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Bio 216
Crazy Chordates
Objectives:Discuss the differences between
traditional and cladistic classification of
chordates, present the major unifying
characteristics of the Phylum Chordata, survey
the Subphyla Urochordata and Cephalochordata.
Phylogenetic tree of chordates
Indicates
time scale.
1
Protostomes vs Deuterstomes
Deuterostomes
Blastopore forms anus
Radial cleavage
direct development
Regulative embryo
Evolutionary experimentation
Symmetry
Fig. 3.10
Bilateral symmetry- can be divided along
saggital plane into two mirrored portions
Organism must have a head
(cephalization)
2
Echinodermata
Phylogeny
Hemichordata
Deuterostomes
Chordata
Embryonic development patterns indicate
shared common ancestor with
echinoderm-hemichordate group.
Phylum Chordata
Five distinguishing characteristics of
Phylum Chordata.
1. Notochord
2. Dorsal hollow nerve cord (DHNC)
3. Pharyngeal pouches
4. Post-anal tail
5. Endostyle
Sometimes only present in larval stage.
Notochord
Notochord = flexible, rodlike body of cells
enclosed by a fibrous sheath. First part of
endoskeleton to appear in the embryo.
Supports and stiffens body.
http://courses.biology.utah.edu/nielsen/3315/html/skeletal/STnotochord.html
3
Serves as an axis for muscle
attachment, bends without
shortening
Undulatory movements
Notochord is center of developmental
organization
No notochord = no development
In many organisms, notochord disappears
Vertebrates, notochord serves as an axis around
which vertebrae develop.
Notochord dictates dorsal orientation.
Notochord removed
Ventral become dorsal
Two dorsal sides!
Dorsal Hollow Nerve Cord
Anterior enlarged to form brain,
ectodermal origin.
Invertebrates: ventral and solid
Chordates: dorsal and hollow
4
Pharyngeal pouches and slits
Endoderm
Ectoderm
Pharyngeal
pouches/slits lead
from pharynx to
outside environment.
Ectoderm inpockets
Endoderm outpockets
Pouches
Slits
Original function: filter
feeding in protochordates
Watermouthgill slits
Food captured in slits by
mucous.
Derived function:
respiration in aquatic
vertebrates.
Internal gills
Eustachian tubes, middle
ear cavity, glands in
tetrapod vertebrates
Endostyle
Glandular groove
Mucous
Thyroid gland
http://www.luc.edu/depts/biology/111/amphioxs.jpg
5
Post-anal tail
Adaptation for swimming
Seen in all developmental stages of
chordates.
Urochordata - sea squirts
Sessile as adults, surrounded by tough, nonliving
'tunic' of cellulose
Adults have two and
larvae have all
chordate
characteristics.
Filter-feeders
Endostyle secretes
mucous
Anus empties into
excurrent siphon
Fig. 2.2 B
6
Circulatory system
Open
Bidirectional blood
flow
Transports nutrients
Nervous system
Adults have a dorsal
ganglion
No DHNC
Cross-siphon reflex
Development
Free-swimming tadpole-like larvae metamorphose
into sessile adult
Fig. 15.5
7
Tunicate Larva
POST-ANAL
TAIL
PHARYNGEAL
GILL SLITS
NOTOCHORD
DHNC
SIPHONS
Subphylum Cephalochordata
marine lancelets (Amphioxus)
exhibit all 5 chordate characteristics.
Habitat = sandy bottoms of coastal waters
Filter feeders
eat plankton
Cilia move food, mucous traps food in
pharyngeal slits
Hepatic cecum = intracellular digestion
Water in mouth, out atriopore
8
Circulatory system: closed
Ventral aortaBranchial arteries Dorsal aorta Body tissue
No heart, blood pigments or blood cells
Nutrient transport
Dorsal aorta
Branchial arteries
Ventral aorta
Nervous system
Nervous system centered around hollow
nerve cord above notochord.
No brain.
Muscle segments innervated
Muscle segment= myomere
Amphioxus a direct ancestor of vertebrates?
NO HEAD!
Cephalochordata is a sister group.
Synapomorphies
1. Myomeres
2. Closed circulatory system
3. Kidney-like cells
4. Caudal fin
5. Unique embryonic features
9
Notochord
Post anal tail
Dorsal Hollow Nerve cord
Pharyngeal gill slits, endostyle
Urochordata (sea squirts)
Chordata
Cephalochordata (lancelets)
Sister Groups
Myomeres
Vertebrata
Summary
Phylum Chordata is named for rodlike
notochord that forms a stiffening body axis
at some stage in the life cycle of every
chordate.
Chordates share 5 defining characteristics.
Urochordate free-swimming larvae possess
all chordate characteristics.
Cephalochordates are vertebrate sister
group.
10
Reproduction
Diecious, gametes pass out the atriopore
Fertilization external
Larvae planktonic
Gas exchange at metapleural folds.
Metapleural
folds
11
12
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