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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.