Animals belonging to phylum Chordata are fundamentally characterized by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits (Figure). These are bilaterally symmetrical, triploblastic, coelomate with the organ-system level of organization. They possess a post-anal tail and a closed circulatory system.
Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrate.
Subphyla Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine. In Urochordata, the notochord is present only in the larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life. Examples: Urochordata – Ascidia, Scapa, Doliolum; Cephalochordata – Branchiostoma (Amphioxus or Lancelet).
The members of subphylum Vertebrata possess notochord during the embryonic period. The notochord is replaced by a cartilaginous or bony vertebral column, in the adult. Thus all vertebrates are chordates but all chordates are not vertebrates. Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation and paired appendages which may be fins or limbs.