Phylum Chordata

The phylum Chordata contains all animals that have a dorsal notochord at some stage of
development; in most cases, this is the backbone.for Biology

Animals in the phylum Chordata share four key features that appear at some stage of their
development:

• A notochord, or a longitudinal, flexible rod between the digestive tube and the nerve
cord. In most vertebrates, it is replaced developmentally by the vertebral column. This is
the structure for which the phylum is named.
• A dorsal nerve cord which develops from a plate of ectoderm that rolls into a tube located
dorsal to the notochord. Other animal phyla have solid nerve cords ventrally located. A
chordate nerve cord splits into the central nervous system: the brain and spinal cord.
• Pharyngeal slits, which allow water that enters through the mouth to exit without
continuing through the entire digestive tract. In many of the invertebrate chordates, these
function as suspension feeding devices; in vertebrates, they have been modified for gas
exchange, jaw support, hearing, and other functions.
• A muscular, postanal tail which extends posterior to the anus. The digestive tract of most
nonchordates extends the length of the body. In chordates, the tail has skeletal elements
and musculature, and can provide most of the propulsion in aquatic species.

In some groups, some of these traits are present only during embryonic development. In addition
to containing vertebrate classes, the phylum Chordata contains two clades of invertebrates:
Urochordata (tunicates) and Cephalochordata (lancelets). However, even though they are
invertebrates, they share characteristics with other chordates that places them in this phylum. For
example, tunicate larvae have both a notochord and a nerve cord which are lost in adulthood.
Most tunicates live on the ocean floor and are suspension feeders. Cephalochordates, or
lancelets, have a notochord and a nerve cord (but no brain or specialist sensory organs) and a
very simple circulatory system. Lancelets are suspension feeders that feed on phytoplankton and
other microorganisms.

Structures present in a tunicate larva: While tunicates are invertebrates and may seem very
different from the more familiar members of Chordata, the tunicate larva possesses both a
notochord and a dorsal nerve cord, although both are lost in adulthood.
The phylum Chordata contains all of the animals that have a rod-like structure used to give them
support. In most cases this is the spine or backbone. Within Chordata there are five classes of
animals: fish, amphibians, reptiles, birds, and mammals. Three dividing factors separate these
classes:

• Regulation of body temperature: animals are either homeothermic (can regulate their
internal temperature so that it is kept at an optimum level) or poikilothermic (cannot
regulate their internal temperature, the environment affects how hot or cold they are)
• Oxygen Absorption: the way in which oxygen is taken in from the air, which can be
through gills, the skin (amphibians), or lungs
• Reproduction: this factor is particularly varied. Animals can be oviparous (lay eggs) or
viviparous (birth live young). Fertilization can occur externally or internally. In
mammals, the mother produces milk for the young.

Notochord: All chordates possess a notochord, or a type of flexible support rod, at some point in
their development. In this dissected lungfish, which is a member of the chordates, tissues have
been dissected away to display the notochord.

Characteristics of Chordata

Animals in the phylum Chordata share four key features: a notochord, a dorsal hollow nerve
cord, pharyngeal slits, and a post-anal tail.

Animals in the phylum Chordata share four key features that appear at some stage during their
development (often, only during embryogenesis) (:
Defining characteristics of chordates: In chordates, four common features appear at some point
during development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal
tail.

1. a notochord
2. a dorsal hollow nerve cord
3. pharyngeal slits
4. post-anal tail

Notochord

The chordates are named for the notochord: a flexible, rod-shaped structure that is found in the
embryonic stage of all chordates and also in the adult stage of some chordate species. It is
located between the digestive tube and the nerve cord, providing skeletal support through the
length of the body. In some chordates, the notochord acts as the primary axial support of the
body throughout the animal’s lifetime.

In vertebrates, the notochord is present during embryonic development, at which time it induces
the development of the neural tube which serves as a support for the developing embryonic body.
The notochord, however, is replaced by the vertebral column (spine) in most adult vertebrates.

Dorsal Hollow Nerve Cord

The dorsal hollow nerve cord derives from ectoderm that rolls into a hollow tube during
development. In chordates, it is located dorsally (at the top of the animal) to the notochord. In
contrast to the chordates, other animal phyla are characterized by solid nerve cords that are
located either ventrally or laterally. The nerve cord found in most chordate embryos develops
into the brain and spinal cord, which comprise the central nervous system.

Pharyngeal Slits

Pharyngeal slits are openings in the pharynx (the region just posterior to the mouth) that extend
to the outside environment. In organisms that live in aquatic environments, pharyngeal slits
allow for the exit of water that enters the mouth during feeding. Some invertebrate chordates use
the pharyngeal slits to filter food out of the water that enters the mouth. In vertebrate fishes, the
pharyngeal slits develop into gill arches, the bony or cartilaginous gill supports.

