Lecture #14
Phylum Chordata
Phylum Chordata
• only 45,000 species
• characteristics:
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1. bilaterally symmetrical
2. notochord
3. pharyngeal gill slits
4. dorsal, hollow nerve cord
5. post-anal tail
6. complete digestive system
7. thyroid gland
8. ventral, contractile heart
Numbers 2 – 5 may be in
a unique combination and
are found at some stage
in development
Chordates
Craniates
Vertebrates
Gnathostomes
Osteichthyans
Lobe-fins
Tetrapods
Amniotes
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Milk
Amniotic egg
Legs
Lung derivatives
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Lobed fins
Mineralized skeleton
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Jaws
Vertebral column
Head
Brain
Notochord
Ancestral deuterostome
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Chordate classification
characteristics:
Notochord? No  echinoderms
Brain? No  Urochordate
(tunicate)
Head/Cranial cavity? No 
Cephalochordate (lancelet)
Vertebral column? No  Hagfish
Jaws? No  Lampreys
Bony skeleton? No  Sharks,
Rays
Lobed fins? No?  Ray finned
fish
Lungs? lung derivatives? 
Coelocanth
Legs? No  Lungfish
Amniotic egg? No  Amphibian
Milk? No  Reptile
Phylum Chordata
• notochord:
– supportive rod that extends most of the animal’s
length – extends into the tail
– dorsal to the body cavity
– flexible to allow for bending but resists
compression
– composed of large, fluid-filled cells encased in a
fairly stiff fibrous tissue
– will become the vertebral column in many
chordates
Phylum Chordata
• dorsal, hollow nerve cord:
– runs along the length of the body – dorsal to the
notochord
– expands anteriorly as the brain
– develops from ectoderm
– BUT: in most vertebrates – nerve cord is solid and
is ventral to the vertebral column
Phylum Chordata
• pharyngeal gill slits:
– series of openings in the pharyngeal region of the
embryo
– develop as a series of pouches separated by
grooves
– in some embryos – grooves develop into slits
– used in primitive chordates for filter feeding
– in aquatic vertebrates – transformed these
slits/pouches into gills
– embryonic in terrestrial chordates
Phylum Chordata
• SubPhyla:
– Urochodata: sea squirts (tunicates)
• notochord, pharyngeal gill slits, and tail present in freeswimming larvae
– Cephalochordata: amphioxus
• all four chordate traits persist through life
– Hyperotreti: hagfishes
• jawless, no paired appendages
– Vertebrata: vertebrates
Subphylum Cephalochordata
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known as the lancelets
earliest diverging group of chordates
get their name (Lancelet) from their blade-like shape
embryos develop: a notochord, a dorsal, hollow nerve
cord, pharyngeal gill slits and a post-anal tail
• filter-feeders – cilia draw water into the mouth
• swim like fishes – chevron shaped muscles on either side
of the notochord
Muscle
segments
Notochord
Dorsal,
hollow
nerve cord
Brain
Mouth
Muscular,
post-anal tail
Anus
Pharyngeal
slits or clefts
Subphylum Urochordata
• tunicates
• embryonic/larval stage has the
characteristics of the chordate
• larva swims to a new substrate
and undergoes metamorphosis
– to form the adult tunicate
• retain the pharyngeal gill slits in
the adults
• water flows in through an
incurrent siphon - filtered by a
net of mucus on the pharyngeal
gill slits
Incurrent
siphon
to mouth
Excurrent
siphon
Atrium
Pharynx
with
numerous
slits
Tunic
Excurrent
siphon
Anus
Intestine
Esophagus
Stomach
Early Chordate Evolution
• research on lancelets had provided information on the
chordate brain
• the same Hox genes that organize the vertebrate brain
into forebrain, midbrain and hindbrain are found in
lancelets
• tunicates have also been sequenced – numerous
similarities with vertebrates BF1 Otx
Hox3
Nerve cord of lancelet embryo
BF1
Hox3
Otx
Brain of vertebrate embryo
(shown straightened)
Forebrain
Midbrain
Hindbrain
Craniates
• chordates with a head
• head – consists of a brain, surrounded by a skull,
and other sensory organs
• living craniates all share a series of unique
characteristics
• one characteristic is the development of neural
crest cells
• aquatic craniates possess pharyngeal gill slits not
clefts or pouches
• most basic craniate – hagfish
Neural Crest Cells
• collection of cells that form
as bilateral bands of cells
near the developing neural
tube
• migrate throughout the body
• major roles in forming the
skull
• also play roles in forming
many kinds of nervous cells
– certain neurons
– sensory structures
The cells give rise to some of the
anatomical structures unique to
vertebrates, including some of the bones
and cartilage of the skull.
