Name: ___________
Date:_________
Biology 11 Enriched: Chordates (Chapter 34)
Domain:___________
Kingdom:____________
What are chordates?
Biology 11 Enriched: Chordates
Page 1
The diagram below illustrates the characteristics common to all chordates:
Write a note about each of the four characteristics.
1. Notochord:
Dorsal cartilage rod providing support.
Located between the gut and nerve cord.
In vertebrates has become bony and surrounds the nerve cord.
2. Dorsal hollow nerve cord:
-Develops from a section of the embryo rolling into a tube.
-Unique to chordates: Nerve Cords in Annelids/Arthropods are ventral and solid.
3. Pharyngeal slits:
-Found in larval, but not all adult forms.
-Allow water to pass over gills without passing through digestive tract.
-Modified for suspension feeding, gas exchange (gill slits), ear/head/neck structures.
4. Post-anal tail:
-Often only present in embryo/larval stages.
Biology 11 Enriched: Chordates
Page 2
Below is a summary of chordate phylogeny. Why are the chordates grouped close to the
echinoderms?
-They are both deuterostomes
Biology 11 Enriched: Chordates
Page 3
Invertebrate Chordates
Sea Squirts (Sub-Phylum Urochordata)
In what stage do sea squirts (tunicates) have all the characteristics of a chordate?
-Larval and is motile. Lack post anal tail, notochord, nerve cord as adults and are sessile.
Adult
Adult
Larval
Larval pharyngeal slits become a basket for filter feeding in the adult. Sea water inters
through incurrent siphon and passes through the pharyngeal slits into the main body
cavity. Food particles are trapped in an net of mucous and digested. Water is pushed out
of the excurrent siphon.
Biology 11 Enriched: Chordates
Page 4
Lancelets (Sub Phylum Cephalochordate)
Lancelets (Amphioxus) are invertebrate chordates that retain all of the chordate
characteristics as adults.
-Lancelets are very simply invertebrate chordates living in the sands in estuary areas.
Their head sticks of the sand and a net of mucous secreted over the pharyngeal gills traps
plankton. The water that enters the gill slits exits through an opening in front of the anus.
-Molecular evidence indicates that lancelets are the closest living non-vertebrate chordate
relative of vertebrates. This is also demonstrated by similarity in the genes controlling
lancelet and vertebrate
embryo development:
-Hox genes control
the development of
the nervous system in
both the lancelet and
in vertebrate
embryos.
Biology 11 Enriched: Chordates
Page 5
Craniates
-The next major development in chordate
evolution was the evolution of Craniates,
organisms with a brain enclosed by a cranium.
-Craniate embryos also produce neural crest cells
that migrate around the embryo to give rise to
various adult structures, such as:
-Reproductive organs and their related structures,
teeth, bones, inner skin layer of the face, eyes and other sensory organs.
Other features of craniates include:
-A high metabolic rate, extensive muscle system, heart with at least 2 chambers, red
blood cells with hemoglobin and kidneys.
The evolution of craniates is preserved in the fossil record:
Biology 11 Enriched: Chordates
Page 6
Hagfish
-Hagfish are jawless, invertebrate
craniates that have skulls made of
cartilage.
-What provides support to the hagfish’s
body and permits movement if it lacks
vertebrae?
-Cartilage notochord. Segmented muscles contract against it.
-Hagfish are scavangers and feed by creating a hole
in their prey using tooth-like structures made of
keratin. They can also secrete slime to deter
competitors when feeding and as a defensive
mechanism.
Vertebrates
Vertebrates are craniates that possess backbones of
cartilage or bone. As you will see, this allowed organisms to capture food and
avoid being eaten more efficiently.
Lampreys
-Lampreys are the oldest lineage of vertebrate, as indicated by their being jawless
and having cartilaginous vertebrae.
-Lampreys are parasitic and use their rasping tongues to penetrate their prey’s skin
and feed on its blood and tissues.
-Lamprey larva resemble lancelets, showing evolutionary links between chordate groups.
Biology 11 Enriched: Chordates
Page 7
Gnathostomes (Jawed Vertebrates)
-Gnathostomes are vertebrates with jaws. Below is a diagram illustrating how these
structures are thought to have developed from gill arches:
-Gill arches are skeletal support for gill slits. They became jays and the remaining gill
slits were modified for gas exchange as they were no longer necessary for suspension
feeding.
-What are some benefits to having jaws?
-Can eat a wider variety of, and larger, prey as can now tear them into bite sized pieces.
Class Chondrichtyes (Cartilaginous Fish)
-Sharks, rays and skates are cartilaginous fish with jaws. Having a cartilaginous skeleton
is a derived characteristic, so it is important not to think that this group was a precursor to
bony fish.
Biology 11 Enriched: Chordates
Page 8
Class Osteichthyes (Bony Fish)
-Fish with ossified endoskeletons. They also possess a swim bladder for buoyancy and a
two-chambered heart:
-Most fish with which you are familiar are ray-finned fish, which have bony supporting
rays in their fins.
Biology 11 Enriched: Chordates
Page 9
Tetrapods
-Tetrapods are jawed vertebrates with legs and feet that can support their weight on land.
-Land-dwelling tetrapods are thought to have evolved from lobe-finned fish like the
Coelacanth.
-Other modern fish also
show adaptations to
living on land, such as
the Lungfish.
-The diagram right illustrates
the evolution of the tetrapod
lineage. Notice the evolution
and modification of
homologous bones.
