Phylum Chordata:

Phylogenetic Relationships: Animals in the phylum Chordata share deuterostome

characteristics with echinoderms and hemichordates.
Characteristics of the phylum Chordata:
Characteristics of the phylum Chordata include:
1. Occupy almost every type of habitat, can live in marine, fresh water and terrestrial
habitat, many are capable to fly
2. Free living but few fishes are ectoparasites
3. Bilaterally symmetrical, deuterostome animals, triploblastic, coelom well developed
4. Epidermis present in all, dermis in vertebrates, keratinized or bony structures often
present in vertebrate’s integument (skin) glands often diverse and abundant in
vertebrates
5. Complete digestive tract, muscular gut in vertebrates
6. Ventral, contractile blood vessel (heart)
Diagnostic Characters of Chordates:
• Notochord
• Dorsal Hollow Nerve Cord
• Pharyngeal gill slits
• Post anal tail (if present).
• Endostyle or thyroid gland
Notochord:
• The phylum is named after the notochord (Gr. noton, the back + L. chorda, cord)
• It is a supportive rod that extends most of the length of the animal dorsal to the body
cavity and into the tail.
• It is first part of the endoskeleton to appear in an embryo
• It consists of a connective-tissue sheath that encloses cells, each of which contains a
large, fluid-filled vacuole.
• This arrangement gives the notochord some turgidity, which prevents compression
along the anteroposterior axis.
• At the same time, the notochord is flexible enough to allow lateral bending, as in the
lateral undulations of a fish during swimming.
• In most adult vertebrates, cartilage or bone partly or entirely replaces the notochord.
Pharyngeal slits:
• Pharyngeal slits are a series of openings in the pharyngeal region between the digestive
tract and the outside of the body
• They are formed by in pocketing of outside ectoderm (pharyngeal grooves) and out
pocketing of the endodermal lining of pharynx (pharyngeal pouches)
• In aquatic chordates the two pockets break through the pharyngeal cavity where they
meet to form pharyngeal slits
• In amniotes some pockets do not break through the pharyngeal cavity and only pouches
are formed instead of slits
• In tetrapod (terrestrial) vertebrates the pharyngeal pouches give rise to several
different structures including Eustachian tubes, middle ear cavity, tonsils and
parathyroid glands
• The perforated pharynx evolved as a filter feeding apparatus and is used as such in
protochordates
• Water with suspended food particles is drawn by ciliary action through the mouth and
flows out through pharyngeal slits where food is trapped in mucus
• In vertebrates’ ciliary action is replaced by muscular pharyngeal contraction that drive
water through the pharynx
Tubular nerve cord:
• The nerve cord runs along the longitudinal axis of the body, just dorsal to the notochord,
and usually expands anteriorly as a brain.
• This central nervous system is associated with the development of complex systems for
sensory perception, integration, and motor responses.
Postanal tail:
• A postanal tail extends posteriorly beyond the anal opening
• Either the notochord or vertebral column supports the tail.
• A postanal tail together with somatic musculature and the stiffening notochord provides
the motility that larval tunicates and amphioxus need for their free swimming existence
Endostyle or thyroid gland:
• An endostyle is present on the ventral aspect of the pharynx in tunicates,
cephalochordates, and larval lampreys.
 • It secretes mucus that helps trap food particles during filter feeding.
• Some cells in the endostyle secrete iodinated proteins. These cells are homologous with
the iodinated hormone secretory thyroid gland of adult lamprey and all other vertebrates
• In adult lampreys and other chordates, the endostyle is transformed into an endocrine
structure, the thyroid gland.
Advancement:
• Living endoskeleton which grows with the body.
• More efficient respiratory organs (gills and lungs) with enormous surface area for rapid
exchange of gases.
• Efficient circulatory system with well-developed heart.
• Better sense organs.
• Well-developed endocrine glands.

Chordata

Acrania Craniata
(Protochordata) (Vertebrata)

