Orders: Ichthyornes, Colymbiformes, Sphenisciformes, Procellariiformes, Ciconiiformes, (Suborders: Steganopodes, Ardeae, Ciconiae, Phoenicopteri), Anseriformes (Sub-orders: Palamedeae, Anseres), Falconiformes (Sub-orders: Cathartae, 1 Craniata may be usefully divided into 3 grades: (a) Branchiata Heterodactyla, which includes Pisces except Cyclostomes.
In all the Craniata a = z ' ? ?
This notochord represents the persistent primordial skeletal axis which, in the higher Craniata (though not so in the lower), gives way by substitution to the segmented vertebral column.
