Largest of the deuterostome phyla. All the chordates possess three unique characteristics at some stage in their life history. These three diagnostic features are:
(i) The dorsal hollow or tubular nerve cord.
(ii) A longitudinal supporting notochord.
(iii) A series of pharyngeal gill slits.
Phylum chordata is divided into two groups: Acraniata (Protochordata) & Craniata (Euchordata).
Acraniata : All marine, small, primitive chordates. Lacking a head, a skull or cranium, a vertebral column, jaws and brain. It is divided into three subphyla-Hemichordata, Urochordata and Cephalochordata chiefly on the character of notochord present (Recent opinion removes Hemichordata as a separate phylum of invertebrates).
Craniata : Includes single subphyla - Vertebrata. Divided into two subdivisions:
(a) Agnatha (Jawless vertebrates) : has two classes - Ostracodermi and Cyclostomata.
(b) Gnathostomata : Further divided into two superclasses :
(i) Pisces : Divided into three classes - Placodermi, Chondrichthyes and Osteichthyes.
(ii) Tetrapoda : Divided into four classes - Amphibia, Reptilia, Aves and Mammalia.