Urochordata Cephalochordata Agnatha Gnathostomata

Cyclostomata Pisces Tetrapoda

(Fins) (limbs)
1. Chondrichthyes 1. Amphibia
2. Osteichthyes 2. Reptilia
3. Aves
4. Mammalia
Urochordata:
• Common name: Tunicates because of presence of tough non-living tunic surround
animals and contains cellulose
• Body is generally enclosed in a leathery test (tunic) composed largely of tunicin (also
called animal cellulose).
Species: 3000 species
Habitat:
• These are exclusively marine animals.
• These are mostly sessile, filter-feeding animals.
• Notochord occurs only in the tail of the larva and disappears in the adult.
Why Urochordata are named so/characteristics:
• Tunicates are highly specialized chordates for most species only larval form, which
resemble a microscopic tadpole, bears all chordates hall marks.
• During adult metamorphosis, notochord (which in larva, is restricted to tail that`s why
they are called as Urochordata).
• Tail disappear while dorsal nerve cord becomes reduced to single ganglion
• Nerve cord is present in the larva, but is replaced by a single dorsal ganglion in the
adult.
• Pharyngeal slits are numerous, persist in the adult, and open into an ectoderm-lined
cavity, the atrium.
• There are no gills.
• Larva is called tadpole and it changes into a degenerate adult.
• This transformation is called retrogressive metamorphosis.
Classification of Urochordata:
Urochordata have 3 classes
1. Class ascidiacea
2. Class appendicularia
3. Class thaliacea
Class ascidiacea:
Common name: Sea squirts because some species forcefully discharge a jet of water from
excurrent siphon when irritated
Habitat: few are sessile animals attached to rocks or other hand substances pilings or bottoms
of ships, abundantly found as intertidal animals, they may be solitary or colonial (have separate
tunicate) when they are in compound they share (many members share same tunicate) in which
they have separate incurrent siphon but excurrent siphon is common
Body shape: they have spherical or cylindrical body form
Body consists of:
Mantle: inner membrane of tunicate lining
Two projections: incurrent and excurrent
• Incurrent siphon also called is oral siphon and anterior end of body
• Excurrent siphon also called is atrial siphon that marks dorsal side
• Water enters the incurrent siphon and passes into ciliated pharynx that is minutely
perforated by slits to form an elaborate basketwork
• Water passes through slits into an atrium and out through excurrent siphon
Feeding and nutrition:
• Feeding depends on formation of mucus net that is secreted by a glandular groove, the
endostyle, located along the mid-ventral side of pharynx
• Cilia on bars of pharynx pull the mucus into a sheet that spreads dorsally across the
inner face of pharynx
• Food particles brought in the incurrent opening are trapped on mucus net which is then
worked into a rope and carried posteriorly by cilia into the esophagus
• Nutirnts are absorbed in midgut and indigestible wastes are discharged from anus
located near excurrent siphon
Circulatory system:
• Consists of dorsal heart and two large vessels one on either side of heart
 • These vessels connect to a diffuse system of smaller vessels and spaces serving
pharyngeal basket (where respiratory exchange occurs) digestive organs, gonads and
other structure
Odd features:
1. An odd feature found in no other chordates is that heart drives blood 1st in one direction
for few beats, then pauses, reverse its action and drives blood in opposite direction for
a few beats
2. 2nd remarkable feature of this phyla, is presence of rare elements in blood such as
vanadium and niobium
Vanadium concentration in sea squirt ciona may reach 2 million times it concentration in
seawater
Nervous system: Nerve ganglion and plexus of nerves, lie on dorsal side of pharynx
Sensory organs: distinctive sensory organs are absent in adult sea squirt, although larval sea
squirts have eyespot and otocyst (balance organ)
Reproduction:
• they are hermaphroditic, single ovary and single testis in same animals
• gametes are carried by ducts into the atrial cavity and then release into the surrounding
water where fertilization occur
Fertilization: external
Development: indirect and larva is called tadpole
Five chief chordates characteristics adult sea squirts have only two
a. Pharyngeal slits
b. Endostyle
However larval stage has all five characteristics ‘tadpole’ larva is an elongated, transparent
form with all five chordate characteristics
i. Notochord
ii. Hollow dorsal nerve cord
iii. Propulsive postanal tail
iv. Large pharynx with endostyle
v. Pharyngeal slits
2. Class thaliacea:
• Tunicates of thaliacea ----------thaliaceans / salps are barrel / lemon shaped,
gelatinous bodies
• May be found singly/ colonial
• Cylindrical thaliacean body is typically surrounded by bands of circular muscles
with incurrent and excurrent siphons at opposite ends
• Water pumped through the body by muscular contraction is used for locomotion
by a sort of jet propulsion, for respiration and as a source of particulate food that
is filtered on mucus surfaces
• Many are provided with luminous organs which give brilliant light at night
3. Class appendicularia:
• Tiny, pelagic creature shaped like bent tadpole
• Feeding method is unique
• They build a delicate house a transparent hollow sphere of mucus interlaced with
filters and passages through which water enters
• Tiny phytoplankton and bacteria trapped on a feeding filter inside house are draw
into animals’ mouth through straw like tube
• When filters become clogged with waste which happens about every 4 hours,
appendicularian abandons its house and builds a new house, aprocess that takes
only few minutes
• Like thaliaceans, appendicularian can quickly build dense population when food
is abundant