CHAPTER 17

Chordata: Urochordata and Cephalochordata

EVOLUTIONARY PERSPECTIVE

 Phylum Chordata includes animals that are familiar to us, including humans, as we are
members of this phylum.
 Other members of Chordata are less familiar.
 Example: During a walk along the seashore, you might see sea squirts (also known as
tunicates) that appear as jellylike masses with two openings.
o Some tunicates live as solitary individuals, while others live in colonies.
o When handled, they may squirt water from their openings.
o These animals are small filter feeders and might not seem like chordates at first
glance.
 However, detailed studies have confirmed that tunicates and lancelets are invertebrate
chordates because they lack a vertebral column.

Phylogenetic Relationships

 Animals in the phylum Chordata share deuterostome characteristics with echinoderms

and hemichordates.
o These characteristics include pharyngeal slits and various molecular features.
 There is strong evidence suggesting that Chordata and other deuterostomes share a
common ancestor from the early to mid-Cambrian period.
 Despite the diverse adult body forms within deuterostomia, the evolutionary pathways
leading to Chordata are difficult to trace.
 Paleontological, developmental, and molecular data are providing increasing evidence that
helps clarify the origin of the chordate lineage.

PHYLUM CHORDATA

 The phylum Chordata has about 65,000 species but has been highly successful in adapting
to both aquatic and terrestrial environments globally.
 Subphyla within Chordata include:
o Urochordata (sea squirts)
o Cephalochordata (lancelets)
o Craniata (vertebrates)
 Major morphological characteristics of Chordata include:
o Bilaterally symmetrical, deuterostomate animals.
o A unique combination of five characteristics present at some stage in development:
 Notochord
 Pharyngeal slits or pouches
 Dorsal tubular nervous system
 Paired trunk muscles extending into a postanal tail
 Endostyle or thyroid gland
o Complete digestive tract
o Ventral, contractile blood vessel (heart)
o Muscles organized into paired blocks called myomeres.
 Paired trunk muscles extend posterior to the anus into a tail, which is modified for functions
like locomotion. These muscles are organized into blocks called myomeres.
 Notochord:
o A supportive rod that runs the length of the animal, dorsal to the body cavity and tail.
o Provides turgidity to prevent compression and is flexible for lateral bending, aiding in
movement.
o In adult vertebrates, the notochord is often replaced by cartilage or bone.
 Pharyngeal slits:
o Openings in the pharyngeal region between the digestive tract and outside.
o Used for filter feeding and, in some chordates, for gas exchange through gills.
o In terrestrial vertebrates, these are mainly embryonic features.
 Tubular nerve cord:
o Runs just dorsal to the notochord and usually expands anteriorly as a brain.
o Supports the development of complex sensory, integration, and motor systems.
 Endostyle or thyroid gland:
 o The endostyle is present in tunicates, cephalochordates, and larval lampreys. It
secretes mucus to trap food during filter feeding.
o The endostyle can bind the amino acid tyrosine, and in metamorphosis, it becomes the
thyroid gland in adult lampreys.
o The thyroid gland regulates metabolism and metamorphosis in craniates.

Subphylum Urochordata

 Subphylum Urochordata (tunicates or sea squirts) includes six clades grouped into three
classes.
o Ascidiacea is the largest class (2,300 species).
 Ascidians are sessile as adults and can be either solitary or colonial.
 They attach to rocks, pilings, and other solid substrates.
 Two siphons allow seawater to circulate through the body:
 Oral siphon: Inlet for water and serves as the mouth.
 Atrial siphon: Outlet for excurrent water.
 The body is covered by a tunic (connective-tissue-like covering) made of
proteins, salts, and cellulose. Tunicates are the only animals that produce
cellulose, known as tunicin.
 Tunicin is a pure form of cellulose that may have commercial uses, such as in
tissue engineering and nanofiltration.
 The tunic also contains blood vessels and blood cells and has stolons that
help anchor tunicates and connect colony members.
o Appendicularia (approximately 70 species):
 Planktonic, living in the pelagic zone (near surface to just above the seafloor).
 Adults retain the tadpole body form typical of larval urochordates.
 They secrete a test (protein and cellulose) around themselves, including a filter-
feeding mesh.
 Tests are frequently discarded and rebuilt, contributing to marine snow.
o Thaliacea (approximately 70 species):
 Free-swimming, pelagic filter feeders.
 Some species (e.g., pyrosomes) form bioluminescent colonies.
 Thaliaceans are an important part of marine food webs.