In most terrestrial animals, including mammals and birds, pharyngeal slits are present only
during embryonic development. In these animals, the pharyngeal slits develop into the jaw and
inner ear bones.

Post-anal Tail

The post-anal tail is a posterior elongation of the body, extending beyond the anus. The tail
contains skeletal elements and muscles, which provide a source of locomotion in aquatic species.
In some terrestrial vertebrates, the tail also helps with balance, courting, and signaling when
danger is near. In humans and other apes, the post-anal tail is present during embryonic
development, but is vestigial as an adult.

Key Takeaways
Key Points

• The phylum chordata is named for the notochord, a longitudinal, flexible rod between the
digestive tube and the nerve cord; in vertebrates, this is the spinal column.
• The chordates are also characterized by a dorsal nerve cord, which splits into the brain
and spinal cord.
• Chordata contains two clades of invertebrates: Urochordata (tunicates) and
Cephalochordata (lancelets), both of which are suspension feeders.
• The phylum chordata includes all animals that share four characteristics, although they
might each possess some of them at different stages of their development: a notochord, a
dorsal nerve cord, pharyngeal slits, and a postanal tail.
• Chordata contains five classes of animals: fish, amphibians, reptiles, birds, and mammals;
these classes are separated by whether or not they can regulate their body temperature,
the manner by which they consume oxygen, and their method of reproduction.

Key Terms

• dorsal nerve cord: a hollow cord dorsal to the notochord, formed from a part of the
ectoderm that rolls, forming a hollow tube.
• notochord: a flexible rodlike structure that forms the main support of the body in the
lowest chordates; a primitive spine
• pharyngeal slit: filter-feeding organs found in non-vertebrate chordates (lancelets and
tunicates) and hemichordates living in aquatic environments

Key Points

• These characteristics are only present during embryonic development in some chordates.
• The notochord provides skeletal support, gives the phylum its name, and develops into
the vertebral column in vertebrates.
 • The dorsal hollow nerve cord develops into the central nervous system: the brain and
spine.
• Pharyngeal slits are openings in the pharynx that develop into gill arches in bony fish and
into the jaw and inner ear in terrestrial animals.
• The post-anal tail is a skeletal extension of the posterior end of the body, being absent in
humans and apes, although present during embryonic development.

Key Terms

• notochord: a flexible rodlike structure that forms the main support of the body in the
lowest chordates; a primitive spine
• nerve cord: a dorsal tubular cord of nervous tissue above the notochord of a chordate
• pharyngeal slit: filter-feeding organs found in non-vertebrate chordates (lancelets and
tunicates) and hemichordates living in aquatic environments

Chordates and the Evolution of Vertebrates

The most familiar group of chordates is the vertebrates. However, in addition to the subphylum
Vertebrata, the phylum Chordata also contains two subphylums of invertebrates: Urochordata
and Cephalochordata. Members of these groups also possess the four distinctive features of
chordates at some point during their development: a notochord, a dorsal hollow nerve cord,
pharyngeal slits, and a post-anal tail. Unlike vertebrates, urochordates and cephalochordates
never develop a bony backbone.

Urochordata

Members of Urochordata are also known as tunicates. The name tunicate derives from the
cellulose-like carbohydrate material, called the tunic, which covers the outer body of tunicates.
Although tunicates are classified as chordates, only the larval form possesses all four common
structures. Adults only maintain pharyngeal slits and lack a notochord, a dorsal hollow nerve
cord, and a post-anal tail.
Urochordates: (a) This photograph shows a colony of the tunicate Botrylloides violaceus. (b)
The larval stage of the tunicate possesses all of the features characteristic of chordates: a
notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. (c) In the adult
stage, the notochord, nerve cord, and tail disappear.

Most tunicates are hermaphrodites. Tunicate larvae hatch from eggs inside the adult tunicate’s
body. After hatching, a tunicate larva swims for a few days until it finds a suitable surface on
which it can attach, usually in a dark or shaded location. It then attaches via the head to the
surface and undergoes metamorphosis into the adult form, at which point the notochord, nerve
cord, and tail disappear.

Most tunicates live a sessile existence on the ocean floor and are suspension feeders. The
primary foods of tunicates are plankton and detritus. Seawater enters the tunicate’s body through
its incurrent siphon. Suspended material is filtered out of this water by a mucous net (pharyngeal
slits) and is passed into the intestine via the action of cilia. The anus empties into the excurrent
siphon, which expels wastes and water. Tunicates are found in shallow ocean waters around the
world.

