Introduction to Phylum Chordata

1.      Bilaterally symmetrical, coelomate, triploblastic with organ level organization.

2.      Characteristic feature – presence of notochord, which is a dorsal hollow nerve cord and paired pharyngeal gill slits

3.      Presence of post-anal tail

4.      Closed circulatory system is seen

Phylum Chordata is further classified into three sub-phyla namely, Urochordata or Tunicata, Cephalochordata and Vertebrata. Sub-phyla Urochordata and Cephalodhordata is together referred to as protochordata. Notochord is seen in larval tail of Urochordata organisms. While in Cephalochordata organisms, the notochord extends from head to tail region and exists throughout their life cycle.

Organisms belonging to the sub-phyla Urochordata are Ascidia, Salpa, Doliolium

Organims belonging to the sub-phyla Cephalochordata are Branchiostoma (Amphioxus or Lancelet)

Sub-phyla vertebrata organisms develop notochord during the embryonic development period. The notochord is derived from mesodermal germinal layer. The notochord is replaced by a cartilaginous or bony vertebral column in an adult. Thus, all vertebrates are chordates but not all chordates are vertebrates. The organisms of the sub-phyla vertebrata have a ventral muscular heart with either two, three or four chambers. Vertebrata organisms show a presence of kidneys for excretion and osmoregulation purposes and paired appendages which may be fins or limbs.

The sub-phyla vertebrata is further divided as follows,

Sub-Class Cyclostomata

1.      Found as ectoparasites on some fishes. Usually marine habitats but migrate to fresh water for spawning (reproduction)

2.      Presence of sucking and circular mouth absent of jaws.

3.      Elongated body with 6-15 pairs of gill slits to carry out respiration process

4.      Absence of scales and paired fins

5.      Closed circulatory system

6.      Presence of cartilaginous cranium and vertebral column

7.      Spawning of cyclostomes results in their death. The larval cyclostomes metamorphose and return to ocean.

8.      Organisms belonging to the sub-class cyclostomata – Petromyzon (Lamphrey) and Myxine (Hag fish)