Animal Diversity: Chordata

I. INTRODUCTION
Members of the phylum Chordata are commonly referred to as chordates. Four characters, of
prime diagnostic importance, are possessed by all chordates: 1) A primitive endoskeletal
structure called the notochord is present during early embryonic life. This pliant, rod-like
structure, composed of a peculiar type of connective tissue, is located along the mid-dorsal
line, where it forms the axis of support for the body. In some animals it persists as such
throughout life, but in most chordates it serves as a foundation around which the vertebral
column is built. 2) A hollow, dorsal nerve tube is present sometime during life. The central
nervous system, made up of the brain and the spinal cord, is located in a dorsal position just
above the notochord. It is a hollow canal from one end to the other. 3) Gill slits or traces of
them connecting to the pharynx are present at some stage of life. Most aquatic chordates
respire by gills made up of vascular lamellae or filaments lining the borders of the gill slits,
which connect to the pharynx and open directly or indirectly to the outside. Even terrestrial
chordates, which never breathe by gills, nevertheless have traces of gill slits present as
transient structures, during early embryonic life. No vascular lamellae line these temporary
structures, nor do they open to the outside, but the fact that they are present in all chordates is
of prime importance in denoting close relationship. 4) Chordates possess a post-anal tail in
some stage of their life that represents a posterior elongation of the body extending beyond
the anus. The tail is primarily an extension of the chordate locomotor apparatus, the
segmental musculature and notochord. Apart from the above four features, chordates also
have certain characteristics common to some other phyla as well. 5) They are bilaterally
symmetrical; 6) are metameric; 7) have a true body cavity or coelom, lined with mesoderm;
8) show cephalization or the concentration of nervous tissue and specialized sense organs in
or towards the head; 9) the blood is pumped anteriorly from the ventrally located heart and
forced to the dorsal side. It then moves posteriorly and returns to the heart by veins.
The phylum Chordata is usually subdivided into four main groups or subphyla. The first three
of these include a few relatively simple animals, which lack a cranium and brain. These
organisms are sometimes collectively referred to as the Acrania. The animals included in this
category are believed to show similarities to the chordate ancestors, hence are frequently
known as the protochordates. These are the subphyla Hemichordata (acorn worms),
Urochordata (tunicates), and the Cephalochordata (amphioxus). Vertebrata (Craniata) is a
large group, embracing chordates having a brain; endoskeleton; notochord not extending
forward under the brain; paired eyes; presence of red blood cells; a ventrally placed heart;
presence of a sympathetic nervous system; and presence of a hepatic portal system.
Vertebrates include the jawless forms, lacking vertebrae (Super class Agnatha), and the jawed
vertebrates (Super class Gnathostomata). Furthermore, the latter include the series, Pisces
embracing the lower forms commonly known as fishes. The remaining vertebrates are
included in the group Tetrapoda, which are basically four-footed animals, although in some
the limbs have been lost or modified secondarily.
Tetrapods are those members of the subphylum Vertebrata having paired appendages in the
form of limbs rather than fins, though in some forms the limbs either degenerate completely
or show modifications. Among other characteristics which distinguish tetrapods from fishes
are a cornified outer layer of skin; nasal passages which communicate with the mouth cavity
and which transport air; lungs used in respiration; and a bony skeleton along with a reduction
in the number of skull bones.
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