The chordates are a phylum that includes vertebrates as well as some non-vertebrate subphyla like tunicates and lancelets. Chordates are defined by five characteristics - a notochord, dorsal nerve cord, pharyngeal pouches/slits, endostyle/thyroid gland, and postanal tail. While some chordate lineages retain these traits as adults, in others like vertebrates the notochord is replaced by a vertebral column in embryos. Chordates are further divided into protostomes and deuterostomes based on their early embryonic development.

1. Phylum Chordata
 The chordates are a group of particular
interest to us as we belong to it, being
members of the subphylum Vertebrata.
 The chordates include all of the
vertebrates (fish, amphibians, reptiles,
mammals and birds), but also two non-
vertebrate subphyla: the Urochordata and
the Cephalochordata.

2. Phylum Chordata
 The chordates were in the 19th century
considered to have been derived from
protostome ancestors (the annelid, mollusc,
arthropod group).
 However, a better understanding of embryology
shows that chordates are deuterostomes and
the invertebrates they are most closely related to
are the Echinodermata (sea stars, sand dollars,
sea urchins) and the Hemichordata (acorn
worms).

3. Protostomes and Deuterostomes
 Within the eucoelomates there are two
major evolutionary lineages that split early
in the history of animals and follow quite
different developmental pathways.
These are the protostomes “mouth first” and
deuterostomes “mouth second”.

4. Important differences in development
between protostomes and deuterostomes
 The differences in development that distinguish
the protostomes and deuterostomes include:
 Whether cleavage of cells in the early zygote is spiral
or radial.
 Whether or not, if the early blastomere is separated,
each cell can develop into a normal larva or not.
 Whether the blastopore ultimately forms the mouth or
anus of the organism.
 Whether or not the organism possesses a coelom
and how that coelom is formed.

6. Protostomes and Deuterostomes
 Protostomes include the annelids,
mollusks, and arthropods.
 Deuterostomes include the echinoderms
and Chordates, which includes the
vertebrates.

7. Figure 23.02

8. Characteristics of the Chordata
 Chordates are:
 bilaterally symmetrical
 triploblastic
 have a well developed coelom
 have a complete digestive system

9. Coeloms
 The coelom is a cavity entirely surrounded
by mesoderm.
 A coelom provides a tube-within-a-tube
arrangement which has many advantages:

10. Advantages of a coelom
 In mammals the pericardial, peritoneal, and
pleural cavities are formed from the coelom.
 Allows flexibility in arranging visceral organs
 Permits greater size and complexity of
internal organs as there is space for them to
grow
 Fluid-filled coelom can act as a hydrostatic
skeleton
 Cushions organs against impact
 Allows organs to grow and move
independently of your outer body wall.

11. Five distinctive characteristics of
the chordates
 Five distinctive characteristics separate the
chordates from all other phyla:
 Notochord
 Single, dorsal, tubular nerve cord
 Pharyngeal pouches or slits
 Endostyle
 Postanal tail
 Not all of these characteristics are apparent in
adult organisms and may appear only in the
embryonic or larval stages.

12. Notochord
 Notochord: the notochord is a flexible,
rodlike structure. It extends the length of
the body and is an anchor point for
muscles.
 The notochord bends without shortening
so it permits the animal to undulate.

13. Figure 23.01

14. Notochord
 In nonvertebrates and the jawless
vertebrates the notochord is present
throughout life.
 However, in the jawed vertebrates it is
replaced by the vertebral column; the
remnants of the notochord being found in
the intervertebral disks.

15. Single, dorsal, tubular nerve
cord
 In most invertebrates the nerve cord, if present,
is ventral to the gut.
 In chordates, in contrast, the nerve cord is dorsal
to the gut and notochord. The nerve cord
passes through the neural arches of the
vertebrae, which protect it.
 The nerve cord is enlarged in vertebrates into a
brain, which is surrounded by a bony or
cartilaginous cranium.

16. Pharyngeal pouches and slits
 Pharyngeal slits occur in aquatic chordates and
lead from the pharyngeal cavity to the outside.
 The pharyngeal slits are used as a filter feeding
device in protochordates (i.e., Urochordata
(Tunicates)) and Cephalochordata (lancelets
e.g. Amphioxus).
 Water containing food is drawn in through the
mouth by cilia and exits via the pharyngeal slits
where the particles are trapped in mucus.

17. Figure 23.09b
Amphioxus

18. Pharyngeal pouches and slits
 In vertebrates the pharyngeal arches have
been modified into gills by the addition of a
rich blood supply and thin gas permeable
walls.
 The contraction of muscles in the pharynx
drive water through the gills.

19. Pharyngeal pouches and slits
 In amniotes an opening may not form and
rather than slits only grooves called
pharyngeal pouches develop.
 In tetrapods these pouches give rise
during development to a variety of
structures including the middle ear cavity,
eustachian tube, and tonsils.

20. Endostyle or thyroid gland
 The endostyle is found in protochordates
and in lamprey larvae. It is located on the
floor of the pharynx and secretes mucus,
which is used to trap particles.
 The endostyle works with the pharyngeal
slits in filter feeding.

21. Endostyle or thyroid gland
 Some cells in the endostyle secrete
iodinated proteins and are homologous
with the iodinated-hormone secreting
thyroid gland, which is found in adult
lampreys and vertebrates.

22. Postanal tail
 The postanal tail, some musculataure and the
notochord enable larval tunicates and
amphioxus to swim.
 The postanal tail evolved to allow organisms to
swim and its efficiency has been enhanced by
the addition of fins. The postanal tail is present
only in vestigial form in humans (the coccyx)
although tails as a whole are widespread among
vertebrates.