Here are definitions of the key terms: Chordate - An animal belonging to the phylum Chordata, which includes all animals with a notochord or backbone at some stage of their lives. Notochord - A flexible rod of cells that forms the basic structure along the back in early embryonic chordates. It is replaced by the vertebral column in most vertebrates. Postanal tail - The muscular structure behind the anus that is present in early chordate embryos. Nerve cord - A tube-like structure above the notochord that develops into the central nervous system. Pharyngeal pouch - Sac-like projections in the throat region that filter feed in some

1. Chordates and Vertebrates

2. Introduction to Chordates
Chordates are animals that have four structures present at
some stage of their development.
postanal tail
pharyngeal
pouches or
“gill slits”
nerve cord
notochord
cat embryo at 2 weeks
Animals in this group include tunicates, lancets, fish,
amphibians, reptiles, birds, and mammals.

3. All vertebrate embryos look very similar too each other
because they are all chordates.
Note that all of these chordates have: a notochord,
postanal tail, nerve cord, and pharyngeal pouches.

4. Notochord
All chordates have an internal notochord that supports the
animal and extends along the upper part of the body (what
we think of as the back.
- The notochord is a firm but flexible structure made up of fluid-
filled cells that are enclosed in a stiff covering.
- The notochord extends into the postanal tail which is a
muscular structure at the end of the developing chordate.
- In most chordates (fish, amphibians, reptiles, birds and
mammals) the notochord gets replaced by the back bones or
vertebrae
- Other chordates such as the tunicate and the lancet, the
notochord is kept into adulthood.

5. Tunicates and Lancets
Lancets a.k.a. amphioxus
are primitive eel-like
chordates that spend most
Tunicates a.k.a. sea squirts are of their time buried in sand.
sessile marine chordates. They contain typical
As larvae they are free chordate features.
swimming and contain Both tunicates and lancets
notochord, nerve cord, and
other chordate features. are invertebrates.

6. Nerve Cord
Above the notochord is a tube-like structure called the
nerve cord.
- As most chordates develop, the
nerve cord enlarges and becomes
the central nervous system.
- The front end of the nerve cord
enlarges and becomes the brain. nerve cord
- The remainder of the nerve nerve cord
cord becomes the spinal cord.

7. Pharyngeal Pouches
Pharyngeal pouches are sac-like projections found in the
region between the mouth and digestive tube.
- Many chordates have several pairs
of pharyngeal pouches.
- Ancient invertebrate chordates
such as tunicates and lancets
use them for filter feeding.
- In fish pharyngeal pouches
become gills
Pharyngeal
- In humans pharyngeal pouches pouches
are present only during
embryonic development.

8. Vertebrates
Vertebrates make up the largest group of animals in the
chordate phylum. They include jawless fish, jawed
cartilaginous fish, bony fish, amphibians, reptiles,
birds, and mammals.
Vertebrates contain all chordate characteristics plus …
- endoskeleton made up of bones and cartilage
- backbones (vertebrae) that protects the spinal cord
- skull that protects the brain
Because vertebrates are animals, they also have
all the characteristics of animals that we learned
earlier.

9. Ectotherms and Endotherms
Vertebrates can be divided as ectotherms (cold-blooded)
or as endotherms (warm-blooded).
Ectotherms include Endotherms include
fish, amphibians, and birds and mammals.
reptiles.
- Their internal body - Their core body
temperature changes temperature is kept
with the temperature of nearly constant
its surroundings. through thermal
homeostasis.

10. Origin of Chordates
Chordates are believed to have descended from
echinoderm-like ancestors during the Precambrian period.

11. Homework
Define the words: chordate, notochord, postanal tail,
nerve cord, pharyngeal pouch, endoskeleton, cartilage,
vertebrae, ectotherm, endotherm