Vertebrates
• branching off from the chordates involved innovations in
the nervous system and skeleton
– more extensive skull
– development of the vertebral column composed of vertebrae
• most vertebrates – vertebrae enclose a spinal cord (replaces the
notochord)
– development of fin rays in aquatic vertebrates
• adaptations in respiration and circulation
– more efficient gas exchange system – gills are modified
– more efficient heart
• adaptations in thermal regulation
– warm blooded vs. cold blooded
• adaptations in reproduction
– amniotic egg
– placental animals
Vertebrate Taxonomy
• most basal vertebrate – lamprey
– jawless
• development of jaws marked the
evolution of the gnathostomes
• development of lungs marked
the evolution of ray-finned
fishes
• development of lobed fins
marked the evolution of lobefinned fishes
• development of limbs marked
the development of amphibians
and reptiles
The Jaw
• lamprey have no jaws
• evolution of the jaw marked the
development of the gnathostomes
• gnathostomes have jaws that
evolved from skeletal supports of
the pharyngeal slits
• gnathostome characteristics:
– 1. hinged jaws with teeth
– 2. duplication of Hox genes – four
sets of Hox genes vs. one set in early
chordates
– 3. enlargened forebrain – with highly
developed sensory structures
– 4. lateral line system – in aquatic
gnathostomes
• for the detection of vibration
Gill slits
Cranium
Mouth
Skeletal rods
Vertebrates & Thermoregulation
• thermoregulation in vertebrates has two sources
• 1. internal metabolism – internal source of heat
• 2. external environment – external source of
heat
• animals can be classified based on the heat that influences
their body temperature
– ectotherms – animals whose body temperatures are determined by
external sources of heat – can also be considered to be poikilotherms
(variable body temperature)
– endotherms – animals whose body temperatures are determined by
internal sources of heat – usually also considered homeotherms
(constant body temperature)
Thermoregulation in Vertebrates
• when energy from food is transformed into ATP and ATP is
transferred into work – energy is lost in the form of heat
– seen in both ectotherms and endotherms
• endotherms produce more heat – cells are less efficient at
using energy vs. ectotherms
– endotherm cells are “Leaky” to ions
– endotherm must spend energy to keep its ionic “balance”
– this causes an increase in the production of heat via ATP hydrolysis
• ectotherms = e.g. amphibians & reptiles
• endotherms = e.g. mammals & birds
Thermoregulation in Vertebrates
• ectotherms regulate their body temperature through
behavioral mechanisms
– known as behavioral thermoregulation
• endotherms regulate their body temperature by
altering internal metabolic heat production
– can also use behavioral thermoregulation
Thermoregulation in Vertebrates
• ectotherms and endotherms can influence their body
temperature using 4 ways of heat exchange:
– 1. Radiation - heat transfer from a warmer medium to a cooler one
via the exchange of infrared radiation
– 2. Convection – heat transfer to a surrounding medium (e.g. air or
water) as it flows over a surface
– 3. Conduction – heat transfer directly between two objects
– 4. Evaporation – heat transfer away from a surface as water
evaporates
Balancing Heat Loss and Gain
• thermoregulation depends on the animal’s ability to
control the exchange of heat
• essence of thermoregulation is to maintain rates of heat
loss with equal rates of heat gain
• animals do this by either:
– reducing overall heat exchange
– favoring heat exchange in a particular direction
• many mechanisms involve the integumentary system
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1. Insulation – fat, feathers & fur
2. Circulatory Adaptations
3. Evaporative Heat Loss – sweating & panting
4. Behavioral Responses – hibernation, basking
5. Adjusting Metabolism
Circulatory Adaptations
• heat exchange between the internal environment
and the skin is through blood flow
• as body temp rises – blood flow to the skin increases
– heat in the blood is lost to the environment through the 4
methods described in the previous slide
