Phylum: Chordata: (Part – I)
At some stages of their life all chordates possess a stiff rod-like structure called notochord, close to the dorsal surface. They have a dorsal, tubular nerve cord close to notochord. They are triploblastic animals with three germ layers and have paired gill pouches at some stages of their life. They are true coelomate animals.
Phylum Chordata has three subphylum; Urochordata, Cephalochordata and Vertebrata.
Subphylum: Urochordata: Marine, mostly sessile, filter-feeders. Notochord occurs only in the tail of the larva and disappears in the adult. The gill slits are numerous, persist in the adult and open into an ectoderm-lined cavity, the atrium, instead of to the exterior. The tail not persists throughout life, Example: Herdmania, Doliolum, and Ciona.
Subphylum: Cephalochordata: Notochord extends to the tip of the snout and persists throughout life. The nerve cord persists throughout life but no brain is formed. The gill slits are numerous and persist in the adult. The body wall consists of myotomes. Tail persists throughout life Example: Amphioxus.
Subphylum: Vertebrate: Notochord replaced by vertebral column (back bone). Body is with well-developed head, neck, trunk and paired fins or limbs. Dorsal tubular nerve cord present which is divided into brain and spinal cord.