1. Phylum Chordata: Class Amphibia & Class Reptilia 17.1
Lab #17 -- Biological Sciences 102 – Animal Biology
Evolution of Amphibians
The evolution of characteristics that allow animals to survive in a variety of terrestrial
habitats will be an important topic for the remaining vertebrate groups we will discuss.
Amphibians were the first tetrapods to evolve from fish ancestors during the
late Devonian period, some 360-370 million years ago. The name
amphibian, suits its nature - being derived from the Greek amphibios
meaning "a double life". Despite this distinction, however, some species are
confined to the land, while other species have a completely aquatic existence
Amphibians most likely evolved from either the lobe-fin fishes
(Crossopterygii) or the lungfishes (Dipnoi) of the Early Devonian Period
(lasting from 408 to 387 million years ago). These fishes had an advantage
over other fishes by the fact that they had lungs. In time of shortage of
water, these fishes probably came out of the pools which were drying out on
their muscular fins to search for other water. They might have added insects
and other small arthropods to their diet while moving and over time became
less dependent on water. A caecilian fossil from the Paleocene Epoch (66.4
to 57.8 million years ago) has been found in Brazil.
In water, there is little problem with support. However, on land, support
structures must be modified to allow adequate support of the body, to prevent lungs from
collapsing under the weight of the body, and to permit locomotion.
The first land animals included the labyrinthodont amphibians, in which the pectoral and
pelvic fins were modified into short, powerful legs. The vertebral column was strengthened by
the development of interlocking processes and the replacement of the notochord by bony
ringlike vertebrae.
Gills were replaced by lungs, which evolved from the swim bladder, an organ to control
buoyancy. Tear glands prevent drying of eyes. The terrestrial ear may have evolved from
portions of the lateral line structure of fish.
The oldest known land vertebrates, the ichthyostegids, retained a tail fin and a lateral line
system, suggesting that they spent most of their time in water.
Fifty million years after the first appearance of amphibians, a major breakthrough in
vertebrate
evolution occurred: a group of small labyrinthodont amphibians evolved the ability to lay their
eggs on dry land, away from water. The early reptiles developed a lighter, more flexible body
covering consisting of the protein keratin, rather than the bony scales of their amphibian and
fish ancestors.
Although a few amphibian species live their lives in water, most live a portion of their lifecycle
on land. They are distributed worldwide, but the majority is found in the tropical regions of the
Earth. Most amphibians have an aquatic larval, or tadpole, stage that metamorphoses into an
adult.
Only one of the 3 major groups of ancient amphibians evolved into the modern salamanders,
frogs, toads, and caecilians. Today amphibians have invaded almost every environment where
fresh water is available for some period of time. Some toads survive in deserts in underground
burrows and only emerge during the occasional monsoon rains that fill temporary ponds.
Ç
Phylum Chordata: Class Amphibia & Class Reptilia 17.2
Lab #17 -- Biological Sciences 102 – Animal Biology
Amphibians have many unique characteristics that distinguish them from other
vertebrate classes. Amphibian skin is glandular and lacks scales, feathers, or hairs. The

2. skin is also permeable allowing for water and gas exchange. This restricts amphibian
activity to wet and/or humid conditions, but this trait also allows amphibians to respire
and absorb water through the skin from the surrounding environment. This is only one
of the methods of amphibian breathing. Respiration is also accomplished by the use of
lungs, gills, and buccal pumping, where the animals swallow air and force it into the
lungs (frogs are positive pressure breathers). The amphibian egg is also a distinctive
feature. It is shell-less and lacks the embryonic membranes of the amniotic egg of the
other tetraopod classes, therefore development is restricted to a moist environment.
Go to the following websites for more information about Acanthostega and Ichthyostega
which
are two different intermediate fossil forms between fish and amphibians.:
http://tolweb.org/Acanthostega
http://www.devoniantimes.org/Order/re-ichthyostega.html
Classification of Amphibians
Class Amphibia
Order Gymnophiona (jim'no-fy'o-na) (Gr. gym nos, naked, +
ophioneos, of a snake) (Apoda): caecilians. Body elongate;
limbs and limb girdle absent; mesodermal scales present in
skin of some; tail short or absent; 95 to 285 vertebrae;
pantropical, 5 families, 33 genera, approximately 160
species.
