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 ' ? ?
On account of the absence of anything in the nature of a skull, Amphioxus has been regarded as the type of a division, Acrania, in contrast with the Craniata which comprise all the higher Chordata.
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.