Deuterostome Animals—
Part 2: The Chordates
(Part I, Fishes and Amphibians)
(Chapter 34)
Phylum Chordata— chordates
The chordates (phylum Chordata) comprise three major subphyla and about 56,000
living species… 3,000 species of tunicates or sea squirts (urochordates), 24 species
of lancelets (cephalochordates), and over 50,000 species of craniates (sometimes
referred to as the vertebrates) (Figures 2 & 3).
The craniates (vertebrates) are comprised of the fishes, amphibians, reptiles
(including birds), and the mammals (Figure 3).
While all vertebrates (craniates) are chordates, not all chordates are vertebrates
(see Figure 3).
Based on our contemporary understanding of phylogeny amongst the
deuterostomes (Figure 2), chordates seem closely related to the echinoderms
(discussed in a previous lecture) and hemichordates (acorn worms; Figure 4).
Four morphological features characterize chordates (Figure 5) and together,
these 4 traits seem to have played an important role in chordate evolution.
1) Possession of pharyngeal slits or pharyngeal pouches (Figure 5)— these
structures connect the pharynx (a muscular tube linking the mouth cavity and
esophagus) with the outside of the body.
The pharyngeal slits typically become the gill slits in animals with gills (e.g., fishes)
or they close-up in chordates without gills.
1
, The presence of pharyngeal pouches at some point in the development of all
chordates provides evidence of the phylum’s aquatic ancestry.
2) Possession of a notochord (Figure 5)— the notochord is a flexible rod that
forms above the gut and below the nerve cord early in embryonic development.
3) Possession of a single hollow nerve cord (Figure 5)— this nerve cord runs just
beneath the dorsal surface of the animal.
In vertebrates, this nerve cord differentiates into the brain and spinal cord.
4) Possession of a muscular post-anal tail (Figure 5)— this tail extends beyond
the anus (at least during embryonic development).
Nearly all other animals have terminal tails.
All chordates exhibit the aforementioned 4 characteristics at some point in their
lives (Figure 6).
In addition, chordates also have muscles that are arranged in segmented blocks
that affect the basic organization of the body (Figure 7).
Most chordates also have an internal skeleton (endoskeleton) against which the
muscles work (Figure 7).
The chordate notochord or internal skeleton make possible the extraordinary
powers of locomotion that characterize the chordates.
Non-vertebrate chordates
Non-vertebrate chordates such as tunicates (Figure 8) and lancelets (Figure 9),
have notochords but no vertebrae.
2
Part 2: The Chordates
(Part I, Fishes and Amphibians)
(Chapter 34)
Phylum Chordata— chordates
The chordates (phylum Chordata) comprise three major subphyla and about 56,000
living species… 3,000 species of tunicates or sea squirts (urochordates), 24 species
of lancelets (cephalochordates), and over 50,000 species of craniates (sometimes
referred to as the vertebrates) (Figures 2 & 3).
The craniates (vertebrates) are comprised of the fishes, amphibians, reptiles
(including birds), and the mammals (Figure 3).
While all vertebrates (craniates) are chordates, not all chordates are vertebrates
(see Figure 3).
Based on our contemporary understanding of phylogeny amongst the
deuterostomes (Figure 2), chordates seem closely related to the echinoderms
(discussed in a previous lecture) and hemichordates (acorn worms; Figure 4).
Four morphological features characterize chordates (Figure 5) and together,
these 4 traits seem to have played an important role in chordate evolution.
1) Possession of pharyngeal slits or pharyngeal pouches (Figure 5)— these
structures connect the pharynx (a muscular tube linking the mouth cavity and
esophagus) with the outside of the body.
The pharyngeal slits typically become the gill slits in animals with gills (e.g., fishes)
or they close-up in chordates without gills.
1
, The presence of pharyngeal pouches at some point in the development of all
chordates provides evidence of the phylum’s aquatic ancestry.
2) Possession of a notochord (Figure 5)— the notochord is a flexible rod that
forms above the gut and below the nerve cord early in embryonic development.
3) Possession of a single hollow nerve cord (Figure 5)— this nerve cord runs just
beneath the dorsal surface of the animal.
In vertebrates, this nerve cord differentiates into the brain and spinal cord.
4) Possession of a muscular post-anal tail (Figure 5)— this tail extends beyond
the anus (at least during embryonic development).
Nearly all other animals have terminal tails.
All chordates exhibit the aforementioned 4 characteristics at some point in their
lives (Figure 6).
In addition, chordates also have muscles that are arranged in segmented blocks
that affect the basic organization of the body (Figure 7).
Most chordates also have an internal skeleton (endoskeleton) against which the
muscles work (Figure 7).
The chordate notochord or internal skeleton make possible the extraordinary
powers of locomotion that characterize the chordates.
Non-vertebrate chordates
Non-vertebrate chordates such as tunicates (Figure 8) and lancelets (Figure 9),
have notochords but no vertebrae.
2
