal body structures (listed in bold) on a diagram or specimen;
4. Know the locations and functions of these body structures.
External Anatomy
Lay your rat on its back on the dissecting pan. The head is where the majority of sensory organs are
located. The ears, eyes, nose, and tongue have function like our own do, for hearing, sight, smell and
taste. In addition, rats and many other animals have mechanosensory vibrissae, or whiskers, on their
face.
Identify the sex of the specimen. To do so, first find the anus at the base of the tail. If the rat is male,
the scrotum, which houses the sperm-producing organs, will be an obvious bulge near the tail. The
penis is internal but the opening (preputial orifice) is visible as a small flap of skin anterior to the
scrotum. If the rat is female, there will be no bulge and a second opening, the vulva, anterior to the anus
and posterior to the last pair of teats.
Head and Neck Region
The regions of the nose and mouth also function as parts of the respiratory and digestive systems, in
some cases, both. Open the rat’s mouth as far as you can. Observe the tongue with visible taste buds,
and teeth. The roof of the mouth is called the palate. The palate separates the mouth cavity from the
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Lab Exercise 4 – Comparative Vertebrate Structures 18
nasal cavity. At the back of the mouth are openings of the esophagus, where food goes when it is
swallowed, and the trachea, which takes air to the lungs. The opening of the trachea, called the glottis,
is covered with a flap of tissue, the epiglottis, when food is swallowed to prevent it from passing into the
lungs.
Opening the Body Cavities
CAUTION: Do not cut deeply! Do not remove any organs or other body parts unless instructed to
do so. You will need them to orient yourself in order to identify all the structures. Try not to
pierce any of the organs with your tools; especially the cecum or liver, or bad-smelling fluids will
flood the specimen and obscure the structures.
Pinch the skin in the area of the belly button and make a small horizontal cut with the scissors. The skin
is thick and tough and covered in fur. If you cut carefully, you can separate the skin from the abdominal
body wall muscles and peel it back to observe these. Eventually you will need to cut all the way through
these muscles and the peritoneum, and shiny membrane lining the inside of the body wall.
Lift the skin and make a cut anteriorly to the throat and posteriorly to near the anus. (If you have a male,
cut to one side of the midline to avoid the penis.) Make lateral cuts to the inner thighs, across the bottom
of the rib cage, to the fore arm pits and up toward the ears from this central cut. (Shown in diagram
above.) Pour out any fluid from the body cavity and pin back the flaps of skin to your dissecting pan.
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Lab Exercise 4 – Comparative Vertebrate Structures 19
The empty space around the internal organs is the coelom or body cavity. It is divided into two sections,
the thoracic and abdominal cavities by a large sheet of muscle in a dome shape, the diaphragm.
Contraction of the diaphragm muscle creates negative pressure in the thoracic cavity and causes
inhalation of air into the lungs. Organs are suspended in the body cavity and attached to each other and
the internal body wall by thin bands of tissue called mesenteries. These also carry the blood vessels to
the organs.
Thoracic Cavity
The thoracic cavity contains the heart and lungs, which are protected inside the rib cage. You will have
to cut through the sternum and separate the rib cage to see the organs.
Respiratory System
Follow the trachea from the neck to where they branch into the two bronchi, leading into the lungs.
Notice that the lungs have several lobes.
Circulatory System
The heart is in the center of the thoracic cavity. It is encased in a thin membrane, the pericardium.
Tear the pericardium away and identify the 2 atria and ventricles of the heart. The remainder of the
circulatory system is the blood vessels and blood. Observe the major blood vessels of the
latex-injected display specimen, showing arteries in red and veins in blue. Notice that the largest
vessels are near the heart, and that they sequentially branch and split, getting smaller and smaller to
supply blood to every part of the body. Large vessels also lead directly to the kidneys and liver and
connect these organs to the intestine.
Abdominal Cavity
The abdominal cavity contains the digestive organs and the urogenital system, which consists of the
excretory and reproductive organs.
Digestive system
The most obvious organ in the abdominal cavity is the dark, multi-lobed liver just posterior to the
diaphragm. The liver secretes bile into the intestine for digestion of fats and also has many other
functions, including converting toxic nitrogen-containing molecules in the blood into urea for
excretion. Underneath the liver and to the rat’s left is the stomach, for food storage and digestion.
The esophagus connects the mouth to the stomach but is obscured by the organs of the thoracic
cavity. The spleen appears to be a flat, oval flap connected to the left side of the stomach (from the
rat’s perspective). The spleen functions in production and destruction of blood cells, including white
blood cells that are part of the immune system. As such, it is properly part of both the circulatory
system and the immune system. The small intestine appears to be a bundle of tubes filling the
middle of the abdominal cavity and is the site of further digestion and absorption of nutrients from
food. The pancreas is a diffuse region of whitish tissue at the junction of the stomach and small
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Lab Exercise 4 – Comparative Vertebrate Structures 20
intestine, which secretes digestive enzymes. The cecum is a large pouch off of the lower right side of
the small intestine, which contains bacteria that help the animal digest plant matter. The large
intestine or colon descends from the small intestine to the rectum, where feces are temporarily
stored and compacted before defecation via the anus. The large intestine is where water is
reabsorbed from the undigested food waste before it is defecated.
Excretory System
The main organ of the excretory system is the kidney. To find the paired kidneys, located in the
lower abdominal cavity near the spine, move the intestines to one side and gently cut through the
membrane lining the body cavity. The kidneys are actually outside the coelom. Trace the connection
to the urinary bladder, the ureter, and the connection from the bladder to the body wall, the
urethra.
Reproductive System
The female reproductive system consists of the ovaries, oviducts, uterus, and vagina. Trace the
system from the body wall, where the elastic vagina connects the reproductive system to the external
environment and functions in copulation and birthing. The uterus has two extensions or “horns”
(termed bicorn) and is where the embryos implant and develop. The oviducts connect the uterus to
the ovaries, which are embedded in fat in the dorsal wall of the body cavity and are where the eggs
are produced.
Begin your examination of the male reproductive system by cutting into the scrotum to expose the
testes, where sperm are made. The sperm is released into the epididymis, where it matures and is
stored. During ejaculation, sperm pass through the vas deferens to the penis. Along the way,
substances are secreted into the vas deferens by the dark, granular seminal vesicles (a fructose
solution to nourish sperm) and the white, lobed structure near the bladder, the prostate gland (a pH
buffer), to make the semen.
Exchange your rat with a lab group that has the other sex and observe the other kind of reproductive
structures.
Important Clean-Up and Disposal Procedures
1. When you are done, remove all pins and tools from your rat.
2. Place the animal and all body parts in the large, clearly labeled “Rat Disposal” bin.
3. Using ONLY the sink by the dissection materials, WASH and DRY all dissecting tools and pins,
and return them to the supply bins.
4. Rinse and dry the dissecting pans and return them to the side counter.
5. Throw paper towels and gloves away in the trash.
6. Wipe down your work area with the yellow cleaner in the spray bottles by the sinks.
7. Wash your hands with hot water and soap (blue squeeze bottles) before leaving the lab.
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Acknowledgements
This lab activity was based on material in:
Hickman, C. P., Jr. & L. B. Kats (2003) Laboratory Studies in Animal Diversity, 3rd ed. McGraw Hill,
NY.
Krogh, Richardson & Richardson (2005) A Laboratory Guide to the Natural World, 3rd ed. Benjamin
Cummings.
Smith, D. G. (2002) Exercises for the Zoology Laboratory, 2
nd ed. Morton Publishing Co., Englewood
Colorado.
Grossmont College Lab Manual, Spring 2021
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