Cephalochordata

Members of Cephalochordata possess a notochord, dorsal hollow nerve cord, pharyngeal slits,
and a post-anal tail in the adult stage. They do not have a true brain, but the notochord extends
into the head, which gives the subphylum its name (“cephalo” is Greek for head). Extinct
members of this subphylum include Pikaia, which is the oldest known cephalochordate. Pikaia
fossils were recovered from the Burgess shales of Canada and dated to the middle of the
Cambrian age, making them more than 500 million years old.

Extant members of Cephalochordata are the lancelets, named for their blade-like shape.
Lancelets are only a few centimeters long and are usually found buried in sand at the bottom of
warm temperate and tropical seas. Like tunicates, they are suspension feeders. With notochord
and paired muscle blocks, the lancelet and Pikaia may belong to the chordate group of animals
from which the vertebrates have descended.
Cephalochrodates: The lancelet, like all cephalochordates, has a head. Adult lancelets retain the
four key features of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a
post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles.
The filtered water then collects in the atrium and exits through the atriopore.

The Evolution of Craniata and Vertebrata

Both genomic and fossil evidence suggests that vertebrates evolved from craniates, which
evolved from invertebrate chordates.

Craniata and Vertebrata

The clade Craniata is a subdivision of Chordata. Members of Craniata posses a cranium, which is
a bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones. The clade
Craniata includes all vertebrates and the hagfishes (Myxini), which have a cranium but lack a
backbone. Hagfish are the only known living animals that have a skull, but not a vertebral
column.
Hagfish: Although it lacks a backbone, the hagfish is a member of the Craniata clade because it
possesses a bony skull.
Clade Craniata: Craniata, including this fish (Dunkleosteus), are characterized by the presence
of a cranium, mandible, and other facial bones.

Vertebrates are members of the subphylum Vertebrata, the clade Craniata, and the phylum
Chordata. Vertebrates display the four characteristic features of chordates, but they are named
for the vertebral column composed of a series of bony vertebrae joined together as a backbone.
In adult vertebrates, the vertebral column replaces the embryonic notochord.

Vertebrates: Vertebrata are characterized by the presence of a backbone, such as the one that
runs through the middle of this fish. All vertebrates are in the Craniata clade and have a cranium.

Vertebrate Evolution

In the phylum Chordata, the closest relatives of the vertebrates are the invertebrate chordates.
Based on the molecular analysis of vertebrate and invertebrate genomes (genomics), scientists
can determine the evolutionary history of different phylogenetic groups.

According to these genomic analyses, vertebrates appear to be more closely related to the
lancelets (cephalochordates) than to the tunicates (urochordates). This suggests that the
cephalochordates first diverged from urochordates, and that vertebrates subsequently diverged
from the cephalochordates. This hypothesis is further supported by the fossil of a 530 million-
year-old organism with a brain and eyes like a vertebrate, but without the skull found in a
craniate. A comparison of the genomes of a lancelet, tunicate, lamprey, fish, chicken, and human
confirmed that two whole-genome duplications occurred in the early history of the Vertebrata
subphylum.

Both fossil and genomic evidence suggests that vertebrates arose during the Cambrian
explosion.The Cambrian explosion was the relatively brief span of time during the Cambrian
period during which many animal groups appeared and rapidly diversified. Most modern animal
phyla originated during the Cambrian explosion.
Characteristics of Vertebrates

Vertebrata is a subphlyum of Chordata that is further defined by their bony backbone.

Characteristics of Vertebrates

Vertebrates are members of the subphylum Vertebrata, under the phylum Chordata and under the
kingdom Animalia. Animals that possess bilateral symmetry can be divided into two groups,
protostomes and deuterostomes, based on their patterns of embryonic development. The
deuterostomes, whose name translates as “second mouth,” consist of two phyla: Chordata and
Echinodermata. Echinoderms are invertebrate marine animals that have pentaradial symmetry
and a spiny body covering; the phylum includes sea stars, sea urchins, and sea cucumbers. The
phylum Chordata contains two groups of invertebrate chordates, but the most conspicuous and
familiar members of Chordata are the vertebrates.

Phylum chordata: All chordates are deuterostomes, possessing a notochord. Vertebrates are
differentiated by having a vertebral column.

As chordates, all vertebrates have a similar anatomy and morphology with the same qualifying
characteristics: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.
However, the subphylum Vertebrata is distinguished from the phylum Chordata by the
development of the notochord into a bony backbone. Vertebrates include the amphibians,
reptiles, mammals, and birds, as well as the jawless fishes, bony fishes, sharks, and rays.
Diversity of vertebrates: animals with backbones: The subphylum Vertebrata contains all
animals that possess backbones, gills, and a central nervous system in at least one phase of
development. Vertebrates include amphibians, reptiles, mammals, and birds, as well as the
jawless fishes, bony fishes, sharks, and rays.

