Classification of Animal Chordates of Class 11
Largest of the deuterostome phyla. All the chordates possess three unique characteristics at some stage in their life history. These three diagnostic features are:
- The dorsal hollow or tubular nerve cord.
- 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 :
- Pisces : Divided into three classes - Placodermi, Chondrichthyes and Osteichthyes.
- Tetrapoda : Divided into four classes - Amphibia, Reptilia, Aves and Mammalia.