What is Phylum Chordata?
1. Bilaterally symmetrical, coelomate, triploblastic with the organ-level organization.
2. Characteristic feature – the presence of a notochord, which is a dorsal hollow nerve cord and paired pharyngeal gill slits
3. Presence of post-anal tail
4. The 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 are together referred to as protochordate. The notochord is seen in the 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
Organisms belonging to the sub-phyla Cephalochordata are Branchiostoma (Amphioxus or Lancelet)
Sub-phyla vertebrate organisms develop notochord during the embryonic development period. The notochord is derived from the 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 vertebrate have a ventral muscular heart with either two, three, or four chambers. Vertebrata organisms show the presence of kidneys for excretion and osmoregulation purposes and paired appendages which may be fins or limbs.
The sub-phyla vertebrate is further divided as follows,
What are the Sub-Class Cyclostomata
1. Found as ectoparasites on some fishes. Usually, marine habitats 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 the respiration process
4. The 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 the ocean.
8. Organisms belonging to the sub-class Cyclostomata – Petromyzon (Lamprey) and Myxine (Hagfish)