More than 64,000 species of vertebrates have been described, but the extant vertebrate species
represent only a small portion of all the vertebrates that have existed. Vertebrates range in size
from the frog species Paedophryne amauensis (as small as 7.7 mm (0.3 inch)) to the blue whale
(as large as 33 m (110 ft)). Vertebrates comprise about 4 percent of all described animal species;
the remainder are invertebrates, which lack backbones.

Anatomy and Morphology

All vertebrates are built along the basic chordate body plan: a stiff rod running through the length
of the animal (vertebral column), with a hollow tube of nervous tissue (the spinal cord) above it
and the gastrointestinal tract below. In all vertebrates, there is a mouth at anterior end of the
animal and an anus before the posterior end of the body. There is a tail posterior to the anus
during at least one phase of the animal’s development.

The Vertebral Column

Vertebrates are defined by the presence of the vertebral column. In vertebrates, the notochord
develops into the vertebral column or spine: a series of bony vertebrae each separated by mobile
discs. These vertebrae are always found on the dorsal side of the animal. However, a few
vertebrates have secondarily lost their vertebrae and, instead, retain the notochord into adulthood
(e.g., the sturgeon fish).

Vertebral column: A fossilized skeleton of the dinosaur Diplodocus carnegii shows an extreme
example of the backbone that characterizes vertebrates.
Central Nervous System

Vertebrates are also the only members of Chordata to possess a brain. In chordates, the central
nervous system is based on a hollow nerve tube that runs dorsal to the notochord along the length
of the animal. In vertebrates, the anterior end of the nerve tube expands and differentiates into
three brain vesicles.

Vertebrate Classification

Vertebrates are the largest group of chordates, with more than 62,000 living species. Vertebrates
are grouped based on anatomical and physiological traits. The traditional groups include
Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia.

Animals that possess jaws are known as gnathostomes, meaning “jawed mouth.” Gnathostomes
include fishes and tetrapods (amphibians, reptiles, birds, and mammals). Tetrapods can be further
divided into two groups: amphibians and amniotes. Amniotes are animals whose eggs are
adapted for terrestrial living; this group includes mammals, reptiles, and birds. Amniotic
embryos, developing in either an externally-shelled egg or an egg carried by the female, are
provided with a water-retaining environment and are protected by amniotic membranes.

Key Takeaways
Key Points

• Urochordata (tunicates) and Cephalochordata (lancelets) are invertebrates because they

lack a backone.
• Larval tunicates (Urochordata) posses all four structures that classify chordates, but adult
tunicates retain only pharyngeal slits.
• Larval tunicates swim for a few days after hatching, then attach to a marine surface and
undergo metamorphosis into the sessile adult form.
• Lancelets (Cephalochordata) are marine organisms that possess all features of chordates;
they are named Cephalochordata because the notochord extends into the head.
• Lancelets may be the closest-living relatives to vertebrates.

Key Terms

• Urochordata: a taxonomic subphylum within the phylum Chordata: the tunicates or sea
squirts
• Cephalochordata: a taxonomic subphylum within the phylum Chordata: the lancelets
• sessile: permanently attached to a substrate; not free to move about; “an attached oyster”

Key Takeaways
Key Points

• The clade Craniata includes animals that have a cranium: a bony, cartilaginous, or fibrous
structure that surrounds the brain, jaw, and facial bones.
 • Members of Craniata include the vertebrates and hagfish.
• Genomic evidence suggests that vertebrates diverged from cephalochordates (lancelets),
which had previously diverged from urochordates (tunicates).
• Fossil evidence suggests that most vertebrate diversity originated in the Cambrian
explosion 540 million years ago.
• Two whole- genome duplications occurred in early vertebrate history.

Key Terms

• cranium: the part of the skull enclosing the brain, the braincase
• genomics: the study of the complete genome of an organism
• Cambrian explosion: the relatively rapid appearance (over a period of many millions of
years), around 530 million years ago, of most major animal phyla as demonstrated in the
fossil record

Key Takeaways
Key Points

• As chordates, vertebrates have the same common features: a notochord, a dorsal hollow
nerve cord, pharyngeal slits, and a post-anal tail.
• Vertebrates are further differentiated from chordates by their vertebral column, which
forms when their notochord develops into the column of bony vertebrae separated by
discs.
• Vertebrates are the only chordates that have a brain as part of their central nervous
system.

Key Terms

• vertebral column: the series of vertebrae that protect the spinal cord; the spinal column
• chordate: a member of the phylum Chordata; numerous animals having a notochord at
some stage of their development; in vertebrates this develops into the spine
• notochord: a flexible rodlike structure that forms the main support of the body in the
lowest chordates; a primitive spine