Lecture #14
Phylum Chordata: The vertebrate
Phylum
Phylum Chordata
• only 45,000 species
• characteristics:
–
–
–
–
–
–
–
–
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 1 – 5 may be in
a unique combination and
are found at some stage
in development
Chordates
Craniates
Vertebral column
Head
Brain
Notochord
Ancestral deuterostome
• Chordate classification
characteristics:
• Notochord? No  Echinoderms
• Notochord? Yes  keep
evolving
• Brain? No  Urochordate
(tunicate)
• Brain? Yes  keep evolving
• Head/Cranial cavity? No 
Cephalochordate (lancelet)
• Cranial cavity? Yes  keep
evolving
• Vertebral column? No 
Hagfish
• Vertebral column? Yes 
Lampreys & keep evolving
baby!!!
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
•
•
•
•
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
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
• most basic craniate – hagfish
https://www.youtube.com/watch?v=t5PGZRxh
AyU
Vertebrates
• branching off from the primitive chordates involved innovations in the
nervous system and skeleton
– vertebraes have a 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
– development of limbs in terrestrial vertebrates
• adaptations in respiration and circulation
– more efficient gas exchange system – gills are modified in aquatic
vertebrates; lungs in the terrestrial vertebrates
– more efficient heart – 2 to 4 chambered
• 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 ray-finned
fishes
lungs marked the evolution
• development of lobed fins
marked the evolution of lobefinned fishes
• development of limbs marked
the development of amphibians
and reptiles
• development of mammary
glands marked the development
of mammals
• Vertebrate classification
requirements:
• Vertebral column? No 
Hagfish
• Jaws? No  Lampreys
• Bony skeleton? No  Sharks,
Milk
Rays
Amniotic egg •
Lobed fins? No?  Ray finned
Legs
fish
Lung derivatives
• Lung derivatives? No
Lobed fins
Coelocanth
Mineralized skeleton
• Legs? No  Lungfish
Jaws
• Amniotic egg? No 
Vertebral column
Amphibian
Head
• Milk? No  Reptile
Brain
• What’s left??? MAMMALS!!
Chordates
Craniates
Vertebrates
Gnathostomes
Osteichthyans
Lobe-fins
Tetrapods
Amniotes
Notochord
Ancestral deuterostome