Biology 11 Enriched: Chordates
Page 10
-Early tetrapods would have looked something like Tiktaalik (375 myo), a lobe-finned,
four legged organism possessing a mixture of fish and tetrapod characteristics.
Amphibians
Amphibians are tetrapods that are still tied to the water for at least part
of their life-cycle. Why is this?
-Need to reproduce in water as have external fertilization and eggs
would desiccate on land.
Biology 11 Enriched: Chordates
Page 11
Amniotes
Amniotes are vertebrates with adaptations for reproducing and living on land. They
include reptiles, birds and mammals:
Biology 11 Enriched: Chordates
Page 12
This diagram shows the features of the amniotic egg, which allows amniotes to reproduce
on land:
*Make sure that you know that names and functions of the extraembryonic membranes
above!
-What function does the leathery shell have?
-Prevents mechanical damage and desiccation.
Biology 11 Enriched: Chordates
Page 13
Reptiles
-Reptiles were the first organisms to live completely on land. What adaptations do they
have that permitted this?
-Internal fertilization, amniotic egg, scales to prevent desiccation, lungs.
-Rely on their environment for body temperature (ectothermic) rather than metabolism,
which allows them to consume 10% less food energy than a mammal of the same size.
Biology 11 Enriched: Chordates
Page 14
Birds
-Birds evolved from
reptiles and have adaptions
for flight. What are some
adaptations that you know?
-Adaptations to reduce
weight: Lack teeth, reduced
tail vertebrae, hollow
shafted feathers,
hollow/honeycombed
bones.
-Keel-like breast bone for attachment of large breast muscles necessary for flight.
-Air foil wing.
-Warm blooded (endothermic) with a 4 chambered heart as flight is energetically
expensive.
-Archaeopteryx is the earliest known bird and it possesses a mixture of reptile and avian
characteristics.
-New techniques are allowing researchers to discover the colour of Archaeopteryx’s
feathers!
Biology 11 Enriched: Chordates
Page 15
Mammals
-Mammals are tetrapods that have hair and milk. There are several groups of mammals:
1. Monotremes (Egg-Laying Mammals)
-Are most closely related to the
mammal common ancestor and include
four Echidna species and one Platypus
species.
-Monotremes lay hard-shelled eggs,
and secrete milk from glands on the
mother’s belly (no nipples).
2. Marsupials (Pouched Mammals)
-Marsupials and Euthernians have shared-derived characteristics not found in
monotremes, such as: a) Higher metabolic rates; b) Nipples that provide milk; c) Embryo
develops inside the uterus; d) The uterus lining and extraembryonic membranes form a
placenta.
-Marsupials give birth to under-developed live young that complete their development
while nursing in the marsupium (pouch).
Biology 11 Enriched: Chordates
Page 16
3. Eutherians (Placental Mammals)
-Eutherians have longer pregnancies than Marsupials as the young complete their
embryonic development in the uterus before being born.
-Primate is the Eutherian order to which we belong.
Biology 11 Enriched: Chordates
Page 17
Here are the major orders of mammals:
Biology 11 Enriched: Chordates
Page 18
Humans are in the primate order:
Biology 11 Enriched: Chordates
Page 19
Circulation
In vertebrates, the heart is either two-chambered, three-chambered, or four-chambered:
http://www.hhmi.org/biointeractive/circulatorium/key.html The closed cicrulatory system
in vertebrates is often called the cardiovascular system.
Comment on the vertebrate cardiovascular system.
Comment on each of these forms of double circulation.
Biology 11 Enriched: Chordates
Page 20
Biology 11 Enriched: Chordates
Page 21
Gas exchange
Efficiency of gas exchange in the gills of fish is increased through a ‘counter-current
exchange’ mechanism.
The arrangement of blood capillaries in fish gills and the flow of water over the gills allow
countercurrent exchange, maximizing the efficiency of gas exchange.
o Blood flows in the opposite direction to the water flowing over the gills.
o As blood enters a gill capillary, it meets water that has already passed over the gill.
o Although it has lost much of its dissolved oxygen, this water still has a higher PO2
(percentage dissolved oxygen) than incoming blood, and oxygen is exchanged from
water to blood.
o As blood moves over the gill, its PO2 increases, but so does the PO2 of the water it
encounters.
o A partial pressure gradient favors the diffusion of oxygen from water to blood along
the length of the capillary.
Countercurrent exchange mechanisms enable fishes to remove more than 80% of the
oxygen dissolved in water.



Among the vertebrates, amphibians have relatively small lungs that do not provide a
large surface area, and many lack lungs altogether.
Amphibians rely heavily on diffusion across other body surfaces, especially their moist
skin, for gas exchange.
In contrast, most reptiles (including all birds) and all mammals rely entirely on lungs for
gas exchange.
o Turtles may supplement lung breathing with gas exchange across moist epithelial
surfaces in their mouth and anus.
Biology 11 Enriched: Chordates
Page 22
o Lungs and air breathing have evolved in a few fish species (lungfishes) as adaptations
to living in oxygen-poor water or to spending time exposed to air.
 In general, the size and
complexity of lungs are correlated
with an animal’s metabolic rate (and
hence rate of gas exchange).
o For example, the lungs of
endotherms have a larger area of
exchange surface than the lungs of
similar-sized ectotherms.
Biology 11 Enriched: Chordates
Page 23