Order Urodela (yur'uh-del'uh) (Gr. oura, tail
+ delos, evident) (Caudata): salamanders.
Body with head, trunk, and tail; no scales;
usually two pairs of equal limbs; 10 to 60
vertebrae; predominantly holarctic; 10 living
families, 61 genera, approximately 500
species.
Order Anura (uh-nur'uh) (Gr. an, without, + oura, tail)
(Salientia): frogs, toads. Head and trunk fused; no tail; no
scales; two pairs of limbs; large mouth; lungs; 6 to 10
vertebrae induding urostyle (coccyx); cosmopolitan,
predominantly tropical; 29 families; 352 genera;
approximately 4840 species. Phylum Chordata: Class Amphibia & Class Reptilia 17.3
Lab #17 -- Biological Sciences 102 – Animal Biology
Classification of Reptiles
Class Reptilia
Subclass Anapsida (a-nap'se-duh) (Gr. an, without, + apsis, arch): anapsids. Amniotes having
some primitive features, such as a skull with no temporal opening.
Order Testudines (tes-tu'din-eez) (L. testudo, tortoise) (Chelonia): turtles. Body in a bony
case of dorsal carapace and ventral plastron; jaws with keratinized beaks instead of teeth;
vertebrae and ribs fused to overlying carapace; tongue not extensible; neck usually
retractable; approximately 300 species.
Subclass Diapsida (di-ap'se-duh) (Gr. di, double, + apsis, arch): diapsids. Amniotes having a
skull with two temporal openings.
Superorder Lepidosauria (lep-i-do-sor' ee-uh) (Gr. lepidos, scale, + sauros, lizard). Diapsid
lineage appearing in the Triassic; characterized by sprawling posture; no bipedal
specializations; diapsid skull often modified by loss of one or both temporal arches;
transverse cloacal slit, skin shed in one piece.
Order Squamata (skwa-ma'ta) (L. squamatus, scaly, + ata, characterized by): snakes,
lizards, amphisbaenians. Skin of horny epidermal scales or plates, which is shed;

3. quadrate movable; skull kinetic (except amphisbaenians); vertebrae usually concave in
front; paired copulatory organs.
Suborder Lacertilia (lay-sur-till' ee-uh) (L. lacerta, lizard) (Sauria): lizards. Body
slender, usually with four limbs; rami of lower jaw fused; eyelids movable; external ear
present; this paraphyletic suborder contains approximately 4600 species.
Suborder Amphisbaenia (am'fis-bee'nee-a) (L. amphis, double, + baina, to walk): worm
lizards. Body elongate and of nearly uniform diameter; no legs (except one genus with
short front legs); skull bones interlocked for burrowing (not kinetic); limb girdles
vestigial; eyes hidden beneath skin; only one lung; approximately 160 species.
Suborder Serpentes (sur-pen'tes) (L. serpere, to creep): snakes. Body elongate; limbs,
ear openings, and middle ear absent; mandibles joined anteriorly by ligaments; eyelids
fused into transparent spectacle; tongue forked and protrusible; left lung reduced or
absent; approximately 2900 species.
Order Sphenodonta (sfen'o-don'tuh) (Gr. sphen, wedge, + odontos, tooth): tuatara
(Rhynchocephalia). Primitive diapsid skull; vertebrae biconcave; quadrate immovable;
parietal eye present; two extant species in the genus Sphenodon
Phylum Chordata: Class Amphibia & Class Reptilia 17.4
Lab #17 -- Biological Sciences 102 – Animal Biology
LAB PROCEDURE
NAME: LAB SCORE:
You must answer ALL questions in the lab procedure for full credit.
Finish them at home if you do not have time to complete them in lab.
What is a tetrapod?