Maintenance Functions

 Muscles and body structure:

o Longitudinal and circular muscles below the body-wall epithelium help change the
shape of the adult tunicate.
o These muscles work against the elasticity of the tunic and the hydrostatic skeleton
created by seawater in internal chambers.
 Nervous system:
o The nervous system is largely confined to the body wall, forming a nerve plexus with a
single ganglion located on the wall of the pharynx.
o The ganglion is not essential for coordinating bodily functions.
o Tunicates are sensitive to mechanical and chemical stimuli, with receptors mainly
around the siphons, but they lack complex sensory organs.
 Internal structures:
o The pharynx is very large, and an atrium surrounds it laterally and dorsally.
o The pharynx is continuous with the digestive tract, and its oral margin has tentacles to
block large objects from entering.
o Numerous pharyngeal slits (stigmas) allow water to circulate through the pharynx and
into the atrium, where it exits through the atrial siphon.
 Digestive system:
o The endostyle is a ventral ciliated groove that secretes a mucous sheet, which moves
dorsally across the pharynx.
o Food particles are trapped in the mucous sheet, moved into the digestive tract, and
digested by enzymes in the stomach. Absorption happens in the intestine.
o Excurrent water carries digestive wastes out through the anus and atrial siphon.
 Pharynx function:
o In addition to feeding, the pharynx is also involved in gas exchange as water circulates
through the tunicate.
 Circulatory system:
 o The heart is located at the base of the pharynx, with one vessel running anteriorly and
another running posteriorly.
o Blood flow through the heart is not unidirectional; it reverses direction periodically,
which may help distribute oxygen and nutrients more evenly throughout the body.
o Tunicate blood plasma is colorless and contains amoeboid cells.
 Excretion:
o Ammonia diffuses into the water passing through the pharynx and is excreted.
o Amoeboid cells in the circulatory system accumulate uric acid and store it in the
intestinal loop.
o Pyloric glands outside the intestine are believed to have excretory functions.

Reproduction and Development

 Urochordates are monoecious, meaning each individual has both male and female
reproductive organs.
 Gonads are located near the intestinal loop, and genital ducts open near the atrial siphon.
 Gametes are released through the atrial siphon for external fertilization, or eggs may be
retained in the atrium for fertilization and early development.
 While self-fertilization occurs in some species, cross-fertilization is the common mode of
reproduction.
 Development leads to the formation of a tadpole-like larva that possesses all five chordate
characteristics.
 Metamorphosis begins after a short free-swimming larval period, during which the larva
does not feed.
 The larva settles onto a solid substrate and attaches using adhesive papillae below the mouth.
 During metamorphosis:
o The outer epidermis shrinks, pulling the notochord and tail structures internally.
o Internal structures rotate 180°, positioning the oral siphon opposite the adhesive
papillae and bending the digestive tract into a U-shape.

Subphylum Cephalochordata

 Cephalochordates (lancelets) are members of the subphylum Cephalochordata, named for