• ectotherms and endotherms can use blood flow to
the skin to control their internal temperatures
Counter Current Exchange
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seen in many birds and mammals
transfer of heat between fluids flowing in opposite directions
same principle as the exchange of respiratory gases seen in fish
arteries and veins are adjacent to one another
warm blood moves from the body core into the arteries – transfers its heat
to the cooler blood leaving via the veins
• heat is exchanged along the entire length of these vessels – maximizes heat
exchange
• also keeps the heat localized to that specific body area
Evaporative Heat Loss
• endotherms must also be able to dissipate heat as environmental
temperatures rise
• 1. increase of blood flow to the skin
• 2. evaporation of moisture off the skin’s surface through
sweating or across the oral mucosa through panting
• BUT water falling from the body in the form of saliva or excess
sweat does not evaporate and does not cool the body
• thus, when the need for heat loss is greatest – excess sweating is
a waste of that water
• sweating and panting are also active processes and require
expending metabolic energy
– so a sweating animal generates heat when it needs to dissipate heat!!
Metabolic Heat Production
• heat production =
thermogenesis
• chemical energy is derived
from food
• nutrients from food are used
to generate ATP
• the production and use of
ATP generates heat
• the more ATP produced/used
– the more heat generated
• metabolic heat is used to
establish core body
temperature in endotherms
Physiologic Thermostats
• regulation of body temperature in
mammals is brought about by a
complex system based on feedback
mechanisms
• sensors for thermoregulation found
in the hypothalamus
• functions as a thermostat
• activates mechanisms that will
promote heat loss
– dilation of surface blood vessels
– production of sweat
• activates mechanisms that will
promote heat gain
– shivering heat production
Energy Allocation and Use
• bioenergetics = overall flow and transformation
of energy in an animal
– determines the animal’s nutritional needs
– ATP production for : cellular work + biosynthesis,
growth, storage and reproduction
• metabolic rate = sum of all the energy used in
biochemical reactions over a given time interval
– energy is measured in Joules or in calories/kilocalories
– 1 kilocalorie = 4,184 joules
– calorie use by nutritionists is actually a kilocalorie
Metabolic Rate
• physiologists can determine an animal’s metabolic
rate by
• 1. measuring its consumption of O2 (or production
of CO2)
• 2. measuring heat loss
– e.g. using a calorimeter
• 3. measuring the rate of food consumption and
waste production
– used over the long term
Metabolism
• within a narrow range of environmental
temps – the metabolic rate of an
endotherm is at a low level and
independent of external temperature =
thermoneutral zone
• the thermoneutral zone is bounded by an
upper and a lower critical environmental
temperature (UCT and LCT)
• when environmental temperature is
within the zone – the animal does not
need to expend much energy to regulate
its temp
– its thermoregulatory responses are passive
– e.g. fluffing fur, controlling blood flow to the skin
• but outside this zone – the animal must
expend metabolic energy
– thermoregulatory responses are active
– e.g. shivering and non-shivering heat production
Metabolism
• the metabolic rate of a resting endotherm in
the thermoneutral zone is called the basal
metabolic rate or BMR
• BMR is measured when the animal is quiet but
awake and not using energy for digestion,
reproduction or growth
• BMR = minimal amount of energy needed to
carry out minimal body functions
• Standard metabolic rate (SMR) is the
metabolic rate of an ectotherm at rest at a
specific temperature
Metabolism
• BMR correlates to body size – increased size = increased BMR
• BUT increased size = decreased BMR per gram of body tissue
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BMR of an elephant – 7,000 times greater than that of a mouse
BUT per gram tissue – mouse uses energy 15X faster than the elephant
why?