Refer to textbook and Internet for diagrams, illustrations and reference text for this lab.
Specifically, pay close attention to the frog dissection photos on the course website.
Answer the following questions using supplementary lab atlas, textbook, instructor
feedback, the Internet and collective discussions with your classmates.
Class Amphibia: Frogs, Toads, Salamanders and Caecilians
¾ In the space below, briefly describe some important characteristics of this Class of
vertebrates.
Your instructor will briefly review with you the basic lifecycle of a typical frog.
¾ List a few characteristics that differentiate frogs from toads.
¾ With regard to the musculoskeletal system, how does a salamander differ from a frog?
¾ Which order of amphibians is most diverse? Provide an hypothesis for why this group is
the most diverse order? Phylum Chordata: Class Amphibia & Class Reptilia 17.5
Lab #17 -- Biological Sciences 102 – Animal Biology
FROG DISSECTION
Class Amphibia
Order Anura (Salientia)
Family Ranidae
Genus Rana (or similar genus)
AFTER YOU HAVE COMPLETED THE CARDIAC PHYSIOLOGY PORTION OF THE
LAB ON PAGES 17.10 TO 17.13 THEN YOU MAY COMPLETE THE DISSECTION
TO OBSERVE THE STRUCTURES LISTED BELOW. All students at each lab table
should observe one live frog. You need not write out your observations, but you
should think about the importance of the various frog adaptations you observe.
Your instructor will double-pith a frog for your dissection which you will do in
groups of 3 or 4 students.

4. External Structures (entire frog)
¾ head
¾ trunk
¾ sacral hump
¾ cloacal opening
¾ forelimbs (how many digits on each forefoot/hand?)
¾ hindlimbs (how many digits on each hindfoot?)
¾ eyes
¾ nictating membrane
¾ tympanic membrane
¾ external naris (nares)
Skeletal Structures
(on frog skeleton and diagram in lab)
¾ skull
¾ axial skeleton
¾ appendicular skeleton
¾ nasal fossa
¾ orbital fossa
¾ auditory capsule
¾ maxilla
¾ dentary
(middle portion of each side of the mandible)
¾ vertebrae (ending in urostyle)
¾ suprascapula and scapula
¾ clavicle
¾ humerus
¾ radioulna
¾ carpals
¾ metacarpals
¾ pelvis (urostyle, ischium and ilia;
pubis is difficult to see)
¾ femur
¾ tibiofibula
¾ calcaneus (with astragalus = tarsals)
¾ metatarsals
¾ phalanges (on all forefeet and hindfeet) Phylum Chordata: Class Amphibia & Class Reptilia
17.6
Lab #17 -- Biological Sciences 102 – Animal Biology
Muscles (see lab diagrams - can be removed from the frog once identified)
¾ pectoralis major (ventral chest)
¾ rectus abdominis (ventral abdomen)
¾ external oblique (abdomen)
¾ sartorius (ventral thigh)
¾ gracilis major (ventral thigh)
¾ adductor magnus (ventral thigh)
¾ gastrocnemius (leg/shank) & Achilles tendon
¾ latissimus dorsi (dorsal thorax)
¾ triceps femoris (dorsal thigh)
¾ biceps femoris (dorsal thigh)
Head - Internal Structures
¾ internal naris (nares)
¾ teeth

5. ¾ tongue
¾ opening to esophagus
Thoracic & Abdominal Organs – Internal Structures
¾ liver
¾ gallbladder
¾ stomach
¾ small intestines (supported by mesenteries)
¾ large intestine
¾ pancreas (as visible)
¾ spleen (as visible)
¾ mesonephric kidneys
¾ urinary bladder
¾ cloaca
¾ gonads (ovary or testes)
¾ oviducts (female)
¾ lungs
¾ heart
¾ pericardial sac (around heart)
¾ atria of heart
¾ ventricle of heart
¾ dorsal aorta (as visible)
¾ posterior vena cava (as visible)
¾ remove heart to view sinus venosus and interior of heart chambers
Phylum Chordata: Class Amphibia & Class Reptilia 17.10
Lab #17 -- Biological Sciences 102 – Animal Biology
¾ VERTEBRATE CARDIAC PHYSIOLOGY
The heart undergoes a constant cycle of contractions and relaxations called the cardiac cycle.