their head and notochord (from Greek "kephalo" = head, Latin "chorda" = cord).
 This group consists of three extant genera: Epigonichthys, Branchiostoma (amphioxus), and
Asymmetron, with about 32 species.
 Distribution: They are found in shallow, clean sand-substrate waters across the world's oceans.
 Size and shape: Lancelets are small (up to 5 cm long), fishlike, elongate, laterally
flattened, and nearly transparent.
o Despite their streamlined shape, they are weak swimmers and spend most of their time
in a filter-feeding position, partly or mostly buried in the sand with their anterior end
exposed.
 Notochord:
o It extends from the tail to the head, which is why they are named cephalochordates.
o Unlike other chordates, their notochord has mostly muscle cells, making it somewhat
contractile, an adaptation likely for burrowing.
o Contraction increases rigidity, supporting the lancelet when pushing into sandy
substrates, while relaxation increases flexibility for swimming.
 Muscle and movement:
o Myotomic muscle bundles on both sides of the notochord cause undulations for
propulsion in the water.
o Longitudinal, ventrolateral folds on the body wall stabilize the lancelet during
swimming.
o They also have a median dorsal fin and a caudal fin that aid in swimming.
 Feeding and mouth structure:
o Oral hood projects from the anterior end, with ciliated, fingerlike projections called
cirri on the ventral side used for feeding.
o The mouth opening at the back of the oral hood leads to a large pharynx, perforated
by numerous pairs of pharyngeal slits supported by cartilaginous gill bars.
 Body structure:
o Folds of the body wall extend ventrally around the pharynx, fusing at the ventral
midline to form the atrium, a chamber that surrounds the pharyngeal region.
o The atrium helps protect the delicate filtering surfaces of the pharynx from bottom
sediments.
 o The atriopore is the opening from the atrium to the outside.

Maintenance Functions

Cephalochordates (lancelets) are filter feeders and employ a fascinating feeding mechanism:

 Feeding position: They are partially or mostly buried in sandy substrates with their mouths
pointed upward.
 Ciliary bands: On the lateral surfaces of the gill bars, there are ciliary bands positioned on
folded epithelium. These bands beat in waves, creating a "rotating wheel" effect, known as the
"wheel organ", which helps draw water into the mouth.
 Feeding process:
o Water flows from the mouth into the pharynx, passes through the pharyngeal slits,
into the atrium, and exits the body through the atriopore.
o Cirri, which are ciliated, fingerlike projections at the mouth, sort food. Larger particles
are filtered out by the cilia, and larger particles are discarded via cirri contractions.
o Smaller edible particles are carried into the mouth with water, where they are trapped
by cilia on the gill bars and in mucus secreted by the endostyle.
 Endostyle:
o The endostyle is a ciliated groove along the ventral midline of the pharynx. It helps
move food and mucus dorsally, forming a food cord that is then rotated by cilia to
dislodge food.
 Digestion:
o Digestion is both extracellular (in the gut) and intracellular (within cells).
o The midgut cecum is a diverticulum extending from the gut, secreting digestive
enzymes. It ends blindly along the right side of the pharynx.
o The anus is located on the left side of the ventral fin.
 Circulatory system:
o Cephalochordates do not have a true heart. Instead, contractile waves in the walls of
major vessels propel blood.
o The blood contains amoeboid cells and bathes tissues in open spaces.
 Excretory system:
o Excretory tubules are modified coelomic cells, closely associated with blood vessels,
indicating active transport between the blood and excretory tubules.
 Coelom:
o The coelom of cephalochordates is reduced compared to other chordates. It is confined
to canals near the gill bars, the endostyle, and the gonads.

Reproduction and Development

Cephalochordates are dioecious, meaning they have separate sexes. Here's how their reproductive
process works:

 Gonads: The gonads bulge into the atrium from the lateral body wall.
 Gamete release: Gametes (eggs and sperm) are shed into the atrium and leave the body
through the atriopore.
 Fertilization: External fertilization occurs, where sperm fertilize the eggs outside the body.
 Larval development:
o The fertilized eggs develop into a bilaterally symmetrical larva.
o These larvae are free-swimming but eventually settle on a substrate.
 Metamorphosis: After settling, the larvae undergo metamorphosis to transform into adult
cephalochordates.

EVOLUTIONARY CONNECTIONS

 Evidence Supporting the Monophyly of Chordates:

o Chordates originate from early deuterostomes, supported by multiple types of
evidence, including paleontological, developmental, and molecular data.
o Chordates possess pharyngeal slits and deuterostome developmental traits, with
vertebrate chordates having a schizocoelous coelom, a derived feature tied to the
evolution of yolky egg cells.
o Molecular data like 16s and 18s rRNA sequencing, HOX genes, and phylogenomic
studies back the deuterostome affinities of chordates.
  Gene Evidence for Divergence:
o Homologous genes controlling dorsoventral axis development (e.g., dpp and bmp)
help differentiate protostomes from deuterostomes.
o In protostomes, dpp is active dorsally; in deuterostomes, bmp is active ventrally,
leading to differences in the nervous system and pharyngeal bars between
chordates and ambulacrarians.
o Hemichordates and echinoderms have dorsally positioned pharyngeal bars, while
chordates have ventral pharyngeal bars.
 Molecular and Paleontological Evidence for Chordate Origins:
o Molecular studies suggest cephalochordates are basal to both Urochordata and
Craniata.
o Fossil evidence like Pikaia (520 mya) and Haikouella (530 mya) supports the
Cephalochordata lineage. Haikouella shows evidence of all chordate characteristics.
 Early Urochordate and Craniate Evolution:
o Urochordate fossil Shankouclava (520 mya) resembles modern solitary ascidians,
marking the divergence of urochordates and the loss of some chordate traits (e.g.,
notochord, post-anal tail, tubular nervous system).
o The first craniates show early protective cranium development, evidenced by
myllokunmingiids (530-520 mya).
 Vertebrate Evolution:
o Vertebrates (infraphylum Vertebrata) have bony or cartilaginous vertebrae,
replacing the notochord.
o Their three-part brain (forebrain, midbrain, hindbrain) and specialized sense organs
represent high cephalization.
o Fossil record of vertebrates, including jawless fishes (500 mya), shows that
vertebrates became dominant by the Devonian period (~400 mya).
o Terrestrial vertebrates appeared at the end of the Devonian period.

Classification of the Chordata

 Phylum Chordata
o Occupy marine, freshwater, and terrestrial habitats
o Key Characteristics:
 Notochord
 Pharyngeal slits
 Dorsal tubular nerve cord
 Postanal tail
 Endostyle or thyroid gland
o About 65,000 species

Subphylum Urochordata

 Larvae: Have notochord, nerve cord, and postanal tail

 Adults: Sessile or planktonic, enclosed in a tunic containing cellulose
 Marine: Sea squirts or tunicates
o Class Ascidiacea: Sessile as adults, solitary or colonial, interconnected by stolons
(e.g., Ascidia, Ciona)
o Class Appendicularia: Planktonic, retain tail and notochord as adults, lack cellulose
tunic, secrete gelatinous covering (e.g., Fritillaria)
o Class Thaliacea: Planktonic, tailless and barrel-shaped adults, muscular contractions
produce water currents (e.g., Salpa, Thetys)

Subphylum Cephalochordata

 Fishlike, laterally compressed, transparent

 All five chordate characteristics persist throughout life
 Example: Amphioxus (Branchiostoma)

Subphylum Craniata

 Skull surrounds the brain, olfactory organs, eyes, and inner ear
 Neural crest contributes to various adult structures
o Infraphylum Hyperotreti: Fishlike, cartilaginous skull, jawless, no paired
 appendages, 4 pairs of tentacles, slime glands (e.g., Hagfishes)

Infraphylum Vertebrata

 Key Characteristics:
o Notochord, nerve cord, postanal tail, and pharyngeal slits at least in embryonic
stages
o Vertebrae surround nerve cord and provide axial support
o Class Petromyzontida: Jawless, no paired appendages, cartilaginous skeleton,
sucking mouth with teeth (e.g., Lampreys)
o Class Chondrichthyes: Jawed, paired appendages, cartilaginous skeleton, no swim
bladder (e.g., Skates, Rays, Sharks)
o Class Actinopterygii: Bony fishes, paired fins with dermal rays, homocercal tail,
pneumatic sacs as swim bladder (e.g., Ray-finned fishes)
o Class Sarcopterygii: Bony fishes with muscular lobes, pneumatic sacs as lungs (e.g.,
Lungfishes, Coelacanths, Tetrapods)

Classes of Vertebrates

 Class Amphibia: Moist skin, lungs/gills, aquatic developmental stages followed by

metamorphosis (e.g., Frogs, Toads, Salamanders)
 Class Reptilia: Dry skin with scales, amniotic eggs, terrestrial embryonic development (e.g.,
Snakes, Lizards, Alligators)
 Class Aves: Feathers for flight, endothermic, amniotic eggs (e.g., Birds)
 Class Mammalia: Hair, endothermic, young nursed from mammary glands, amniotic eggs
(e.g., Mammals)