solitary and colonial forms. Body is unsegmented and covered by a test or tunic. Adult forms are sac like. Coelom is absent, but has an atrial cavity surrounding the pharynx.
Notochord is present only in the tail region of the larval stage, hence named urochordata. Alimentary canal is complete and circulatory system is of open type. The heart is ventral and tubular. Respiration is through gill slits and clefts.
Dorsal tubular nerve cord is present only in the larval stage and a single dorsal ganglion is present in the adults. Mostly hermaphrodites, development indirect and includes a free swimming tadpole larva with chordate characters. Retrogressive metamorphosis is seen (Figure . ).
Examples: Ascidia, Salpa, Doliolum . . Subphylum: Cephalochordata ( L . Cephalo- ‘head’ ; G.
chorda ‘cord’ .) Cephalochordates are marine forms, found in shallow waters, leading a burrowing mode of life. They are small Table. Comparison of chordates and non-chordates Chordates Non-chordates Notochord is present Notochord is absent Dorsal, hollow and single nerve cord Double ventral solid nerve cord Pharynx perforated by gill slits Gill slits absent Heart is ventrally placed Heart is dorsal or laterally placed or absent A post anal tail is present A post anal tail is absent Alimentary canal is placed ventral to the nerve cord Alimentary canal is placed dorsal to the nerve cord Figure . Examples of Urochordata Ascidia Salpa Doliolum XI Std Biology-Zoology Chapter- without paired appendages.
Notochord persists in the adult. Gnathostomata includes jawed vertebrates with paired appendages. Notochord is replaced partly or wholly by the vertebral column. Agnatha includes one important class – Cyclostomata.
Gnathostomata includes jawed fishes (Pisces) and Tetrapoda ( amphibia, reptilia, aves and mammals). The superclass Pisces includes all fishes which are essentially aquatic forms with paired fins for swimming and gills for respiration. Pisces includes cartilaginous fishes (Chondrichthyes) and bony fishes (Osteichthyes). .
. Class: Cyclostomata (G. cyklos –circle; stomata -mouth) All members of cyclostomata are primitive, poikilothermic, jawless aquatic vertebrates and are ectoparasites on some fishes. Body is slender and eel-like bearing six to fifteen pair of gill slits for respiration