The period of ventricular contraction is called systole, while the period of ventricular relaxation
is called diastole. Diastole begins as the ventricles start to relax. Soon the pressures within the
aorta and pulmonary artery exceed ventricular pressures, causing the semilunar valves to
close. As the ventricular pressure falls below the atrial pressure the AV valves open and the
ventricles fill with blood. The ventricles fill to about 80% of capacity prior to contraction of the
atria, the last event in diastole. As the ventricles start to contract, the ventricular pressure
soon exceeds the atrial pressure, causing the AV valves to close. As the ventricles continue to
contract, the ventricular pressure exceeds the arterial pressures causing the semilunar valves
to open. Blood is forcefully ejected out of the ventricles and into the aorta and pulmonary
artery.
The heart is autorhythmic, meaning it generates its own rhythmic action potentials
independent of the nervous system. The rhythmic beating of the heart is controlled by a small
group of cells in the wall of the right atrium, collectively called the sinoatrial node (SA node).
Since the SA node controls heart rate, it is called the pacemaker of the heart. The electrical
impulse that arises in the SA node travels through gap junctions to the atrial myocardium
and then to the atrioventricular (AV) node. The impulse is delayed slightly in the AV node (AV
nodal delay) before traveling into the bundle of His. The impulse then travels through the
bundle branches, into the purkinje fibers and terminates in the ventricular myocardium
where it stimulates muscle contraction. The specialized structures that conduct the electrical
impulse through the heart are collectively called the conduction system.
¾ Draw a diagram of the conduction system within the heart and label its parts.
Phylum Chordata: Class Amphibia & Class Reptilia 17.11

6. Lab #17 -- Biological Sciences 102 – Animal Biology
¾ Drugs that Effect the Vertebrate Heart
Both branches of the autonomic nervous system enervate the SA node and modify its activity.
Sympathetic input (norepinephrine and epinephrine) increases heart rate, while
parasympathetic input (acetylcholine) decreases heart rate. Also, certain drugs can alter the
rate and strength of cardiac contraction. In today’s experiment you will record a frog’s cardiac
cycle on the Biopac equipment using an exposed frog heart. You will also test the effects of the
following drugs on heart rate.
Epinephrine: Epinephrine is a hormone secreted by the adrenal medulla. Epinephrine works
in concert with norepinephrine (a neurotransmitter secreted by the sympathetic nervous
system) to increase both the strength and rate of cardiac contractions.
Pilocarpine: Pilocarpine stimulates the release of acetylcholine from parasympathetic neurons
that innervate the SA node, thus causing a decrease in heart rate
Acetylcholine: Acetylcholine is a neurotransmitter secreted by the parasympathetic nervous
system at the SA node of the heart. It causes a decrease in cardiac rate.
Atropine: Atropine is a competitive inhibitor of acetylcholine. Atropine outcompetes
acetylcholine for binding to the acetylcholine receptor. Atropine blocks the effects of
acetylcholine, thus inhibiting the parasympathetic activity of the heart.
Eserine: Eserine is an inhibitor of Acetylcholinesterase, the enzyme responsible for the
breakdown of acetylcholine in the synaptic cleft of the neuromuscular junction.
Potassium Chloride (KCl): The initiation and propagation of action potentials in the heart is
dependent, in part, on the steep K+ concentration gradient between the ICF and ECF (higher
concentration of K+ on the inside of the cell than on the outside). An increase in extracellular
K+ above normal (hyperkalemia) disrupts this gradient, causing a decrease in heart rate and
contractility. In extreme hyperkalemia, the conduction rate of action potentials may be so
depressed that ectopic pacemakers appear in the ventricles and fibrillation may develop, often
with fatal results
http://www.biosciweb.net/animal/pdf/pdflabs%20spr11/17ampreplab17.pdf