as an animal increases in size – its surface area to volume decreases
heat dissipation relies on surface area
theorized that larger animals have decreased BMRs per gram tissue to avoid
overheating
Metabolic Adaptations
• when environmental temps fall below the
lower critical level of the TZ – endotherms
must produce heat
• thermogenesis can be through:
– 1. shivering heat production
– 2. non-shivering heat production
• most non-shivering heat production- occurs in
specialized adipose tissue called brown fat
– high numbers of mitochondria and blood vessels
Metabolic Adaptations
• in the mitochondria: ATP production is uncoupled from
metabolic fuel consumption – yet heat is produced (heat is
produced rather than ATP)
• brown fat is prevalent in newborn humans – decreases in
adulthood
– metabolic activity can be stimulated upon cold exposure
– less brown fat activity in obese individuals
• also present in large amounts in certain animals
– cold weather animals
– animals that hibernate
• other adaptations have evolved to help endotherms retain
heat
– thick layers of fur, feathers or fat
– ability to decrease blood flow to the skin
– counter-current exchange of heat in the appendages of many animals
Metabolic Adaptations
• regulated hypothermia can also be used – by many birds and
mammals
• hummingbirds – high metabolic rate – drop their body temps
by 10 to 20C when they are inactive
– lowers their metabolic rate and conserves energy
– called daily torpor
• regulated hypothermia that lasts for days or weeks =
hibernation
– metabolic rate needed for hibernation may be 1/50th of
the animal’s BMR
– many animals can maintain body temp’s close to freezing!
– arousal from hibernation requires the hypothalamus to
reset the body’s internal thermostat
Thermoregulation in Ectotherms
– amphibians
• assume the temperature of the water when
submerged
• on land – the body temp can differ from the
environment
• cooling - evaporative loss across the thin skin
• warming –radiation from the sun & from from
warm surfaces
• to prevent overheating – many amphibians are
nocturnal or will hide in shady areas
Thermoregulation in Ectotherms
– reptiles: wide variety of behaviors
• seen best in the lizards - radiation
– to heat up – bask perpendicular to sun’s rays
– to cool down – parallel to rays
• some reptiles can pant to regulate temp - heat
loss through evaporative cooling across the
mouth
• some reptiles can increased body temp through
increased metabolism – brooding snakes curl
around their eggs
• many reptiles endure cold temps by
“hibernating” in large groups
Thermoregulation in Ectotherms
– fish
• rare in most fish - most assume the temperature of the
surrounding water
• two kinds of thermoregulation strategies
• 1. “hot” fish – tuna, mackarel, sharks
– blood is oxygenated at gills – most of this cold blood is moved to the
body via arteries under the skin
– cool blood flows into muscles and cool blood flows out of muscles
– blood in the arteries of the muscles is warmed by the warm venous
blood– counter- current exchange
– the blood that returns to the heart via veins under the skin is cool
– SO: this strategy keeps the heat within the muscle mass
• 2. “cold” fish – most fish species
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blood is oxygenated and cooled to seawater temperature at the gills
cold blood is carried into the tissues via a large aorta
blood is warmed by the metabolism of muscles
blood flowing out of muscles is warm
veins return warmed blood to the heart
pumped to the gills – re-cooled
Thermoregulation in Ectotherms
– fish
• rare in most fish - most assume the temperature of the
surrounding water
• two kinds of thermoregulation strategies
• 1. “hot” fish – tuna, mackeral, sharks
– blood is oxygenated at gills – most of this cold blood is moved to the
body via arteries under the skin
– cool blood flows into muscles and cool blood flows out of muscles is
warmed by the venous blood flowing out of the muscle – countercurrent exchange
– so blood that returns to the heart via veins under the skin is cool
– SO: this strategy keeps the heat within the muscle mass
• 2. “cold” fish – most fish species
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blood is oxygenated and cooled to seawater temperature at the gills
cold blood is carried into the tissues via a large aorta
veins return warmed blood to the heart
blood is warmed by the metabolism of muscles
pumped to the gills – re-cooled