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 nonvertebrate subphyla: the Urochordata and
the Cephalochordata.
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).
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”.
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.
Protostomes and Deuterostomes
 Protostomes
include the annelids,
mollusks, and arthropods.
 Deuterostomes
include the echinoderms
and Chordates, which includes the
vertebrates.
Figure 23.02
Characteristics of the Chordata
 Chordates




are:
bilaterally symmetrical
triploblastic
have a well developed coelom
have a complete digestive system
Germ layers of a triploblastic
organism

Endoderm: innermost germ layer of an embryo. Forms
the gut, liver, pancreas.

Ectoderm: Outer layer of cells in early embryo.
Surrounds the blastocoel. Forms outer epithelium of
body and nervous system.

Mesoderm: Third germ layer formed in gastrula between
ectoderm and endoderm. The coelom forms in the
mesoderm. Mesoderm gives rise to connective tissue,
muscle, urogenital and vascular systems and
peritoneum.
Coeloms
 The
coelom is a cavity entirely surrounded
by mesoderm.
 A coelom provides a tube-within-a-tube
arrangement which has many advantages:
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.
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.
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.
Figure 23.01
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.
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.
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.
Figure 23.09b
Amphioxus
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.
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.
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.
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.
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.
Figure 23.09b
Amphioxus
Classification of the Chordata
 There



are three subphyla in the Chordata:
Subphylum Urochordata: tunicates
Subphylum Cephalochordata: lancelets
Subphylum Vertebrata: fish, amphibians,
reptiles, birds, mammals, etc.
Subphylum Urochordata

The Urochordata (“tunicates” named for the
tough tunic that surrounds the adult) look like
most unpromising candidates to be chordates
and relatives of the vertebrates.

The largest group, the ascidians or sea squirts
(Class Ascidiacea) as adults are marine, sessile,
filter feeding organisms that live either solitarily
or in colonies.
Ciona intestinalis
(a solitary sea squirt)
0147.jpg
Synoicum pulmonaria a colonial sea squirt
Ascidians
 Adult
ascidians lack a notochord and there
is only a single ganglion in place of the
dorsal nerve cord.
 Of
the five characteristics of chordates
adults possess only two: pharyngeal gill
slits and an endostyle, both of which they
use in filter feeding.
Ascidians
 The
adult sea squirt draws water in
through an incurrent siphon and pushes it
back out an excurrent one.
 Food
particles are filtered out in the
pharyngeal slits with mucus from the
endostyle used to trap particles.
Figure 23.04
15.4
Cilia lining the slits in the branchial sac or pharynx
Ascidian filter feeding process. Water is drawn
through the animal by the beating of cilia.
Branchial sac orpharynx
http://seapeachbio.blogspot.com/2012/06/a
scidian-filter-feeding-mechanism.html
Larval Ascidian
 Even
though the adult ascidian hardly
resembles a chordate its larva does.
 Larval
ascidians are very small and
tadpole-like and possess all five chordate
characteristics previously outlined.
Young larval ascidian
Larval Ascidian
 The
larval ascidians role is to disperse and
to achieve this it is free swimming.
However, it has only a short larval life
(minutes to a couple of days) and does not
feed during this time.
 Instead
it searches for a place to settle
and then attaches and metamorphoses
into an adult.
Ascidian metamorphosis
 During
metamorphosis the notochord
disappears, the nerve cord is reduced to a
single nerve ganglion and a couple of
nerves.
Figure 23.06
15.5
Other Urochordate classes
 Besides
the ascidians there are two other
classes of the Urochordata: the Larvacea
and Thaliacea.
 Both
are small, transparent planktonic
forms. Thaliaceans are cylindrical or
spindle shaped whereas larvaceans are
tadpolelike and resemble an ascidian
larva.
Garstang’s hypothesis of chordate
larval evolution
 In
the 1920’s it was proposed that the
vertebrates were derived from an
ancestral ascidian that retained its
characteristics into adulthood (the process
by which juvenile characteristics are
retained into adulthood is referred to as
paedomorphosis).
Figure 23.12
Garstang’s hypothesis of chordate
larval evolution
 Garstang’s
hypothesis is supported by
embryological evidence, but more recently
molecular analyses have suggested that
sessile ascidians are a derived form and
that the free-living larvaceans are more
likely to be the closest relatives of the
chordates.
Subphylum Cephalochordata

The cephalochordates are the lancelets, which
are small (3-7 cm long) laterally compressed
fishlike animals that inhabit sandy sediments of
coastal waters. They lack a distinct head and
have no cranium.

They are commonly referred to as Amphioxus as
this was the original genus name. There are 29
species, five of which occur in North American
coastal waters.
Figure 23.09b
Amphioxus
 Amphioxus
 Water
is a filter feeder.
enters the mouth and then passes
through the pharyngeal slits, where food is
trapped in mucus. Cilia then move the
food to the gut.
Figure 23.09a
Amphioxus
Amphioxus

Amphioxus is interesting because it displays the
basic chordate characteristics in a simple and
obvious form because of its transparency.

Amphioxus is considered to be the closest living
relative of the vertebrates because it shares
several characteristics with vertebrates that
Urochordates do not possess.
Amphioxus characteristics
shared with vertebrates
 As
well as possessing the chordate
characteristics, amphioxus shares a
number of traits with vertebrates including:



Segmented myomeres (muscle blocks)
Dorsal and ventral aortas
Branchial (gill) arches (blood vessels running
over the gills).
Characteristics of vertebrates
not possessed by Amphioxus
 Amphioxus
however lacks several
characteristics that biologists think the
ancestor of vertebrates possessed. These
include:




Tripartite brain (with forebrain, midbrain and
hindbrain) protected by a cranium
Chambered heart
Muscular gut and pharynx
List continues on next slide
Characteristics of vertebrates
not possessed by Amphioxus


Various special sensory organs (eyes,
chemical and pressure receptors)
Neural crest (ectodermal cells that are found
on the embryonic neural tube and are
engaged in the formation of the cranium, tooth
dentine, some endocrine glands and
Schwann cells, which provide myelin
insulation to nerve cells).