Phylum Chordate is divided into three Sub-Phyla.
- SUB-PHYLUM: UROCHORDATA
- SUB-PHYLUM: CEPHALOCHORDATA
- SUB-PHYLUM: VERTEBRATA
The first two sub phyla are called lower chordates or PROTOCHORDATES. They are usually called Acrania group. The vertebrata sub-phylum is called CRANIATA group.
1. SUB-PHYLUM: UROCHORDATA:
The urochordate animals belonging to this sub-phylum re called sea-squirts. The adults are marine and mostly fixed. Free living forms are also present. The body is covered by test
or tunicin. The notochord and nerve cord are found in the larval stage
. in the adult form they are completely lost. Gill-slits may be permanent. They are’ bisexual forms. The larvae are free swimming.
Ex: Olkopleura, A.scidian, Herdmania, etc.
2. SUB-PHYLUM: CEPHALOCHORDATA:
Cephalochordata are marine organisms. Their body is fish-like with notochord and nerve cord persisting throughout life . They extend the entire length of the body. The eyes, and jaws are absent. The fundamental plan of the chordate body is seen in its most simple form in these animals. Gonads are paired in Amphioxus and unpaired in Asymmetron.
Ex: Amphioxus, Asymmetron.
3. SUB-PHYLUM: VERTEBRATA OR CRANIATA:
are well developed chordates. They show distinct head. Notochord is replaced by the vertebral column completely or partially. The nerve-tubes anterior
and enlarged to form a brain. Cranium protects the brain. Visceral clefts called Gills perform respiration. They are not more than seven pairs. Heart is ventral Andes are present They are formed by several segments.