Download Chordate Information

CHORDATE BACKGROUND INFORMATION
EVOLUTIONARY PERSPECTIVE
The relationship of chordates relates to echinoderms and the relationship of the echinoderm-chordate line to previous phyla
discussed Zoologists have not yet found evidence documenting the origin of chordates from non-chordates. There are three subphyla within
the phylum Chordata: Urochordata, Cephalochordata, and Vertebrata. Of these, members of the subphylum Urochordata have been used in
one hypothesis of chordate origins. Adult urochordates are sessile filter feeders and are most similar to other chordates as larvae. It is
suggested that a urochordate-like ancestor gave rise to the chordate lineage when sexual maturity developed in the larval body form. (This is
called "paedomorphosis" and is seen in a number of animal phyla.) These ancestral chordates then could have given rise to modern
urochordates, cephalochordates, and vertebrates.
THE TRIPLOBLASTIC COELOMATE ANIMALS-CHORDATA
It is relatively easy to characterize the phylum Chordata. Even though they show remarkable diversity of form and function, all
chordates at some time in their life history have four easily recognized, unique features: the notochord, pharyngeal slits (pouches), a dorsal
tubular nerve cord, and a post anal tail.
The notochord is a supportive rod that extends most of the length of the animal along the dorsal body wall. It consists of a connective
tissue sheath packed with vacuolated cells. It supports the body along the anterior-posterior axis, yet is flexible enough to allow freedom of
body movement. In many vertebrate adults, the notochord is partly or entirely replaced by bone.
Pharyngeal slits are a series of openings between the digestive tract, in the pharyngeal region, and the outside of the body. Although
the term "gill slits" is often used to refer to these openings, it is misleading. The earliest chordates used the openings for filter feeding and did
not have gills associated with these pharyngeal slits. In some chordates that function is retained. Other chordates, however, have developed
gills within the slits and use them for gas exchange. Like the notochord, the pharyngeal slits of higher vertebrates are mainly embryonic
features. The eustachian tube of tetrapods, connecting the middle ear to the pharynx, is derived from a pharyngeal slit. The eustachian tube is
used in equalizing pressure within the middle ear to prevent damage to the eardrum.
The dorsal tubular nerve cord and its associated structures are largely responsible for the success of the chordates. It runs along the
dorsal axis of the body, just above the notochord, and is usually modified anteriorly into a brain. These nervous structures have allowed the
development of the most complex animal systems for sensory perception, integration, and motor response.
The fourth characteristic is a tail that extends posterior to the anal opening. The tail is generally supported by the notochord or by the
vertebral column, which replaces the notochord in many adult chordates. In addition, chordates are coelomate, triploblastic, and bilaterally
symmetrical.
CLASSIFICATION
Subphylum Urochordata
Subphylum Cephalochordata
Subphylum Vertebrata
Super Class Agnathans
Class Cephalaspidomorphi
Class Myxini
Super Class Gnathostomes
Class Chondrichthyes
Class Osteichthyes
Class Amphibia
Class Reptilia
Class Aves
Class Mammalia
The vertebrates have traditionally been divided into superclasses based on whether they lack jaws and paired appendages
(Agnatha) or possess these structures (Gnathostomata).
Subphylum Urochordata-Tunicates
Characteristics: Notochord, nerve cord, and postanal tail present only in free-swimming larvae; pharyngeal slits present in larvae and adults;
pharyngeal basket; adults sessile, or occasionally planktonic, and enclosed in a cellulose-containing tunic. Members of the subphylum
Urochordata are the tunicates or sea squirts. Most are sessile as adults and are either solitary or colonial. Sessile urochordates attach their
saclike bodies to rocks, pilings, the hulls of ships, and other solid substrates.
Subphylum Cephalochordata
Characteristics: Body laterally compressed and transparent, fishlike; all four chordate characteristics persist throughout life. The
Cephalochordata (lancelets) is a relatively small group of fishlike chordates. Lancelets are marine, spending most of their life with their
posterior half buried in sandy substrates of shallow water. They feed by filtering suspended organic particles from water in a manner similar to
that described for tunicates.
Subphylum Vertebrata
Characteristics: Notochord, nerve cord, postanal tail, and pharyngeal slits present at least in embryonic stages; vertebrae surround nerve cord
and serve as primary axial support; skeleton modified anteriorly into a skull for protection of the brain. There are eight classes of vertebrates:
Agnathans
Lack jaws and paired appendages. Have cartilaginous skeleton, persistent notochord, and two semicircular canals.
Class Cephalaspidomorphi – Lampreys
Sucking mouth with teeth and rasping tongue. Seven pairs of pharyngeal slits. Blind olfactory sacs.
Class Myxini – Hagfishes
Mouth with four pairs of tentacles. Olfactory sacs open to the mouth cavity. Five to fifteen pairs of pharyngeal
slits.
Gnathostomes
Jaws and paired appendages present. Notochord may be replaced by a vertebral column. Three semicircular canals.
Class Chondrichthyes – Sharks, skates, rays, ratfishes
Tail with large upper lobe. Cartilaginous skeleton. Most lack opercula, swim bladder, and lungs.
Class Osteichthyes – Bony fish
Most have a bony skeleton. Single gill opening covered by a bony operculum. Pneumatic sac(s) function as
lungs or swim bladders.
Class Amphibia – Frogs, toads, salamanders
Skin with mucoid secretions. Usually undergo metamorphosis. Moist skin functions as a respiratory organ.
Class Reptilia – Snakes, lizards, alligators, turtles
Dry skin with epidermal scales. Amniotic egg.
Class Aves – Birds
Feathers. Skeleton modified for flight. Metabolic heat used in temperature regulation (endothermic). Amniotic
egg.
Class Mammalia – Mammals
Have hair, mammary glands. Endothermic temperature regulation. Amniotic egg.